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Multi-branched oligomers F1, F2 and F3 were investigated by spectroscopic experiments as well as quantum chemical calculations to elucidate the structure–property relationships and intramolecular charge transfer (ICT) characteristics of typical nonlinear optical materials. A better ICT property of F2 is obtained by replacing terminal electron donors with stronger electron-donating ability. The degree of ICT is effectively increased, which can be clearly seen from the frontier molecular orbitals contribution of central anthracene between HOMO and LUMO. The central group plays a major role in the direction as well as effective path of ICT process. In the X-shaped oligomers with anthracene as central group, ICT process takes place from terminal groups of 1,8 position branches, to the central anthracene group. However, the electron distribution as well as the ICT of tri-branched oligomer F3 with triazine as central group is not symmetric. ICT process of F3 takes place from terminal triphenylamine groups to the central triazine and two fluorene groups at 4,6 position branches. Y-shaped oligomer F3 has a longer lifetime of ICT state as well as a higher fluorescence quantum yield, which is consistent with the results of time-resolved fluorescence spectroscopy. Z-scan results show that the TPA cross-section value of F3 is 10.6 and 2.9 times higher than that of X-shaped F1 and F2. The dynamics curves of transient absorption spectroscopy also indicate that oligomer F3 structure has relatively stronger ICT properties. To verify the substituent effect, two linear molecules F4 and F5 were also calculated. The quantum chemical calculations result shows that the branches at positions 4 and 11 of the central anthracene group have little effect on the electron cloud distribution, and there are indeed significant differences in the contributions of branches at different positions. Our results may provide some reference for molecular design. [ABSTRACT FROM AUTHOR]

In this study, we present the synthesis and analysis of a novel, air‐stable, and solvent‐resistant phosphaalkene switch. Using this symmetric switch, we have demonstrated degenerate photoisomerization experimentally for the first time. With a combination of photochemical‐exchange NMR spectroscopy, ultrafast transient absorption spectroscopy, and quantum chemical calculations, we elucidate the isomerization mechanism of this symmetric phosphaalkene, comparing it to two other known molecules belonging to this class. Our findings highlight the critical role of the isolobal analogy between C=P and C=C bonds in governing nanoscale molecular motion and break new ground for our understanding of light‐induced molecular processes in symmetric heteroalkene systems. [ABSTRACT FROM AUTHOR]

The (6–4) photolesion is a key photodamage that occurs when two adjacent pyrimidine bases in a DNA strand bond together. To better understand how the absorption of UVB and UVA radiation by the 2‐pyrimidinone moiety in a (6–4) lesion can damage DNA, it is important to study the electronic deactivation mechanism of its 2‐pyrimidinone chromophore. This study employs theoretical (MS‐CASPT2/cc‐pVDZ level) and experimental (steady state and femtosecond broadband spectroscopic) methods to elucidate the photochemical relaxation mechanisms of 2‐(1H)‐pyrimidinone and 1‐methyl‐2‐(1H)‐pyrimidinone in aqueous solution (pH 7.4). In short, excitation at 320 nm leads to the population of the S11(ππ*) state with excess vibrational energy, which relaxes to the S11(ππ*) minimum in one picosecond or less. A trifurcation event in the S11(ππ*) minimum ensued, leading to radiative and nonradiative decay of the population to the ground state or the population of the long‐lived and reactive T13(ππ*) state in hundreds of picoseconds. Collectively, the theoretical and experimental results support the idea that in DNA and RNA, the T13(ππ*) state of the 2‐pyrimidinone moiety in the (6–4) lesion can further participate in photosensitized chemical reactions increasing DNA and RNA damage. [ABSTRACT FROM AUTHOR]

