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Femtosecond laser flash photolysis of 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)tetrahydrofuran produces singlet 3-oxacyclopentylidenecarbene which reacts with acetonitrile solvent to form an ylide. This is the first direct detection of ylide formation by an alkylidenecarbene. This new type of ylide was observed to have a broad absorption band in the visible region with λmax ∼450 nm and a lifetime of ∼13.5 ps. As with other "conventional" carbenes (the divalent carbon atom is separately bound to two substituents), this ylide formation method could be also useful for detecting alkylidenecarbenes, especially those that do not absorb at wavelengths suitable for direct observation. Furthermore, the mechanisms by which 3-oxacyclopentylidenecarbene forms the ylide and the overall favorability of ylide formation, vis-a-vis ring expansion of the carbene to strained 3-oxacyclohexyne, were supported by results from density functional theory calculations.

Maximizing the regeneration of singlet excitons remains a considerable challenge in deep-blue emission systems to obtain low-cost, high-efficiency fluorescent materials. However, the formation of the long-lifetime triplet excitons generally dominates the radiative process, making it greatly difficult to harvest deep-blue emission with high color purity because of the depression of singlet excitons. Here, a very bright deep-blue emission in double perovskite Cs2Na0.4Ag0.6InCl6 alloyed with Bi doping (CNAICB) was successfully achieved by pressure-driven reverse intersystem crossing (RISC), an abnormal photophysical process of energy transfer from the excited triplet state back to the singlet. Therein, the inherently broad emission of CNAICB was associated with the self-trapped excitons (STEs) at excited triplet states, whereas the radiative recombination of STEs populated in excited singlet states was responsible for the observed deep-blue emission. Moreover, the deep-blue emission corresponds to Commission Internationale de L'Eclairage (CIE) coordinates (0.16, 0.06) at 5.01 GPa, which meets the requirement of Rec. 2020 display standards. Likewise, pressure was introduced as an efficient tool to rule out the possibility of the recombination of free excitons and clarify the long-standing conventional dispute over the origin of the low-wavelength emission of Cs2AgInCl6. Our study not only demonstrates that pressure can be a robust means to boost the deep-blue emission but also provides deep insights into the structure-property relationship of lead-free CNAICB double perovskites.

Cyanine (Cy) dyes are among the most useful organic fluorophores that have found a wide range of applications in single-molecule and super-resolution imaging as well as in other biophysical studies. However, recent observations that blueshifted derivatives of Cy dyes are formed via photoconversion have raised concerns as to the potential artifacts in multicolor imaging. Here, we report the mechanism for the photoconversion of Cy5 to Cy3 that occurs upon photoexcitation during fluorescent imaging. Our studies show that the formal C2H2 excision from Cy5 occurs mainly through an intermolecular pathway involving a combination of bond cleavage and reconstitution while unambiguously confirming the identity of the fluorescent photoproduct of Cy5 to be Cy3 using various spectroscopic tools. The carbonyl products generated from singlet oxygen-mediated photooxidation of Cy5 undergo a sequence of carbon-carbon bond-breaking and -forming events to bring about the novel dye-to-dye transformation. We also show that the deletion of a two-methine unit from the polymethine chain, which results in the formation of blueshifted products, commonly occurs in other cyanine dyes, such as Alexa Fluor 647 (AF647) and Cyanine5.5. The formation of a blueshifted congener dye can obscure the multicolor fluorescence imaging, leading to misinterpretation of the data. We demonstrate that the potentially deleterious photoconversion, however, can be exploited to develop a new photoactivation method for high-density single-particle tracking in a living cell without using UV illumination and cell-toxic additives.

Both the sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground state doublet (2S0/D0) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronyl nitroxide, NN) covalently attached to a donor-acceptor chromophore via a m-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4'-di-tert-butyl-2,2'-bipyridine, M = PdII (1-Pd) or PtII (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light produces an initial exchange-coupled, three-spin (bpy•-, CAT•+ = semiquinone (SQ), and NN•), charge-separated doublet 2S1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a 2T1 (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective, and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. It is postulated that zero-field-splitting-induced transitions between the chromophoric 2T1 and 4T1 states (1-Pd and 1-Pt) and spin-orbit-induced transitions between 2T1 and NN-based quartet states (1-Pt) account for the differences in polarization.

Radical pairs generated in crystalline solids by bond cleavage reactions of triplet ketones offer the unique opportunity to explore a frontier of spin dynamics where rigid radicals are highly entangled as the result of short inter-radical distances, large singlet-triplet energy gaps (ΔEST), and limited spin-lattice relaxation mechanisms. Here we report the pulsed laser generation and detection of strongly entangled triplet acyl-alkyl radical pairs generated in nanocrystalline suspensions of 1,1-diphenylmethyl 2-ketones with various 3-admantyl substituents. The sought-after triplet acyl-alkyl radical pairs could be studied for the first time in the solid state by taking advantage of the efficient triplet excited state α-cleavage reactions of 1,1-diphenylmethyl ketones and the slow rate of CO loss from the acyl radicals, which would have to generate highly unstable phenyl and primary alkyl radicals or relatively unstable secondary and tertiary alkyl radicals. With the loss of CO prevented, the lifetime of the triplet acyl-alkyl radical pair intermediates is determined by intersystem crossing to the singlet radical pair state, which is followed by immediate bond formation to the ground state starting ketone. Experimental results revealed biexponential kinetics with long-lived components that account for ca. 87-92% of the transient population and lifetimes that extend to the range of 53-63 μs, the longest reported so far for this type of radical pair. Structural information inferred from the starting ketone will make it possible to analyze the affects of proximity and orientation of the singly occupied orbitals and potentially help set a path for the use of triplet radical pairs as qubits in quantum information technologies.

H-tunneling is a ubiquitous phenomenon, relevant to fields from biochemistry to materials science, but harnessing it for mastering the manipulation of chemical structures still remains nearly illusory. Here, we demonstrate how to switch on H-tunneling by conformational control using external radiation. This is outlined with a triplet 2-hydroxyphenylnitrene generated in an N2 matrix at 10 K by UV-irradiation of an azide precursor. The anti-orientation of the nitrene's OH moiety was converted to syn by selective vibrational excitation at the 2ν(OH) frequency, thereby moving the H atom closer to the vicinal nitrene center. This triggers spontaneous H-tunneling to a singlet 6-imino-2,4-cyclohexadienone. Computations reveal that such fast H-tunneling occurs through crossing the triplet-to-singlet potential energy surfaces. Our experimental realization provides an exciting novel strategy to attain control over tunneling, opening new avenues for directing chemical transformations.

