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While spin-orbit coupling does not play a decisive role in the photophysics of unsubstituted perylene diimides (PDI), this changes dramatically when two phenylselenyl or phenyltelluryl substituents were attached to the PDI bay positions. In the series of PhO-, PhS-, PhSe-, and PhTe-substituted PDIs we observed strongly decreasing fluorescence quantum yield as a consequence of strongly increasing intersystem crossing (ISC) rate, measured by transient absorption spectroscopy with fs- and ns-time resolution as well as by broadband fluorescence upconversion. Time-dependent density functional calculations suggest increasing spin-orbit coupling due to the internal heavy-atom effect as the reason for fast ISC. In case of the selenium PDI derivative we found significant singlet oxygen sensitization

Quantum oscillations in radical pairs The spin dynamics of photoinduced radical pairs, involving an interconversion between singlet and triplet spin states, plays an important role in nature, for example, in avian magnetoreception. The spin interconversion is a truly quantum process with characteristic coherent oscillations (quantum beats) that should be reflected in the reaction kinetics. However, their experimental observation has remained challenging. Mims et al . developed an optical readout technique that can directly monitor the singlet-triplet interconversion quantum beats, as demonstrated for a photoinduced, charge-separated state of an electron donor–acceptor dyad (see the Perspective by Hore). The present work opens a new way to monitor the spin evolution in radical pairs, which will be important not only in biological physics but also in organic solar cells and other practical applications. —YS

Yamamoto homocoupling of two chiral oxindoles led to the atropo-diastereoselective formation of an axially chiral oxindole dimer. This building block served as the starting material for the syntheses of axially chiral squaraine and merocyanine chromophore dimers. These dimers show pronounced chiroptical properties, this is, outstandingly high ECD signals (Δ

Three novel tetracationic bis-triarylboranes with 3, 4-ethylenedioxythiophene (EDOT) linkers, and their neutral precursors, showed significant red-shifted absorption and emission compared to their thiophene-containing analogues, with one of the EDOT-derivatives emitting in the NIR region. Only the EDOT-linked trixylylborane tetracation was stable in aqueous solution, indicating that direct attachment of a thiophene or even 3-methylthiophene to the boron atom is insufficient to provide hydrolytic stability in aqueous solution. Further comparative analysis of the EDOT-linked trixylylborane tetracation and its bis-thiophene analogue revealed efficient photo-induced singlet oxygen production, with the consequent biological implications. Thus, both analogues bind strongly to ds-DNA and BSA, very efficiently enter living human cells, accumulate in several different cytoplasmic organelles with no toxic effect but, under intense visible light irradiation, they exhibit almost instantaneous and very strong cytotoxic effects, presumably attributed to singlet oxygen production. Thus, both compounds are intriguing theranostic agents, whose intracellular and probably intra-tissue location can be monitored by strong fluorescence, allowing switching on of the strong bioactivity by well-focused visible light.

Tetraiodotetraazapentacene I

We theoretically study the dynamics of charge transfer induced by femtosecond laser-pulse excitation. Models involving coupled electronic states of symmetrically bridged organic mixed-valence molecules are investigated, where the motion proceeds along two reaction coordinates. Linear absorption spectra of two species that differ in the energetical position of the bridge, relative to acceptor and donor states, are determined and compared to experimental results. From the wave packet dynamics it emerges that relaxation dominates the charge transfer. This behavior is reflected in transient absorption spectra, which are obtained from a directional decomposition of the time-dependent polarization. Due to the nature of the coupled dynamics the extraction of the relevant contributions needs an extension of well-known techniques for the decomposition.

A series of distinct BODIPY heterooligomers (dyads, triads, and tetrads) comprising a variable number of typical green BODIPY monomers and a terminal red-emitting styryl-equipped species acting as an energy sink was prepared and subjected to computational and photophysical investigations in solvent media. An ethylene tether between the single monomeric units provides a unique foldameric system, setting the stage for a systematic study of excitation energy transfer processes (EET) on the basis of nonconjugated oscillators. The influence of stabilizing β-ethyl substituents on conformational space and the disorder of site energies and electronic couplings was addressed. In this way both the strong (Frenkel) and the weak (Forster) coupling limit could be accessed within a single system: the Frenkel limit within the strongly coupled homooligomeric green donor subunit and the Forster limit at the terminal heterosubstituted ethylene bridge. Femtosecond transient-absorption spectroscopy combined with mixed quantum-classical dynamic simulations demonstrate the limitations of the Forster resonance energy transfer (FRET) theory and provide a consistent framework to elucidate the trend of increasing relaxation lifetimes at higher homologues, revealing one of the fastest excitation energy transfer processes detected to date with a corresponding lifetime of 39 fs.