As the simplest nitroaromatic compound, nitrobenzene is an interesting model system to explore the rich photochemistry of nitroaromatic compounds. Previous investigations of nitrobenzene's photochemical dynamics have probed structural and electronic properties. These investigations paint, at times, a convoluted and sometimes contradictory description of the photochemical landscape. We investigate the ultrafast dynamics of nitrobenzene triggered by photoexcitation at 267 nm for the first time using a structural probe with femtosecond time resolution. Our probe complements previous measurements of nitrobenzene's electronic structure evolution and aids in determining the photochemical dynamics with less ambiguity. We employ megaelectronvolt ultrafast electron diffraction to follow nitrobenzene's structural evolution within the first 5 ps after photoexcitation. We observe ground state recovery within 160 ± 60 fs through nonadiabatic dynamics. Based on comparisons of the experimental signal with molecular dynamics simulations, we exclude a significant population of the triplet manifold. Furthermore, we do not observe fragmentation of nitrobenzene within the investigated time window, which indicates that previously observed photofragmentation reactions take place in the vibrationally 'hot' ground state on timescales considerably beyond 5 ps. [ABSTRACT FROM AUTHOR]

Femtosecond fluorescence upconversion experiments were combined with CASPT2 and time dependent DFT calculations to characterize the excited state dynamics of the mutagenic etheno adduct 1,N2‐etheno‐2'‐deoxyguanosine (ϵdG). This endogenously formed lesion is attracting great interest because of its ubiquity in human tissues and its highly mutagenic properties. The ϵdG fluorescence is strongly modified with respect to that of the canonical nucleoside dG, notably by an about 6‐fold increase in fluorescence lifetime and quantum yield at neutral pH. In addition, femtosecond fluorescence upconversion experiments reveal the presence of two emission bands with maxima at 335 nm for the shorter‐lived and 425 nm for the longer‐lived. Quantum mechanical calculations rationalize these findings and provide absorption and fluorescence spectral shapes similar to the experimental ones. Two different bright minima are located on the potential energy surface of the lowest energy singlet excited state. One planar minimum, slightly more stable, is associated with the emission at 335 nm, whereas the other one, with a bent etheno ring, is associated with the red‐shifted emission. [ABSTRACT FROM AUTHOR]

Organic–inorganic hybrid perovskite quantum dots (QDs) have garnered significant research interest owing to their unique structure and optoelectronic properties. However, their poor optical performance in ambient air remains a significant limitation, hindering their advancement and practical applications. Herein, three amino acids (valine, threonine and cysteine) were chosen as surface ligands to successfully prepare highly luminescent CH3NH3PbBr3 (MAPbBr3) QDs. The morphology and XRD results suggest that the inclusion of the amino acid ligands enhances the octahedral structure of the QD solutions. Moreover, the observed blue-shifted phenomenon in the photoluminescence (PL) aligns closely with the blue-shifted phenomenon observed in the ultraviolet–visible (UV-Vis) absorption spectra, attributed to the quantum confinement effect. The time-resolved spectra indicated that the introduction of the amino acid ligands successfully suppressed non-radiative recombination, consequently extending the fluorescence lifetime of the MAPbBr3 QDs. The photoluminescence quantum yields (PLQYs) of the amino acid-treated MAPbBr3 QDs are increased by 94.8%. The color rendering index (CRI) of the produced white light-emitting diode (WLED) is 85.3, with a correlated color temperature (CCT) of 5453 K. Our study presents a novel approach to enhancing the performance of perovskite QDs by employing specially designed surface ligands for surface passivation. [ABSTRACT FROM AUTHOR]

Methylation of adenine at the N6 position is a crucial epigenetic modification that profoundly influences gene regulation and expression. Moreover, this modification intricately alters the excited state dynamics of adenine nucleobases. To explore the impact of N6‐methyladenine on the excited state dynamics within oligonucleotides, we conducted a comprehensive investigation of two dinucleotides containing N6‐methyladenosine, in conjunction with adenosine or guanosine. Using steady‐state and time‐resolved absorption and fluorescence spectroscopy techniques, we not only observed the customary monomer‐like and charge transfer emissive states, as reported in previous dinucleotides, but also identified an additional low‐energy emissive state. This unique state exhibits an extraordinary Stokes Shift exceeding 2.3 eV and has a relatively long lifetime of 4–5 ns. We propose that this state corresponds to a bonded exciplex state, governed by ground‐state geometries. [ABSTRACT FROM AUTHOR]