Singlet fission (SF), the conversion of one high-energy singlet to two low-energy triplets, provides the potential to increase the efficiency of photovoltaic devices. In the SF chromophores with C2h symmetry, exemplified by polyenes, singlet-to-triplet conversion generally involves a low-lying 21Ag dark state, which serves as either a multiexciton (ME) intermediate to promote the SF process or a parasitic trap state to shunt excited-state populations via internal conversion. This controversial behavior calls for a deep understanding of dark-state-related photophysics involving the higher-lying singlet state. However, the optical "dark" and "transient" nature of these dark states and strong correlation feature of double exciton species make their characterization and interpretation challenging from both experimental and computational perspectives. In the present work combining transient spectroscopy and multireference electronic structure calculations (XDW-CASPT2), we addressed a new photophysical model, i.e., a high-lying 31Ag dark-state-mediated ultrafast SF process in the benzodipyrrolidone (BDPP) skeleton. Such a 31Ag dark state with distinctive double excitation character, described as the ME state, could be populated from the initial 11Bu bright state on an ultrafast time scale given the quasi-degeneracy and intersection of the two electronic states. Furthermore, the suitable optical band gap and triplet energy, high triplet yield, and excellent photostability render BDPP a promising SF candidate for photovoltaic devices. These results not only enrich the arsenal of SF materials but also shed new insights into the understanding of dark-state-related photophysics, which could promote the development of new SF-active materials.

Density functional calculations have provided evidence that a Ag(I)-mediated deconstructive fluorination of N-benzoylated cyclic amines (LH) with Selectfluor [(F–TEDA)-(BF(4))(2)] begins with an association of the reactants to form a singlet state adduct {[(LH)–Ag]–[F–TEDA](2+)}. The subsequent formation of an iminium ion intermediate, [L(+)–Ag]–HF–[TEDA](+), is, formally, a Ag(I)-mediated hydride abstraction event that occurs in two steps: (a) a formal oxidative addition (OA) of [F–TEDA](2+) to the Ag(I) center that is attended by an electron transfer (ET) from the substrate (LH) to the Ag center (i.e., OA + ET, this process can also be referred to as a F-atom coupled electron transfer), followed by (b) H-atom abstraction from LH by the Ag-coordinated F atom. The overall process involves lower-lying singlet and triplet electronic states of several intermediates. Therefore, we formally refer to this reaction as a two-state reactivity (TSR) event. The C─C bond cleavage/fluorination of the resulting hemiaminal intermediate via a ring-opening pathway has also been determined to be a TSR event. A competing deformylative fluorination initiated by hemiaminal to aldehyde equilibration involving formyl H-atom abstraction by a TEDA(2+) radical dication, decarbonylation, and fluorination of the resulting alkyl radical by another equivalent of Selectfluor may also be operative in the latter step.

Herein we report the synthesis and characterization of anionic boron- and carbon-based Kekule diradicaloids spanned by a p-phenylene bridge. In contrast to Thiele’s hydrocarbon, a closed-shell singlet system, they show an appreciable population of the triplet state at room temperature, as evidenced by both NMR and EPR spectroscopy. Moreover, en route to these anionic boron- and carbon-based hetero-diradicaloids, the formation of an isolable diamino(4-diarylboryl-phenyl)methyl radical was observed.

Double-bond transposition in alkenes (isomerization) offers opportunities for the synthesis of bioactive molecules, but requires high selectivity to avoid mixtures of products. Generation of Z-alkenes, which are present in many natural products and pharmaceuticals, is particularly challenging because it is usually less thermodynamically favorable than generation of the E isomers. We report a β-dialdiminate-supported, high-spin cobalt(I) complex that can convert terminal alkenes, including previously recalcitrant allylbenzenes, to Z-2-alkenes with unprecedentedly high regioselectivity and stereoselectivity. Deuterium labeling studies indicate that the catalyst operates through a π-allyl mechanism, which is different from the alkyl mechanism that is followed by other Z-selective catalysts. Computations indicate that the triplet cobalt(I) alkene complex undergoes a spin state change from the resting-state triplet to a singlet in the lowest-energy C-H activation transition state, which leads to the Z product. This suggests that this change in spin state enables the catalyst to differentiate the stereodefining barriers in this system, and more generally that spin-state changes may offer a route toward novel stereocontrol methods for first-row transition metals.

The reaction of (LSi:)2 (1; L = PhC(NtBu)2) with 2 equiv of Me3SiC2C2SiMe3 resulted in the formation of (Me3SiC2)2(Me3Si)2C4Si2(L)2 (2). 2 exhibited a one-electron transfer when treated with 1 equiv of [Ph3C]+[B(C6F5)4]- to yield [(Me3SiC2)2(Me3Si)2C4Si2(L)2]·+[B(C6F5)4]- (3) and Ph3CCPh3, respectively. When compound 2 was treated with 2 equiv of AgOSO2CF3 a transfer of two electrons occurred to produce [(Me3SiC2)2(Me3Si)2C4Si2(L)2]2+·2[OSO2CF3]- (4) and elemental silver. The 1,4-disilabenzene 2 is disclosed of an open-shell singlet diradical character, and 3 and 4 are, respectively, the elusive stable radical cation and dication species of the 1,4-disilabenzene (2). Furthermore, 2 reacted with group 16 elements of O, S, and Se by oxidative addition to form (Me3SiC2)2(Me3Si)2C4Si2(L)2(μ-O2) (5) and (Me3SiC2)2(Me3Si)2C4Si2(L)2(μ-E) (E = S (6) and Se (7)), respectively.

Inspired by natural metalloenzymes that efficiently catalyze a variety of transformations, chemists have developed large numbers of dinuclear transition-metal complexes with extraordinary properties and reactivity patterns. For main-group element compounds, however, metal-metal cooperativity is much less explored. Here we present the synthesis and characterization of a room-temperature-stable compound with two separated two-coordinated gallium(I) centers possessing both a lone pair of electrons and a vacant orbital, reminiscent of singlet carbenes. This species displays enhanced reactivity compared to its mononuclear counterpart due to bimetallic cooperativity that allows for the facile activation of strong C-F bonds across the gallium-gallium bond. Two mechanistic scenarios of the cooperative bond activation have been identified by DFT and DLPNO-CCSD(T) calculations.

Pentacene's extraordinary photophysical and electronic properties are highly dependent on intermolecular through-space interactions. Macrocyclic arrangements of chromophores have been shown to provide a high level of control over these interactions, but few examples exist for pentacene due to inherent synthetic challenges. In this work, zirconocene-mediated alkyne coupling was used as a dynamic covalent C-C bond forming reaction to synthesize two geometrically distinct, pentacene-containing macrocycles on a gram scale and in four or fewer steps. Both macrocycles undergo singlet fission in solution with rates that differ by an order of magnitude, while the rate of triplet recombination is approximately the same. This independent modulation of singlet and triplet decay rates is highly desirable for the design of efficient singlet fission materials. The dimeric macrocycle adopts a columnar packing motif in the solid state with large void spaces between pentacene units of the crystal lattice.