Molecular symmetry is a decisive factor for the two-photon absorption (2PA) properties of an organic chromophore. In order to explore its impact on the 2PA cross section of squaraine dyes, we synth...

A squaraine heterotriad consisting of three different covalently linked squaraine chromophores was synthesized, and its absorption spectra were interpreted in terms of Kasha's exciton coupling theory. Using the exciton couplings derived from model dyads (ca. 700 cm

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.

The interactions between auxochromic groups in π-conjugated functional molecules dictate their electronic properties. From the standpoint of potential applications, understanding and control of suc...

A series of bis‐(4’‐pyridylethynyl)arenes (arene=benzene, tetrafluorobenzene, and anthracene) were synthesized and their bis‐N‐methylpyridinium compounds were investigated as a class of π‐extended methyl viologens. Their structures were determined by single crystal X‐ray diffraction, and their photophysical and electrochemical properties (cyclic voltammetry), as well as their interactions with DNA/RNA were investigated. The dications showed bathochromic shifts in emission compared to the neutral compounds. The neutral compounds showed very small Stokes shifts, which are a little larger for the dications. All of the compounds showed very short fluorescence lifetimes (

Donor–acceptor dyads consisting of perylene bisimide (PBI) and trialkoxyphenyl–oligothiophene (TAPOT) units are reported that self-assemble into helical four-, six-, and seven-stranded hydrogen-bon...

A series of four indolenine squaraines bearing a chiral center at the 3-position of the indolenine moiety, with either an n-propyl or a phenyl group alongside a methyl group, were synthesized and obtained in a high purity of ≥98% for the desired stereoisomer. The indolenine precursors with a phenyl group attached at the chiral center were asymmetrically synthesized using a pericyclic-reaction cascade and obtained in a high ee of 98%, whereas the ones with an n-propyl group were prepared by kinetic resolution through asymmetric hydrogenation, resulting in an ee of up to 98%. X-ray crystallography revealed a slightly twisted geometry for the phenyl-substituted cisoid squaraine derivative, whereas the n-propyl-substituted derivative possessed the expected planar geometry. Variation of the substitution also influenced the optical properties, where the introduction of phenyl groups caused a progressive red-shift and reduction in squared transition moments, as well as reduced fluorescence quantum yields, Stokes shifts, and fluorescence lifetimes. All of the investigated compounds exhibited strong ECD signals, with Δe values of up to 24 M-1 cm-1 for the HOMO-LUMO transition. DFT calculations indicated that this was due to both large electric and magnetic transition moments, although the two vectors were mutually almost orthogonal.

We explored a series of squaraine homodimers with varying π-bridging centres to probe the relationship between the chemical structure and the two-photon absorption (2PA) characteristics. To this end, we designed and synthesised six linear homodimers based on two indolenine squaraine dyes with transoid configuration (SQA) which are connected by diverse bridges. In this regard, we investigated the effect of exciton coupling in these dimeric systems where the variation of the bridging units affects the magnitude of exciton coupling and leads to an alteration of their linear optical properties. Using two-photon absorption induced fluorescence measurements we determined the two-photon absorption cross section in this series of homodimers and found sizable values up to 5700 GM at ca. 11 000 cm−1 and 12 000 GM at 12 500 cm−1. The 2PA strength roughly follows the exciton coupling interaction between the squaraine chromophores which therefore may be used as design criteria to achieve high 2PA cross sections. The results were substantiated by polarization dependent linear and nonlinear optical measurements and by density functional theory calculations based on time dependent and quadratic response theory.