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene (fDAE) derivative were investigated by time-resolved absorption and fluorescence spectroscopies. Prescreening with quantum chemical calculation predicted that a derivative with methylthienyl groups (mt-fDAE) in the closed-ring isomer has a two-photon absorption cross-section larger than 1000 GM, which was experimentally verified by Z-scan measurements and excitation power dependence in transient absorption. Comparison of transient absorption spectra under one-photon and simultaneous two-photon excitation conditions revealed that the closed-ring isomer of mt-fDAE populated into higher excited states deactivates following three pathways on a timescale of ca. 200 fs: (i) the cycloreversion reaction more efficient than that by the one-photon process, (ii) internal conversion into the S1 state, and (iii) relaxation into a lower state (S1' state) different from the S1 state. Time-resolved fluorescence measurements demonstrated that this S1' state is relaxed to the S1 state with the large emission probability. These findings obtained in the present work contribute to extension of the ON–OFF switching capability of fDAE to the biological window and application to super-resolution fluorescence imaging in a two-photon manner. [ABSTRACT FROM AUTHOR]

Organic photosensitizer with both large two‐photon absorption (σ) and efficient intersystem crossing (ISC) offers incomparable advantages in precise two‐photon photodynamic therapy. However, the current design strategy cannot achieve efficient ISC without compromising σ. Here, very efficient ISC and ultrahigh σ in organic photosensitizer (PFBT) for precise cerebrovascular two‐photon photodynamic therapy is simultaneously achieved. A hybridized local and charge‐transfer (1HLCT) excited state in PFBT, formed by incorporating benzothiadiazole into a typical polyfluorene (PF), is the key to initiating efficient ISC while bringing substantial σ enhancement (25 000 GM vs 10 000 GM) compared to PF. Mechanism studies identify that 1HLCT produces large spin‐orbit coupling and tiny singlet‐triplet energy gaps, together generating efficient ISC. These properties afford PFBT nanophotosensitizer a ≈2000‐fold increase in two‐photon photodynamic therapy efficiency than clinically‐used Photofrin, enabling in vivo deep‐brain cerebrovascular imaging and closure with unprecedented precision. [ABSTRACT FROM AUTHOR]

This article reports a detailed theoretical investigation on dual fluorescence properties of p-amino substituted benzaldehyde molecules by taking specifically p-N,N-dimethylaminobenzaldehyde (3° PABA), p-N-methylaminobenzaldehyde (2° PABA), and p-aminobenzaldehyde (1° PABA) molecules. The calculations are performed in gas phase as well as in butanol (BuOH) and dichloromethane (DCM) solvents. Twisted intramolecular charge transfer emission property is particularly looked at by scanning the potential energy curves as a function of -NR2 (R = H/CH3) rotation (twisting) angle. The results suggest that there are dual emission possibilities for 3° and 2° PABA molecules. Experimental validation is available for the former but not for the latter. 1° PABA, on the other hand, does not show any possibility of dual fluorescence except in BuOH. The theoretical absorption and emission spectra are also calculated and compared with experiment where it is available, and they are in close correlation. Excited state molecular dynamics simulations show that for 3° PABA, the molecule reaches to the 90° twisting angle and stays there with a probability higher than the same probability for 2° PABA. 1° PABA, on the other hand, had showed variations in the twisting angle from 0° to above 100°, but no evidence of this molecule to have considerable lifetime at 90° twisting angle.

2,6‐Diaminopurine (2,6‐dAP) is an alternative nucleobase that potentially played a role in prebiotic chemistry. We studied its excited state dynamics in the gas phase by REMPI, IR‐UV hole burning, and ps pump‐probe spectroscopy and performed quantum chemical calculations at the SCS‐ADC(2) level of theory to interpret the experimental results. We found the 9H tautomer to have a small barrier to ultrafast relaxation via puckering of its 6‐membered ring. The 7H tautomer has a larger barrier to reach a conical intersection and also has a sizable triplet yield. These results are discussed relative to other purines, for which 9H tautomerization appears to be more photostable than 7H and homosubstituted purines appear to be less photostable than heterosubstituted or singly substituted purines. [ABSTRACT FROM AUTHOR]