The porphyrin macrocycle can stabilize a set of magnetic metal ions, thus introducing localized net spins near the center. However, it remains elusive but most desirable to introduce delocalized spins in porphyrins with wide implications, for example, for building correlated quantum spins. Here, we demonstrate that metal-free porphyrins host delocalized π-electron magnetism, as revealed by scanning probe microscopy and a different level of theory calculations. Our results demonstrate that engineering of π-electron topologies introduces a spin-polarized singlet state and delocalized net spins in metal-free porphyrins. In addition, the π-electron magnetism can be switched on/off via scanning tunneling microscope manipulation by tuning the interfacial charge transfer. Our results provide an effective way to precisely control the π-electron magnetism in metal-free porphyrins, which can be further extended to design new magnetic functionalities of porphyrin-based architectures.

Photocages are light-sensitive chemical protecting groups that give investigators control over activation of biomolecules using targeted light irradiation. A compelling application of far-red/near-IR absorbing photocages is their potential for deep tissue activation of biomolecules and phototherapeutics. Toward this goal, we recently reported BODIPY photocages that absorb near-IR light. However, these photocages have reduced photorelease efficiencies compared to shorter-wavelength absorbing photocages, which has hindered their application. Because photochemistry is a zero-sum competition of rates, improvement of the quantum yield of a photoreaction can be achieved either by making the desired photoreaction more efficient or by hobbling competitive decay channels. This latter strategy of inhibiting unproductive decay channels was pursued to improve the release efficiency of long-wavelength absorbing BODIPY photocages by synthesizing structures that block access to unproductive singlet internal conversion conical intersections, which have recently been located for simple BODIPY structures from excited state dynamic simulations. This strategy led to the synthesis of new conformationally restrained boron-methylated BODIPY photocages that absorb light strongly around 700 nm. In the best case, a photocage was identified with an extinction coefficient of 124000 M-1 cm-1, a quantum yield of photorelease of 3.8%, and an overall quantum efficiency of 4650 M-1 cm-1 at 680 nm. This derivative has a quantum efficiency that is 50-fold higher than the best known BODIPY photocages absorbing >600 nm, validating the effectiveness of a strategy for designing efficient photoreactions by thwarting competitive excited state decay channels. Furthermore, 1,7-diaryl substitutions were found to improve the quantum yields of photorelease by excited state participation and blocking ion pair recombination by internal nucleophilic trapping. No cellular toxicity (trypan blue exclusion) was observed at 20 μM, and photoactivation was demonstrated in HeLa cells using red light.

The shape of the lowest singlet excited-state (S1) energy profile is of primary importance in photochemistry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S1 energy profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent π-expanded oxepins, the energy decrease accompanying the bent-to-planar conformational change in S1 becomes less pronounced with lower ESA levels. Stabilization energies following from ESA were quantitatively estimated to be 10-20 kcal/mol using photophysical data. Very fast planarization dynamics in S1 was revealed by time-resolved fluorescence spectroscopy. The time constants were estimated to be shorter than 1 ps, regardless of molecular size and level of ESA, indicating barrierless S1 planarization within the oxepin series.

Complex crystal structures with subtle atomic-scale details are now routinely solved using complementary tools such as X-ray and/or neutron scattering combined with electron diffraction and imaging...

The photoexcitation of α-diazocarbonyl compounds produces ketenes by both concerted and stepwise Wolff rearrangements. The stepwise mechanism proceeds through singlet carbene intermediates which can also participate in bimolecular reactions such as ylide formation with nucleophiles. Here, ultrafast transient infra-red absorption spectroscopy is used to show competitive production of singlet carbene and ketene intermediates from the photoexcitation of ethyl diazoacetoacetate. We provide direct spectroscopic evidence for ylide formation by singlet α-carbonyl carbene capture in aprotic nucleophilic solvents (with ylide bands at 1625 cm-1 in acetonitrile, and 1586 cm-1 and 1635 cm-1 in tetrahydrofuran) and report an enol mediated pathway for singlet α-carbonyl carbene reaction with alcohols (ethanol or t-butanol) identified by an absorption band at 1694 cm-1, but find no evidence for a previously proposed ylide pathway. The α-carbonyl carbene is monitored using a band with solvent-dependent wavenumber in the range 1627 – 1645 cm-1. A computed two-dimensional cut of the potential energy surface for the reaction of the singlet α-carbonyl carbene with methanol shows that the enol forms without a barrier, and that this reaction is promoted by an intermolecular hydrogen bond from methanol to the carbonyl oxygen atom. The corresponding ylide structure lies higher in energy, with a barrierless downhill path to isomerization to the enol.

Sigmarene, which is a Kekule hydrocarbon with appreciable singlet biradical character originating from an o-quinodimethane scaffold, is isolated as a doubly σ-bonded dimer. The dimer dissociates into a monomeric sigmarene upon heating or photoirradiation. The monomeric species undergoes a rapid [4 + 4] cycloaddition reaction under dark conditions even at room temperature to produce the dimer. Contrarily, the monomeric sigmarene undergoes a [4 + 2] cycloaddition reaction in the presence of dienophile as an orbital symmetry allowed process. Therefore, the sigmarene shows high reactivity for both symmetry-forbidden and allowed processes in the framework of the orbital symmetry rule. This duality of reactivity of the sigmarene is consistent with the intermediate singlet biradical character (44%) estimated by a density functional theory (DFT) calculation.

Inclusion of quinoidal cores in conjugated hydrocarbons is a common strategy to modulate the properties of diradicaloids formed by aromaticity recovery within the quinoidal unit. Here we describe an alternative approach of tuning of diradical properties in indenoindenodibenzothiophenes upon anti → syn isomerism of the benzothiophene motif. This alters the relationship of the S atom with the radical center from linear to cross conjugation yet retains the same 2,6-naphtho conjugation pattern of the rearomatized core. We conduct a full comparison between the anti and syn derivatives based on structural, spectroscopic, theoretical, and magnetic measurements, showing that these systems are stable open-shell singlet diradicaloids that only access their triplet state at elevated temperatures.