The absorption and emission characteristics of (ppz)2(dipy)IrIII, (ppz)(dipy)PtII and (ppz)(dipy)PdII, where ppz stands for phenylpyrazole and dipy for a phenyl meso-substituted dipyrrin ligand, have been investigated by means of combined density functional theory and multireference configuration interaction including scalar relativistic and spin-orbit coupling effects. These results were compared with experimental spectra. The complexes exhibit a high density of low-lying electronically excited states originating from ligand-centered (LC) and metal-to-ligand charge transfer (MLCT) states involving the dipyrrin ligand. In addition, metal-centered (MC) states are found to be low-lying in the Pd complex. In all three cases, the first strong absorption band and the phosphorescence emission band stem from LC excitations on the dipyrrin ligand with small MLCT contributions. The MLCT states show more pronounced relaxation effects than the LC states, with the consequence that the first excited state with predominant singlet multiplicity is of SMLCT/LC type in the heavier Ir and Pt complexes. Substantial spin-orbit coupling between SMLCT/LC and TLC enables fast and efficient intersystem crossing (ISC) and a high triplet quantum yield. Phosphorescence rate constants are rather small in accord with the dominant LC character of the transitions. Out-of-plane distortion promotes nonradiative decay of the excited state population via the MC states thus explaining the lower phosphorescence quantum yield of the Pt complex. The spectral properties of the Pd complex are different in many aspects. Optimization of the S1 state yields a dipyrrin intraligand charge transfer (ILCT) state with highly distorted nuclear arrangement in the butterfly conformers leading to nonradiative deactivation. In contrast, the primarily excited SLC state and the SMLCT/LC state of the twist conformer have nearly equal adiabatic excitation energies. The lack of a driving force toward the SMLCT/LC minimum, the high fluorescence rate constant of the bright SLC state and its moderately efficient ISC to the triplet manifold explain the experimentally observed dual emission of the Pd complex at room temperature.

Optical two-dimensional electronic spectroscopy (2DES) is now widely utilized to study excitonic structure and dynamics of a broad range of systems, from molecules to solid state. Besides the traditional experimental implementation using phase matching and coherent signal field detection, action-based approaches that detect incoherent signals such as fluorescence have been gaining popularity in recent years. While incoherent detection extends the range of applicability of 2DES, the observed spectra are not equivalent to the coherently detected ones. This raises questions about their interpretation and the sensitivity of the technique. Here we directly compare, both experimentally and theoretically, four-wave mixing coherently and fluorescence-detected 2DES of a series of squaraine dimers of increasing electronic coupling. All experiments are qualitatively well reproduced by a Frenkel exciton model with secular Redfield theory description of excitation dynamics. We contrast the spectral features and the sensitivities of both techniques with respect to exciton energies, delocalization, coherent and dissipative dynamics, and exciton-exciton annihilation. Discussing the fundamental and practical differences, we demonstrate the degree of complementarity of the techniques.

Chemistry - a European journal 28(30), e202200355 (2022). doi:10.1002/chem.202200355, A series of donor-acceptor (D−A) macrocyclic dyads consisting of an electron-poor perylene bisimide (PBI) π-scaffold bridged with electron-rich α-oligothiophenes bearing four, five, six and seven thiophene units between the two phenyl-imide substituents has been synthesized and characterized by steady-state UV/Vis absorption and fluorescence spectroscopy, cyclic and differential pulse voltammetry as well as transient absorption spectroscopy. Tying the oligothiophene strands in a conformationally fixed macrocyclic arrangement leads to a more rigid π-scaffold with vibronic fine structure in the respective absorption spectra. Electrochemical analysis disclosed charged state properties in solution which are strongly dependent on the degree of rigidification within the individual macrocycle. Investigation of the excited state dynamics revealed an oligothiophene bridge size-dependent fast charge transfer process for the macrocyclic dyads upon PBI subunit excitation., Published by Wiley-VCH, Weinheim

Reversible conversion between excited-states plays an important role in many photophysical phenomena. Using 1-(pyren-2′-yl)-o-carborane as a model, we studied the photoinduced reversible charge-transfer (CT) process and the thermodynamic equilibrium between the locally-excited (LE) state and CT state, by combining steady state, time-resolved, and temperature-dependent fluorescence spectroscopy, fs- and ns-transient absorption, and DFT and LR-TDDFT calculations. Our results show that the energy gaps and energy barriers between the LE, CT, and a non-emissive ‘mixed’ state of 1-(pyren-2′-yl)-o-carborane are very small, and all three excited states are accessible at room temperature. The internal-conversion and reverse internal-conversion between LE and CT states are significantly faster than the radiative decay, and the two states have the same lifetimes and are in thermodynamic equilibrium.

We introduce a new approach to transient absorption spectroscopy based on measurement at specific excitation intensities. A combination of datasets isolates individual perturbative nonlinear orders, allowing systematic high-intensity measurement of clean single- and multi-particle dynamics.