Isocytosine, having important applications in antivirus and drug development, is among the building blocks of Hachimoji nucleic acids. In this report, we present an investigation of the excited state dynamics of isocytosine in both protic and aprotic solvents, which was conducted by a combination of methods including steady‐state spectroscopy, femtosecond broadband time‐resolved fluorescence, and transient absorption. These methods were coupled with density functional and time‐dependent density functional theory calculations. The results of our study provide the first direct evidence for a highly efficient nonradiative mechanism achieved through internal conversion from the ππ* state of the isocytosine keto‐N(3)H form occurring within subpicoseconds and picoseconds following photo‐excitation. Our study also unveils a crucial role of solvent, particularly solute–solvent hydrogen bonding, in determining the tautomeric composition and regulating the pathways and dynamics of the deactivation processes. The deactivation processes of isocytosine in the solvents examined are found to be distinct from those of cytosine and the case known for isocytosine in the gas phase mainly due to different tautomeric forms involved. Overall, our findings demonstrate the high photo‐stability of isocytosine in the solution and showcase the remarkable effect of covalent modification in altering the spectral character and excited state dynamics of nucleobases. [ABSTRACT FROM AUTHOR]

Cyanobacterial photosynthetic apparatus efficiently capture sunlight, and the energy is subsequently transferred to photosystem I (PSI) and II (PSII), to produce electrochemical potentials. PSII is a unique membrane protein complex that photo-catalyzes oxidation of water and majorly contains photosynthetic pigments of chlorophyll a and carotenoids. In the present study, the ultrafast energy transfer and charge separation dynamics of PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus were reinvestigated by femtosecond pump-probe spectroscopic measurements under low temperature and weak intensity excitation condition. The results imply the two possible models of the energy transfers and subsequent charge separation in PSII. One is the previously suggested "transfer-to-trapped limit" model. Another model suggests that the energy transfers from core CP43 and CP47 antennas to the primary electron donor ChlD1 with time-constants of 0.71 ps and 3.28 ps at 140 K (0.17 and 1.33 ps at 296 K), respectively and that the pheophytin anion (PheoD1−) is generated with the time-constant of 43.0 ps at 140 K (14.8 ps at 296 K) upon excitation into the Qy band of chlorophyll a at 670 nm. The secondary electron transfer to quinone QA: PheoD1−QA → PheoD1QA− is observed with the time-constant of 650 ps only at 296 K. On the other hand, an inefficient β-carotene → chlorophyll a energy transfer (33%) occurred after excitation to the S2 state of β-carotene at 500 nm. Instead, the carotenoid triplet state appeared in an ultrafast timescale after excitation at 500 nm. [ABSTRACT FROM AUTHOR]

Resonance Enhanced MultiPhoton Ionization spectroscopic techniques coupled with laser desorption and supersonic cooling have been employed to elucidate the photoactive properties of resveratrol. The observed excitation spectra give evidence for an internal-energy dependent trans–cis isomerisation pathway in the electronically excited state, while pump-probe studies show dynamics that are in line with what is known for the parent compound, trans-stilbene. Similar studies have been performed on a derivative of resveratrol with methoxy instead of hydroxy groups, a compound aimed to reduce previously observed photodegradation pathways of resveratrol. Time-resolved studies of the latter compound under solution conditions have given further insight into its excited-state dynamics and support the isolated-molecule conclusions on the topology of the potential energy surface of the electronically excited state. Spectroscopic studies under prolonged irradiation conditions show that both compounds suffer from photodegradation, although in the alkylated compound other pathways appear to be involved than in resveratrol. [ABSTRACT FROM AUTHOR]

This article epitomizes the design and synthesis of a potential bioactive Semicarbazone Schiff base (E)‐2‐(4‐(diethylamino)‐2‐hydroxybenzylidene)hydrazine‐1‐carboxamide (DHHC) and its water assisted photoinduced tautomerization. DHHC is feebly emissive in non‐polar solvents but the fluorescence intensity intensifies (at 406 nm) in alcohols possibly due to intermolecular H‐bonding. However, in protic polar water, the scenario is markedly different, the fluorescence intensity decreases a bit as compared to alcohols and the emission maxima shifted from 406 nm to 430 nm probably due to solvent (water) assisted excited state proton transfer reaction (ESPT). In contrast to the reported excited state intramolecular proton transfer reaction (ESIPT) in various compounds, DHHC on photoexcitation loses a proton from its phenolic ‐OH group to the water molecule, which insists a subsequent proton transfer from the hydronium ion to the imine nitrogen of DHHC resulting 24 nm red‐shifted fluorescence emission from the tautomeric species of DHHC, in aqueous media. The mechanism of ESPT reaction is explained theoretically through the reaction kinetics of the DHHC‐water complex (in ratio 1 : 3) in an explicit CPCM solvent model. The illustration of the findings may open up enormous possibilities for Semicarbazone Schiff bases to be used as a probe for water sensing inside biomacromolecules like proteins and enzymes. [ABSTRACT FROM AUTHOR]