The oxadi-π-methane rearrangement of 2,4-cyclohexadienones to bicyclic ketones was found to proceed with high enantioselectivity (92-97% ee) in the presence of catalytic amounts of a chiral Lewis acid (15 examples, 52-80% yield). A notable feature of the transformation is the fact that it proceeds on the singlet hypersurface and that no triplet intermediates are involved. Rapid racemic background reactions were therefore avoided, and the catalyst loading could be kept low (10 mol %). Computational studies suggest that the enantioselectivity is determined within a Lewis acid bound singlet intermediate via a conical intersection. The utility of the method was demonstrated by a concise synthesis of the natural product trans-chrysanthemic acid.

Organic diradicaloids have unique open-shell structures and properties and promising applications in organic electronics and spintronics. Incorporation of heteroatoms is an effective strategy to alter the electronic structures of organic diradicaloids. However, B-containing organic diradicaloids are very challenging due to their high reactivities, which are caused by not only diradical nature but also the B atom. In this article, we report a new kind of organic diradicaloids containing boron atoms. Our strategy is to incorporate planarized triarylboranes to antiaromatic polycyclic hydrocarbons (PHs). We synthesized two isomeric B-containing PHs composed of indenofluorene π-skeletons and two dioxa-bridged triphenylborane moieties. As proved by theoretical and experimental results, both of them have excellent ambient stability and open-shell singlet diradical structures, as well as intriguing magnetic and optoelectronic properties, such as thermally accessible triplet species, reversible multiredox ability, and narrow energy gaps. Notably, they possess sufficient Lewis acidity, which has never been observed for organic diradicaloids. In addition, they can coordinate with Lewis bases to form Lewis adducts, achieving unprecedented dynamic modulations of (anti)aromaticity and thus diradical character of organic diradicaloids.

Super-reducing excited states have the potential to activate strong bonds, leading to unprecedented photoreactivity. Excited states of radical anions, accessed via reduction of a precatalyst followed by light absorption, have been proposed to drive photoredox transformations under super-reducing conditions. Here we investigate the radical anion of naphthalene monoimide as a photoreductant and find that the radical doublet excited state has a lifetime of 24 ps, which is too short to facilitate photoredox activity. To account for the apparent photoreactivity of the radical anion, we identify an emissive two-electron reduced Meisenheimer complex of naphthalene monoimide, [NMI(H)]–. The singlet excited state of [NMI(H)]– is a potent reductant (–3.08 V vs Fc/Fc+), is long-lived (20 ns), and its emission can be dynamically quenched by chloroarenes to drive a radical photochemistry, establishing that it is this emissive excited state that is competent for reported C–C and C–P coupling reactivity. These results provide a mechanistic basis for photoreactivity at highly reducing potentials via singlet excited state manifolds and lays out a clear path for the development of exceptionally reducing photoreagents derived from electron-rich closed-shell anions.

Biradicaloids in π-conjugated organic molecules have been extensively studied in recent years because of the fundamental insights into the chemical bonds and unique optical, electrical, and magnetic properties. Several studies have reported that the spin-spin interactions of biradicaloids with flexible molecular frameworks dynamically evolve correlating with molecular structural changes. Although these dynamic behaviors will provide important insights into the relationship between molecular structures and spin properties, studies on such behaviors have been limited to two-spin systems. Here, we investigated the stepwise photochromic properties of biphotochromic molecules involving multiple spin interactions by double-pulse laser flash photolysis. The one-photon photochromic reaction generates the o-biradical form as the open-closed form, which thermally isomerizes to the o-quinoidal form and reaches the thermal equilibrium state between them. The additional absorption of a photon by the open-closed form leads to the photochromic reaction of the other photochromic unit, resulting in the generation of unpaired spins at the p-position of the central aromatic bridge of the biradical or quinoidal form. Under the situation, while the interaction between the unpaired spins and the o-biradical preferentially produces the p-quinoidal form in which the antiferromagnetic interaction at the p-position is dominant, that between the spins and the o-quinoidal form kinetically produces the bis(o-quinoidal) form followed by the thermal isomerization to the thermodynamically stable p-quinoidal form. These dynamic spin-spin interactions along with the rearrangement of chemical bonds will give a deeper understanding of the singlet biradicaloids and that to bridge organic multiradicals in molecular systems to cooperative spin behaviors in bulk materials.

A procedure is described for unbiased identification of all pi-electron chromophore pair geometry choices that locally maximize the rate of conversion of a singlet exciton into a singlet biexciton (triplet pair), using a simplified version of the diabatic frontier orbital model of singlet fission (SF). The resulting approximate optimal geometries provide insight and are expected to represent useful starting points for searches by more advanced methods. The general procedure is illustrated on a pair of ethylenes as the simplest model of a pi-electron system, but it is applicable to pairs of much larger molecules, with dozens of non-hydrogen atoms, and not necessarily planar. We first examine the value of vertical bar T-A vertical bar(2), the square of the electronic matrix element for SF with initial excitation fully localized on partner A, on a grid of several billion geometries within the six-dimensional space of physically realizable possibilities. Several of the optimized pair geometries are somewhat unexpected, but all are found to follow the qualitative guidance proposed earlier. In the neighborhood of each local maximum of vertical bar T-A vertical bar(2), consideration of mixing with charge-transfer configurations and of excitonic interaction between partners A and B determines the SF energy balance and yields squared matrix elements vertical bar T*vertical bar(2) and vertical bar T**vertical bar(2) for the lower and upper excitonic states S* and S**, respectively. Assuming Boltzmann populations of these states, the geometry is further optimized to maximize k, the sum of the SF rates obtained from Marcus theory, and this reorders the suitable geometries substantially. At 87 pair geometries, the vertical bar T*vertical bar(2) and vertical bar T**vertical bar(2) values are compared with those obtained from high-level ab initio nonorthogonal configuration interaction calculations and found to follow the same trend. Finally, the biexciton binding energy at the optimized geometries is calculated. Altogether, 13 significant local maxima of SF rate for a pair of ethylenes are identified in the physically relevant part of space that avoids molecular interpenetration in the hard-sphere approximation. The three best geometries are twist-stacked, slip-stacked, and L-shaped. The maxima occur at the (five dimensional) surfaces of seven six-dimensional "parent" regions of space centered at physically inaccessible geometries at which the calculated SF rate is very large but the two ethylenes interpenetrate. The results are displayed in interactive graphics. The computer code ("Simple") written for these calculations is flexible in that it permits a choice of performing the search for local maxima in six dimensions on vertical bar T-A vertical bar(2), vertical bar T*vertical bar(2), or k.