Angewandte Chemie / International edition 61(1), e202113598 (2022). doi:10.1002/anie.202113598, Two macrocyclic architectures comprising oligothiophene strands that connect the imide positions of a perylene bisimide (PBI) dye have been synthesized via a platinum-mediated cross-coupling strategy. The crystal structure of the double bridged PBI reveals all syn-arranged thiophene units that completely enclose the planar PBI chromophore via a 12-membered macrocycle. The target structures were characterized by steady-state UV/Vis absorption, fluorescence and transient absorption spectroscopy, as well as cyclic and differential pulse voltammetry. Both donor���acceptor dyads show ultrafast F��rster Resonance Energy Transfer and photoinduced electron transfer, thereby leading to extremely low fluorescence quantum yields even in the lowest polarity cyclohexane solvent., Published by Wiley-VCH, Weinheim

Perylene-3,4:9,10-bis(dicarboximides) (PBIs) rank among the most important functional dyes and organic semiconductors, but only recently have their radical anions and dianions attracted interest for a variety of applications. Here, we systematically elucidate the functional properties (redox, absorption, and emission) of five PBI anions and dianions bearing different bay-substituents attached to the chromophore core. Cyclic voltammetry measurements reveal the influence of the substituents ranging from electron-withdrawing cyano to electron-donating phenoxy groups on the oxidation and reduction potentials that relate to the HOMO and LUMO levels ranging from −7.07 eV to −6.05 eV and −5.01 eV to −4.05 eV, respectively. Spectroelectrochemical studies reveal a significant number of intense absorption bands in the NIR-spectral range (750–1400 nm) for the radical anions, whereas the dianionic species are characterized by similar spectra to those for the neutral dyes, however being bathochromically shifted and with increased molar extinction coefficients of approximately 100 000 M−1 cm−1. The increase of the transition dipole moment is up to 56% and accompanied by an almost cyanine-like red-shifted (by 300 nm) absorption spectrum for the most electron-poor tetracyanotetrachloro PBI. Whilst the outstanding fluorescence properties of the neutral PBIs are lost for the radical anions, an appreciable near-infrared (NIR) fluorescence with a quantum yield of up to 18% is revealed for the dianions by utilizing a custom-built flow-cell spectroelectrochemistry setup. Time-dependent density functional theory calculations help to assign the absorption bands to the respective electronic transitions.

The influence of oligosquaraine chain length on the energies and shape of absorption and emission bands and the exciton coherence length is studied in CHCl3 where the oligomers adopt a random coil structure. From the observed fluorescence band narrowing an effective coherence length of Ncoh = 2.5 was estimated for the nonamer. Applying a theoretical Frenkel exciton model the absorption and emission spectra were simulated which confirmed the experimental results. From the relative amplitude of the 00 peak to the vibronic shoulder the coherence length was estimated which yields a somewhat higher saturation value of Ncoh ≈ 3 for the nonamer, which is in very good agreement with the theoretical amplitude ratio. The coherence length is much smaller than the geometrical length because the electronic delocalisation is reduced by structural disorder. Taking into account the energetic (diagonal) and structural (off-diagonal) disorder we observed a different influence on the absorption and fluorescence spectra. For the emission spectra, exciton delocalisation leads to a narrowing of the band caused by averaging over energetic disorder, but for the absorption band the spectra are broadened by excitonic splitting and structural disorder.

We recently reported diethynylarene-linked bis(triarylborane) tetracations which show remarkable fluorimetric and Raman-SERS sensing of DNA/RNA. In the current study, we show that they exhibit promising photodynamic therapy (PDT)-based biological activity on human cell lines and adenovirus type 5 (HAdV5), acting as theranostic agents. All compounds efficiently enter living cells showing negligible antiproliferative activity. Bis-thiophene- and anthracene- analogues bind non-covalently to HAdV5 virus with high affinity, the anthracene-analogue itself causing a moderate antiviral effect, i.e., decreased ability of the virus to infect human cells. Irradiation of bis-thiophene- and anthracene- analogues with visible light (400-700 nm) caused a very rapid (within 1 minute) and strong increase in cytotoxicity, as well as an order of magnitude increase in antiviral activity, attributed to the formation of reactive oxygen species (ROS). Photochemical studies of the compounds revealed that, upon irradiation, they produce singlet oxygen, which correlates with the observed light-induced bioactivity.

One-electron chemical reduction of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthrene-4-amine (3-B(Mes)2-[4]helix-9-N(p-Tol)2) 1 and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthrene-8-amine (3-B(Mes)2-[5]helix-12-N(p-Tol)2) 2 gives rise to monoanions with extensive delocalization over the annulated helicene rings and the boron pz orbital. Two-electron chemical reduction of 1 and 2 produces open-shell biradicaloid dianions with temperature-dependent population of the triplet states due to small singlet-triplet gaps. These results have been confirmed by single-crystal X-ray diffraction, EPR and UV/vis-NIR spectroscopy, and DFT calculations., Stepwise chemical reduction of D–π–A triarylborane-based helicenes gives the corresponding monoanions and dianions with delocalized unpaired electrons. The structures were confirmed by single crystal X-ray diffraction.

We synthesized new pyrene derivatives with strong bis(para‐methoxyphenyl)amine donors at the 2,7‐positions and n‐azaacene acceptors at the K‐region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7‐substituted pyrenes but not for K‐region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin‐Day class‐II and delocalized Robin‐Day class‐III species., Donor–acceptor pyrenes: Three new pyrene derivatives with strong donors at the 2,7‐positions and acceptors at the K‐region were synthesized. This leads to unusual properties such as emission in the red‐to‐NIR region (700 nm) and long intrinsic lifetimes of >100 ns, and the monocations absorb up to 3000 nm.

Exciton transport and exciton–exciton interactions in molecular aggregates and polymers are of great importance in natural photosynthesis, organic electronics, and related areas of research. Both the experimental observation and theoretical description of these processes across time and length scales, including the transition from the initial wavelike motion to the following long-range exciton transport, are highly challenging. Therefore, while exciton dynamics at small scales are often treated explicitly, long-range exciton transport is typically described phenomenologically by normal diffusion. In this work, we study the transition from wavelike to diffusive motion of interacting exciton pairs in squaraine copolymers of varying length. To this end we use a combination of the recently introduced exciton–exciton-interaction two-dimensional (EEI2D) electronic spectroscopy and microscopic theoretical modelling. As we show by comparison with the model, the experimentally observed kinetics include three phases, wavelike motion dominated by immediate exciton–exciton annihilation (10–100 fs), sub-diffusive behavior (0.1–10 ps), and excitation relaxation (0.01–1 ns). We demonstrate that the key quantity for the transition from wavelike to diffusive dynamics is the exciton delocalization length relative to the length of the polymer: while in short polymers wavelike motion of rapidly annihilating excitons dominates, in long polymers the excitons become locally trapped and exhibit sub-diffusive behavior. Our findings indicate that exciton transport through conjugated systems emerging from the excitonic structure is generally not governed by normal diffusion. Instead, to characterize the material transport properties, the diffusion presence and character should be determined., We measure excitation energy transport in conjugated polymers using direct observation of exciton pair dynamics.

Two different chromophores, namely a dipolar and an octupolar system, were prepared and their linear and nonlinear optical properties as well as their bioimaging capabilities were compared. Both contain triphenylamine as the donor and a triarylborane as the acceptor, the latter modified with cationic trimethylammonio groups to provide solubility in aqueous media. The octupolar system exhibits a much higher two‐photon brightness, and also better cell viability and enhanced selectivity for lysosomes compared with the dipolar chromophore. Furthermore, both dyes were applied in two‐photon excited fluorescence (TPEF) live‐cell imaging., Dipole versus octupole: Branching of a cationic triarylborane chromophore leads to a cooperative increase in the two‐photon absorption cross‐section and brightness, as well as higher selectivity towards lysosomes and better cell viability. The octupolar dye was demonstrated to be an ideal candidate for two‐photon excited fluorescence imaging of lysosomes in live cells.

The synthesis and characterization of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthren-4-amine (3-B(Mes)2 -[4]helix-9-N(p-Tol)2 1) and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthren-8-amine (3-B(Mes)2 -[5]helix-12-N(p-Tol)2 2) are reported herein. Their electrochemical and photophysical properties have been studied experimentally and theoretically. The donor and acceptor-substituted helicene derivatives exhibit moderate fluorescence quantum yields in THF (Φf =0.48 and 0.61 for 1 and 2, respectively), which are higher than unsubstituted ones (Φf =0.18 for [4]helicene; Φf

A bis(benzobuta)tetraazapentacene derivative was reduced to its radical anion and its dianion, using potassium [18]crown-6 anthracenide in THF. Both reduced species were characterized by UV/Vis spectroscopy of the isolated species and by spectroelectrochemistry. Two distinct single-crystal structures of the dianion and an EPR spectrum of the radical anion were obtained. Contrary to other azaacenes, the lowest energy absorption in the UV/Vis spectrum of the dianion is redshifted in comparison to that of the neutral compound.