We report an air-stable diradical dication of chiral D(2)-symmetric conjoined bis[5]diazahelicene with unprecedented high spin (triplet) ground state, singlet triplet energy gap, ΔE(ST) = 0.3 kcal mol(−1). The diradical dication possesses closed-shell (Kekulé) resonance forms with 16 π-electron perimeters. The diradical dication is monomeric in dibutylphthalate (DBP) matrix at low temperatures, and it has a half-life of more than two weeks at ambient conditions in the presence of excess oxidant. A barrier of ~35 kcal mol(−1) has been experimentally determined for inversion of configuration in the neutral conjoined bis[5]diazahelicene, while the inversion barriers in its radical cation and diradical dication were predicted by the DFT computations to be within a few kcal mol(−1) of that in the neutral species. Chiral HPLC resolution provides the chiral D(2)-symmetric conjoined bis[5]diazahelicene, enriched in (P,P)- or (M,M)-enantiomers. The enantiomerically enriched triplet diradical dication is configurationally stable for 48 h at room temperature, thus providing the lower limit for inversion barrier of configuration of 27 kcal mol(−1). The enantiomers of conjoined bis[5]diazahelicene and its diradical dication show strong chirooptical properties that are comparable to [6]helicene or carbon-sulfur [7]helicene, as determined by the anisotropy factors, |g| = |Δε|/ε = 0.007 at 348 nm (neutral) and |g| = 0.005 at 385 nm (diradical dication). DFT computations of the radical cation suggest that SOMO and HOMO energy levels are near-degenerate.

High spin (S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with a large singlet-triplet energy gap, ΔE(ST) ≥ 1.7 kcal mol(−1), leading to nearly exclusive population of triplet ground state at room temperature, and good thermal stability with onset of decomposition at ~160 °C under inert atmosphere. Magnetic properties of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 (S = 1) chain of organic radicals with intrachain antiferromagnetic coupling of J′/k = −14 K, which is associated with the N···N and N···O intermolecular contacts. The intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains of 2 most isotropic, and therefore an excellent system for studies of low-dimensional magnetism. In polystyrene glass and in frozen benzene or dibutyl phthalate solution, both 1 and 2 are monomeric. Diradical 2 is thermally robust and is evaporated under ultra-high vacuum to form thin films of intact diradicals on silicon substrate, as demonstrated by X-ray photoelectron spectroscopy. Based on C-K NEXAFS spectra and AFM images of the ~1.5-nm thick films, the diradical molecules form islands on the substrate with molecules stacked approximately along the crystallographic a-axis. The films are stable under ultra-high vacuum for at least 60 h but show signs of decomposition when exposed to ambient conditions for 7 h.

Localized singlet diradicals are key intermediates in bond homolysis. A thorough study of the reactive species is needed to clarify the mechanisms of the homolytic bond cleavage and formation processes. In general, the singlet diradicals are quite short-lived because of the fast radical-radical coupling reactions. The short-lived characteristic has retarded the thorough study on bond homolysis. In this study, a new series of long-lived singlet diradicals, viz., 1,2-diazacyclopentane-3,5-diyl, were identified, and their electronic structures and novel reactivities were thoroughly studied using laser-flash photolysis (LFP), product analysis, and computational studies. A direct observation of the thermal equilibration (fast process) between the singlet diradicals and the corresponding ring-closing compounds was undertaken on the submicrosecond time scale. The solvent and substituent effects on the equilibration constant and rate constants for the ring-closing reaction and ring-opening reaction clarify the novel nitrogen-atom effect on the localized singlet 1,3-diyl diradicals. Two types of alkoxy-migrated compounds, 9 and 10, were isolated with high yields as the final products. Crossover, spin-trapping, and LFP experiments for the formation of alkoxy-group migration products (i.e., 9 versus 10) revealed the unique temperature effect on the product ratio of the two types of alkoxy-migration products. The temperature-insensitive intersystem crossing process (slow process, millisecond time scale) was found to be a key step in the formation of 9, which is an entropy-controlled pathway. An intramolecular migration process was identified for the formation of 10 that was accelerated by a polar solvent in an enthalpy-controlled process. This unique heteroatom effect has opened up a new series of localized singlet diradicals that are crucial intermediates in bond homolysis.

The ability to prepare physical qubits in specific initial quantum states is a critical requirement for their use in quantum information science (QIS). Subnanosecond photoinduced electron transfer in a structurally well-defined donor-acceptor system can be used to produce an entangled spin qubit (radical) pair in a pure initial singlet state fulfilling this criterion. Synthetic DNA is a promising platform on which to build spin qubit arrays with fixed spatial relationships; therefore, we have prepared a series of DNA hairpins in which naphthalenediimide (NDI) is the chromophore/acceptor hairpin linker, variable-length diblock A- and G-tracts are intermediate donors, and a stilbenediether (Sd) is the terminal donor. Photoexcitation of NDI in these DNA hairpins generates high-yield, long-lived, entangled spin qubit pairs at 85 K, and time-resolved and pulse electron paramagnetic resonance (EPR) spectroscopies are used to probe their spin dynamics. Specifically, measurements of the distance-dependent dipolar coupling between the two spins are used to obtain the average spin qubit pair distance in the absence of the terminal Sd donor and reveal that one of the spins is fully delocalized across up to five adjacent guanines in a G-tract on the EPR time scale. We have recently shown that extensive spin hopping between degenerate sites accessible to one spin of the pair may result in spin decoherence. However, we observe a strong out-of-phase electron spin echo envelope modulation (OOP-ESEEM) signal from the NDI•--Sd•+ spin qubit pair in DNA hairpins showing that spin coherence is maintained across a 2 adenine A-tract followed by a 2-4 guanine G-tract as a result of rapid spin transport to Sd. These results demonstrate that pulse-EPR can manipulate coherent spin states in DNA hairpins, which is essential for quantum gate operations relevant to QIS applications.

We report the challenging synthesis of two very long bis(anthraoxa)quinodimethanes with nine (ABA) and ten (ANA) consecutively fused six-membered rings. The former is stable with negligible diradical character, while the latter with a moderate diradical character (y0 = 25.0%) is reactive and an unexpected trifluoroacetic substituted product (ANA-TFA) was isolated. X-ray crystallographic analysis revealed a planar backbone with a typical quinoidal character for both. Their dications can be regarded as the isoelectronic structures of the respective nonacene and decacene. The dication ABA2+ and dianion ABA2– are open-shell singlet diradicaloids, while the longer dication ANA-TFA2+ and dianion ANA2– have closed-shell ground state, which can be explained by the different intramolecular Coulomb interactions. Both dianions have a bent backbone and can be considered as an isoelectronic structure of the tetraanion of nonacene and decacene, respectively.

The photochemical dynamics of three classes of organic photoredox catalysts employed in organocatalyzed atom-transfer radical polymerization (O-ATRP) are studied using time-resolved optical transient absorption and fluorescence spectroscopies. The nine catalysts selected for study are examples of N-aryl and core-substituted dihydrophenazine, phenoxazine and phenothiazine compounds with varying propensities for control of polymerization outcomes. Excited singlet state lifetimes extracted from the spectroscopic measurements are reported in N,N-dimethylformamide (DMF), dichloromethane (DCM) and toluene. Ultrafast (< 200 fs to 3 ps) electronic relaxation of the photocatalysts after photoexcitation at near-UV wavelengths (318-390 nm) populates the first singlet excited state (S1). The S1-state lifetimes range from 130 ps to 40 ns with considerable dependence on the photocatalyst structure and the solvent. Competition between ground-electronic state recovery and intersystem crossing controls triplet state populations and is a minor pathway in the dihydrophenazine derivatives, but is of greater importance for phenoxazine and phenothiazine catalysts. Comparison of our results with previously reported O-ATRP performances of the various photoredox catalysis shows that high triplet-state quantum yields are not a pre-requisite for controlling polymer dispersity. For example, the 5,10-di(4-cyanophenyl)-5,10-dihydrophenazine photocatalyst, shown previously to exert good polymerization control, possesses the shortest S1-state lifetime (135 ps in DMF and 180 ps in N,N-dimethylacetamide) among the nine examples reported here, and a negligible triplet state quantum yield. The results call for a re-evaluation of the excited state properties of most significance in governing the photocatalytic behaviour of organic photoredox catalysts in O-ATRP reactions.

Singlet fission (SF) is a photophysical process capable of boosting the efficiency of solar cells. Recent experimental investigations into the mechanism of SF provide evidence for coherent mixing between the singlet, triplet, and charge transfer basis states. Up until now, this interpretation has largely focused on electronic interactions; however, nuclear motions resulting in vibronic coupling have been suggested to support rapid and efficient SF in organic chromophore assemblies. Further information about the complex interactions between vibronic excited states is needed to understand the potential role of this coupling in SF. Here, we report mixed singlet and correlated triplet pair states giving rise to sub-50 fs SF in a terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer in which the two TDI molecules are covalently linked by a direct N-N connection at one of their imide positions, leading to a linear dimer with perpendicular TDI π systems. We observe the transfer of low-frequency coherent wavepackets between the initial predominantly singlet states to the product triplet-dominated states. This implies a non-negligible dependence of SF on nonadiabatic coupling in this dimer. We interpret our experimental results in the framework of a modified Holstein Hamiltonian, which predicts that vibronic interactions between low-frequency singlet modes and high-frequency correlated triplet pair motions lead to mixing of the pure basis states. These results highlight how nonadiabatic mixing can shape the complex potential energy landscape underlying ultrafast SF.

Ultralong organic phosphorescence holds great promise as an important approach for optical materials and devices. Most of phosphorescent organic molecules with long lifetimes are substituted with heavy atoms or carbonyl groups to enhance the intersystem crossing (ISC), which requires complicated design and synthesis. Here, we report a cyclization-promoted phosphorescence phenomenon by boosting ISC. N-butyl carbazole exhibits a phosphorescence lifetime (τp) of only 1.45 ms and a low phosphorescence efficiency in the solution state at 77 K due to the lack of efficient ISC. In order to promote its phosphorescence behavior, we explored the influence of conjugation. By linear conjugation of four carbazole units, possible ISC channels are increased so that a longer τp of 2.24 s is observed. Moreover, by cyclization, the energy gap between the singlet and triplet states is dramatically decreased to 0.04 eV for excellent ISC efficiency accompanied by increased rigidification to synergistically suppress the nonradiative decay, resulting in satisfactory phosphorescence efficiency and a prolonged τp to 3.41 s in the absence of any heavy atom or carbonyl group, which may act as a strategy to prepare ultralong phosphorescent organic materials by enhancing the ISC and rigidification.

Excitonic effects caused by Coulomb interactions between electrons and holes play subtle and significant roles on photocatalysis, yet have been long ignored. Herein, porphyrinic covalent organic frameworks (COFs, specifically DhaTph-M), in the absence or presence of different metals in porphyrin centers, have been shown as ideal models to regulate excitonic effects. Remarkably, the incorporation of Zn2+ in the COF facilitates the conversion of singlet to triplet excitons, whereas the Ni2+ introduction promotes the dissociation of excitons to hot carriers under photoexcitation. Accordingly, the discriminative excitonic behavior of DhaTph-Zn and DhaTph-Ni enables the activation of O2 to 1O2 and O2•-, respectively, under visible light irradiation, resulting in distinctly different activity and selectivity in photocatalytic terpinene oxidation. Benefiting from these results, DhaTph-Ni exhibits excellent photocatalytic activity in O2•--engaged hydroxylation of boronic acid, while DhaTph-Zn possesses superior performance in 1O2-mediated selective oxidation of organic sulfides. This work provides in-depth insights into molecular oxygen activation and opens an avenue to the regulation of excitonic effects based on COFs.

We analyze correlated-triplet-pair (TT) singlet-fission intermediates toward two-triplet separation (T...T) using spin-state-averaged density matrix renormalization group electronic-structure calculations. Specifically, we compare the triplet-triplet exchange (J) for tetracene dimers, bipentacene, a subunit of the benzodithiophene-thiophene dioxide polymer, and a carotenoid (neurosporene). Exchange-split energy gaps of J and 3J separate a singlet from a triplet and a singlet from a quintet, respectively. We draw two new insights: (a) the canonical tetracene singlet-fission unit cell supports precisely three low-lying TT intermediates with order-of-magnitude differences in J, and (b) the separable TT intermediate in carotenoids emanates from a pair of excitations to the second triplet state. Therefore, unlike with tetracenes, carotenoid fission requires above-gap excitations. In all cases, the distinguishability of the molecular triplets-that is, the extent of orbital overlap-determines the splitting within the spin manifold of TT states. Consequently, J represents a spectroscopic observable that distnguishes the resemblance between TT intermediates and the T...T product.

International audience; Many organic and main group compounds, usually acids or bases, can accelerate chemical reactions when used in substoichiometric quantities, a process known as organocatalysis. In marked contrast, very few of these compounds are able to activate carbon monoxide and, until now, none of them catalyze its chemical transformation, a classical task for transition metals. Herein, we report that a stable singlet ambiphilic carbene activates CO and catalytically promotes the carbonylation of an ortho-quinone into a cyclic carbonate. These findings pave the way for the discovery of metal-free catalyzed carbonylation reactions.