Quantum dynamical model calculations are performed on the optically induced electron transfer in a mixed-valence system interacting with different solvents. The simultaneously occurring processes of population transfer between electronic states and relaxation are studied in detail. Transient absorption traces, as recently recorded in our laboratory, are simulated, and the features of the spectra are related to the dynamics. The agreement with the experiment hints at the fact that the employed one-dimensional models catch the essentials of the photochemistry of the investigated systems and that they can be used for the interpretation of the transient absorption spectra. It is inferred that the ultrafast electron transfer processes take place on a sub-picosecond time scale and afterward relaxation occurs within several picoseconds.

Two shape-persistent star mesogens with oligo(phenylene ethenylene) arms and a phthalocyanine core-one providing free space (2) and one sterically encumbered by four fullerenes attached through spacers (3)-have been successfully synthesized. In contrast to the smaller discotic derivative 1, mesogen 2 forms a columnar liquid crystal (LC), which can only be partially aligned without π-stacking, while 3 is not an LC. Exceptionally, the 1:1 mixture of 2 and 3 forms an alignable columnar LC with strong π-stacking and quadruply helically organized fullerenes by an unprecedented click process that is similar to a ball detent mechanism. The C60 units also interconnect different columns. This is driven by nanosegregation and space-filling of the voids with fullerenes. Photophysical studies confirm the presence of a light-collecting system that generates charge-separated states in solution and in the solid state, which makes such highly organized materials attractive for the study of future photovoltaic devices.

A rigidly bridged squaraine dimer serves as a model compound to study exciton interactions between two chromophores without interfering with conformational or other stereochemical isomers. We describe the synthesis as well as steady state and fs- and ps-time-resolved optical spectroscopic data. The spectra are interpreted using a vibronic coupling model, which considers a single vibrational mode that produces a shallow excited state surface with two minima. These two minima cause symmetry breaking of the excited state, which leads to a partial localization of excitation. The localization of the wave function causes a reduced fluorescence transition moment, although both the absorption and the emission spectra display exchange narrowing typical of excitonically coupled chromophores.

Via regioselective Ir-catalyzed C–H borylation and subsequent reactions (i.e., viaBr4-Per or (BF3K)4-Per intermediates), we have introduced strong π-donors and acceptors at the 2,5,8,11-positions of perylene leading to unusual properties. Thus, incorporation of four donor diphenylamine (DPA) or four acceptor Bmes2 (mes = 2,4,6-Me3C6H2) moieties yields novel compounds which can be reversibly oxidized or reduced four times, respectively, an unprecedented behavior for monomeric perylene derivatives. Spectroelectrochemical measurements show NIR absorptions up to 3000 nm for the mono-cation radical of (DPA)4-Per and a strong electronic coupling over the perylene bridge was observed indicative of fully delocalized Robin-Day Class III behavior. Both (DPA)4-Per and (Bmes2)4-Per derivatives possess unusually long intrinsic singlet lifetimes (τ0), e.g., 94 ns for the former one. The compounds are emissive in solution, thin films, and the solid state, with apparent Stokes shifts that are exceptionally large for perylene derivatives. Transient absorption measurements on (DPA)4-Per reveal an additional excited state, with a long lifetime of 500 μs, which sensitizes singlet oxygen effectively.

A series of tetracationic quadrupolar chromophores containing three-coordinate boron π-acceptors linked by different π-bridges, namely 4,4′-biphenyl, 2,7-pyrene, 2,7-fluorene, 3,6-carbazole and 5,5′-di(thien-2-yl)-3,6-diketopyrrolopyrrole, were synthesized. While their neutral precursors 1–5 displayed highly solvatochromic fluorescence, the water-soluble tetracationic target molecules 1M–5M, did not, but their emission colour could be tuned from blue to pink by changing the π-bridge. Compound 5M, containing the diketopyrrolopyrrole bridge, exhibits the most red-shifted absorption and emission maxima and the largest two-photon absorption cross-section (4560 GM at 740 nm in MeCN). Confocal laser scanning fluorescence microscopy studies in live cells confirm localization of the dye at the lysosome. Moreover, the low cytotoxicity, and high photostability of 5M combined with two-photon excited fluorescence imaging studies demonstrate its excellent potential for lysosomal imaging in live cells.