Generation of electron-hole pairs via symmetry-breaking charge separation (SB-CS) in photoexcited assemblies of organic chromophores is a potentially important route to enhancing the open-circuit voltage of organic photovoltaics. While most reports of SB-CS have focused on molecular dimers in solution where the environmental polarity can be manipulated, here, we investigate SB-CS in polycrystalline thin films of 1,6,7,12-tetra(phenoxy)perylene-3,4:9,10-bis(dicarboximide) having either n-octyl groups (octyl-tpPDI) or hydrogen atoms (H-tpPDI) attached to its imide nitrogen atoms. Structural analyses using various X-ray techniques reveal that while both compounds show π-π stacking in thin films, H-tpPDI is more slip-stacked than octyl-tpPDI and has intermolecular hydrogen bonds to its neighboring molecules. Transient absorption spectroscopy shows that octyl-tpPDI exhibits strong mixing between its singlet excited state and a charge transfer state, yielding an excimer-like state, while H-tpPDI undergoes nearly quantitative SB-CS, making the latter a promising candidate for use in organic photovoltaic devices.

The singlet fission (SF) channels in many systems are controlled by the thermodynamic driving force (Switch-1) and kinetic barrier (Switch-2), both of which could be modulated by chromophore structure dynamics and solvent properties. Using

An olympicenyl radical, a spin 1/2 hydrocarbon radical with C2v symmetry and uneven spin distribution, remains elusive despite the considerable theoretical research interest. Herein, we report syntheses of two air-stable olympicenyl radical derivatives, OR1 and OR2, with half-life times (τ1/2) in air-saturated solution of 7 days and 34 days. The high stability was ascribed to kinetic blocking of reactive sites with high spin densities. X-ray crystallographic analysis revealed unique 20-center-2-electron head-to-tail π-dimer structures with intermolecular distances shorter than the sum of van der Waals radius of carbon. The ground state of the π-dimers was found to be singlet, with singlet-triplet energy gaps estimated to be -2.34 kcal/mol and -3.28 kcal/mol for OR1 and OR2, respectively, by variable-temperature electron spin resonance (ESR) spectroscopy. The monomeric radical species were in equilibrium with the π-dimer in solution, and the optical and electrochemical properties of the monomers and π-dimers in solution were investigated by UV-vis-NIR spectroscopy and cyclic voltammetry, revealing a concentration-dependent nature. Theoretical calculations illustrated that upon formation of a π-dimer the local aromaticity of each monomer was enhanced, and spatial ring current between the monomers was present, which resulted in an increment of aromaticity of the interior of the π-dimer.

Although diazoalkanes are important carbene precursors in organic synthesis, a comprehensive mechanism of photochemi-cal formation of carbenes from diazoalkanes has not been proposed. Synergy of experiments and computations demon-strates the involvement of higher excited singlet states in the photochemistry of diazoalkanes. In all investigated diazo-alkanes, excitation to S1 results in nonreactive internal conversion to S0. On the contrary, excitation to higher-lying singlet states (Sn, n > 1), drives the reaction toward a different segment of the S1/S0 conical intersection seam and results in nitrogen elimination and formation of carbenes.

Two new highly charged [2]catenanes—namely, mHe[2]C·6PF₆ and mHo[2]C·6PF₆—were synthesized by exploiting radical host–guest templation between derivatives containing BIPY•+ radical cations and the meta analogue of cyclobis(paraquat-p-phenylene). In contrast to related [2]catenanes that have been isolated as air-stable monoradicals, both mHe[2]C·6PF₆ and mHo[2]C·6PF₆ exist as air-stable singlet bisradicals, as evidenced by both X-ray crystallography in the solid state and EPR spectroscopy in solution. Electrochemical studies indicate that the first two reduction peaks of these two [2]catenanes are shifted significantly to more positive potentials, a feature which is responsible for their extraordinary stability in air. The mixed-valence nature of the mono- and bisradical states endows them with unique NIR absorption properties, e.g., NIR absorption bands for the mono- and bisradical states observed at ∼1800 and ∼1450 nm, respectively. These [2]catenanes are potentially useful in applications that include NIR photothermal conversion, UV–vis–NIR multiple-state electrochromic materials, and multiple-state memory devices. Our findings highlight the principle of “mechanical-bond-induced stabilization” as an efficient strategy for designing persistent organic radicals.

We report an experimental discovery and computational investigation of the first photochemical stereoselective [1,3]-sigmatropic silyl shift of an allylsilane. An organocatalytic enantioselective cascade annulation generates a trimethylsilyl-o-isotoluene reactant in >99:1 e.r., and this trimethylsilyl-o-isotoluene contains an allylic silane moiety that undergoes a stereoselective photochemical [1,3]-silyl shift to form a benzylsilane with 96:4 e.r. The mechanism of this unprecedented [1,3]-silyl shift has been elucidated by a series of experimental studies and CASSCF, DFT, and TD-DFT calculations on model systems and the experimental system. The highly stereoselective photoreaction is proposed to occur via a singlet silyl/allyl conical intersection. This is a new demonstration of the role of conical intersections in selective photochemistry.

The first hypercoordinated disilene with the longest Si═Si distance (2.623(1) A) reported to date, the 5,6-bis(amidinato silylenyl)acenaphtene (amidinato = PhC(NtBu)2) (1), is presented. It results from a spatially compelled double dative interaction between the two singlet silylene moieties, due to their close proximity. The dynamic behavior of the Si═Si bond contraction and elongation in 1 predicted by quantum chemical calculations could be confirmed experimentally by variable temperature NMR spectroscopy. Compound 1 exhibits an ambivalent reactivity, reacting as disilene with ethylene, diphenylacetylene and benzophenone to give [2+2] cycloaddition products, but as bis(silylene) toward Ni(cod)2 (cod = cycloocta-1,4-diene) to form the corresponding bis(silylene)Ni0(η4-cod) complex.

The photostability of fluorescent labels comprises one of the main limitations in single-molecule fluorescence (SMF) and super-resolution imaging. An attractive strategy to increase the photostability of organic fluorophores relies on their coupling to photostabilizers, e.g., triplet excited state quenchers, rendering self-healing dyes. Herein we report the self-healing properties of trisNTA-Alexa647 fluorophores (NTA, N-nitrilotriacetic acid). Primarily designed to specifically label biomolecules containing an oligohistidine tag, we hypothesized that the increased effective concentration of Ni(II) triplet state quenchers would lead to their improved photostability. We evaluated photon output, survival time, and photon count rate of different Alexa647-labeled trisNTA constructs differing in the length and rigidity of the fluorophore- trisNTA linker. Maximum photon output enhancements of 25-fold versus Alexa647-DNA were recorded for a short tetraproline linker, superseding the solution based photostabilization by Ni(II). Steady-state and time-resolved studies illustrate that trisNTA self-healing role is associated with a dynamic excited triplet state quenching by Ni(II). Here improved photophysical/photochemical properties require for a judicious choice of linker length and rigidity, and in turn a balance between rapid dynamic triplet excited state quenching versus dynamic/static singlet excited state quenching. TrisNTA fluorophores offer superior properties for SMF allowing specific labeling and increased photostability, making them ideal candidates for extended single-molecule imaging techniques.

It is pivotal to achieve efficient triplet-triplet annihilation based photon upconversion (TTA-UC) in the solid-state for enhancing potentials of renewable energy production devices. However, the UC efficiency of solid materials is largely limited by low fluorescence quantum yields that originate from the aggregation of TTA-UC chromophores and also by severe back energy transfer from the acceptor singlet state to the singlet state of the triplet donor in the condensed state. In this work, to overcome these issues, we introduce a highly fluorescent singlet energy collector as the third component of donor-doped acceptor crystalline films, in which dual energy migration, i.e., triplet energy migration for TTA-UC and succeeding singlet energy migration for transferring energy to a collector, takes place. To demonstrate this scheme, a highly fluorescent singlet energy collector was added as the third component of donor-doped acceptor crystalline films. An anthracene-based acceptor containing alkyl chains and a carboxylic moiety is mixed with the triplet donor Pt(II) octaethylporphyrin (PtOEP) and the energy collector 2,5,8,11-tetra- tert-butylperylene (TTBP) in solution, and simple spin-coating of the mixed solution gives acceptor films of nanofibrous crystals homogeneously doped with PtOEP and TTBP. Interestingly, delocalized singlet excitons in acceptor crystals are found to diffuse effectively over the distance of ∼37 nm. Thanks to this high diffusivity, only 0.5 mol % of doped TTBP can harvest most of the singlet excitons, which successfully doubles the solid-state fluorescent quantum yield of acceptor/TTBP blend films to 76%. Furthermore, since the donor PtOEP and the collector TTBP are separately isolated in the nanofibrous acceptor crystals, the singlet back energy transfer from the collector to the donor is effectively avoided. Such efficient singlet energy collection and inhibited back energy transfer processes result in a large increase of UC efficiency up to 9.0%, offering rational design principles toward ultimately efficient solid-state upconverters.

Recent work has shown that diarylmethyl radicals generated by pulsed laser excitation in nanocrystalline (NC) suspensions of tetraarylacetones constitute a valuable probe for the detailed mechanistic analysis of the solid-state photodecarbonylation reaction. Using a combination of reaction quantum yields and laser flash photolysis in nanocrystalline suspensions of ketones with different substituents on one of the α-carbons, we are able to suggest with confidence that a significant fraction of the initial α-cleavage reaction takes place from the ketone singlet excited state, that the originally formed diarylmethyl-acyl radical pair loses CO in the crystal with time constants in the sub-nanosecond regime, and that the secondary bis(diarylmethyl) triplet radical pair has a lifetime limited by the rate of intersystem crossing of ca. 70 ns.

A series of dirhodium(II,II) paddlewheeel complexes of the type cis-[Rh2(μ-DTolF)2(μ-L)2][BF4]2, where DTolF = N,N′-di(p-tolyl)formamidinate and L = 1,8-naphthyridine (np), 2-(pyridin-2-yl)-1,8-naphthyridine (pynp), 2-(quinolin-2-yl)-1,8-naphthyridine (qnnp), and 2-(1,8-naphthyridin-2-yl)quinoxaline (qxnp), were synthesized and characterized. These molecules feature new tridentate ligands that concomitantly bridge the dirhodium core and cap the axial positions. The complexes absorb light strongly throughout the ultraviolet/visible range and into the near-infrared region and exhibit relatively long-lived triplet excited-state lifetimes. Both the singlet and triplet excited states exhibit metal/ligand-to-ligand charge transfer (ML-LCT) in nature as determined by transient absorption spectroscopy and spectroelectrochemistry measurements. When irradiated with low-energy light, these black dyes are capable of undergoing reversible bimolecular electron transfer both to the electron acceptor methyl viologen and ...

Molecular triplet states constitute a crucial gateway in the photochemical reactions of organic molecules by serving as a reservoir for the excess electronic energy. Here, we report the remarkable sensitivity of soft X-ray transient absorption spectroscopy for following the intricate electronic structure changes accompanying the non-adiabatic transition of an excited molecule from the singlet to the triplet manifold. Core-level X-ray spectroscopy at the carbon-1s K-edge (284 eV) is applied to identify the role of the triplet state (T1, 3ππ*) in the ultraviolet-induced photochemistry of pentane-2,4-dione (acetylacetone, AcAc). The excited-state dynamics initiated at 266 nm (1ππ*, S2) is investigated with element- and site-specificity using broadband soft X-ray pulses produced by high harmonic generation, in combination with time-dependent density functional theory calculations of the X-ray spectra for the excited electronic singlet and triplet states. The evolution of the core-to-valence resonances at the ...

Controlling spin–spin interactions in multispin molecular assemblies is important for developing new approaches to quantum information processing. In this work, a covalent electron donor–acceptor–radical triad is used to probe spin-selective reduction of the stable radical to its diamagnetic anion. The molecule consists of a perylene electron donor chromophore (D) bound to a pyromellitimide acceptor (A), which is, in turn, linked to a stable α,γ-bisdiphenylene-β-phenylallyl radical (R•) to produce D-A-R•. Selective photoexcitation of D within D-A-R• results in ultrafast electron transfer to form the D+•-A–•-R• triradical, where D+•-A–• is a singlet spin-correlated radical pair (SCRP), in which both SCRP spins are uncorrelated relative to the R• spin. Subsequent ultrafast electron transfer within the triradical forms D+•-A-R–, but its yield is controlled by spin statistics of the uncorrelated A–•-R• radical pair, where the initial charge separation yields a 3:1 statistical mixture of D+•-3(A–•-R•) and D+•-...

Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI)-based multireference calculations have been performed to better understand the ground state properties and the photodissociation mechanism of SiH2OO, a silicon analogue of the parent Criegee intermediate, CH2OO. The CASSCF/aug-cc-pV(T+d)Z results suggest that the ground state of SiH2OO is severely multireference in nature. This explains why SiH2OO could not be characterized in recently reported coupled cluster calculations. An important implication of this multireference character is the dramatically enhanced reactivity of SiH2OO, i.e., the calculated barrier for the cyclization of SiH2OO is only 4.4 kcal/mol, which is nearly 10 kcal/mol lower than that reported for the CH2OO case. The MRCI/aug-cc-pV(T+d)Z results on the evolution of the low-lying singlet electronic states along the OO bond suggest that SiH2OO absorbs strongly in the near UV–vis region. These results improve our fundamental understandi...