Square‐planar NiII complexes are interesting as cheaper and more sustainable alternatives to PtII luminophores widely used in lighting and photocatalysis. We investigated the excited‐state behavior of two NiII complexes, which are isostructural with two luminescent PtII complexes. The initially excited singlet metal‐to‐ligand charge transfer (1MLCT) excited states in the NiII complexes decay to metal‐centered (3MC) excited states within less than 1 picosecond, followed by non‐radiative relaxation of the 3MC states to the electronic ground state within 9–21 ps. This contrasts with the population of an emissive triplet ligand‐centered (3LC) excited state upon excitation of the PtII analogues. Structural distortions of the NiII complexes are responsible for this discrepant behavior and lead to dark 3MC states far lower in energy than the luminescent 3LC states of PtII compounds. Our findings suggest that if these structural distortions could be restricted by more rigid coordination environments and stronger ligand fields, the excited‐state relaxation in four‐coordinate NiII complexes could be decelerated such that luminescent 3LC or 3MLCT excited states become accessible. These insights are relevant to make NiII fit for photophysical and photochemical applications that relied on PtII until now. [ABSTRACT FROM AUTHOR]

In a recent work, we have shown that photon absorption can cause a chemical bond to be created between the two monomers within a protonated serine dimer, a process known as intra-cluster bond formation, despite this process not occurring following thermal excitation via low energy collision-induced dissociation (LE-CID). Here we show further evidence for non-statistical photon-induced dissociation (PID) of the protonated serine dimer. In addition we discuss LE-CID and PID studies of the protonated serine octamer, showing that in this case as well, PID leads to non-statistical fragmentation and to the formation of two bonds between three neighboring monomers. [ABSTRACT FROM AUTHOR]

Red fluorescent proteins (RFPs) represent an increasingly popular class of genetically encodable bioprobes and biomarkers that can advance next‐generation breakthroughs across the imaging and life sciences. Since the rational design of RFPs with improved functions or enhanced versatility requires a mechanistic understanding of their working mechanisms, while fluorescence is intrinsically an ultrafast event, a suitable toolset involving steady‐state and time‐resolved spectroscopic techniques has become powerful in delineating key structural features and dynamic steps which govern irreversible photoconverting or reversible photoswitching RFPs, and large Stokes shift (LSS)RFPs. The pertinent cis‐trans isomerization and protonation state change of RFP chromophores in their local environments, involving key residues in protein matrices, lead to rich and complicated spectral features across multiple timescales. In particular, ultrafast excited‐state proton transfer in various LSSRFPs showcases the resolving power of wavelength‐tunable femtosecond stimulated Raman spectroscopy (FSRS) in mapping a photocycle with crucial knowledge about the red‐emitting species. Moreover, recent progress in noncanonical RFPs with a site‐specifically modified chromophore provides an appealing route for efficient engineering of redder and brighter RFPs, highly desirable for bioimaging. Such an effective feedback loop involving physical chemists, protein engineers, and biomedical microscopists will enable future successes to expand fundamental knowledge and improve human health. [ABSTRACT FROM AUTHOR]

Epidemiological evidence indicates that damage to DNA/RNA initialized by ultraviolet (UV) radiation is associated with skin cancer. Wavelength dependence of DNA photodamage was proposed as early as 1990s and demonstrated later on. Unraveling the photo-activated dynamics involved in related reactions is essential. However, studies aimed at uncovering the wavelength dependent excited state dynamics in canonical pyrimidine nucleosides have not received enough attention. In this work, excitation wavelength dependent excited state dynamics of 2′-deoxy-thymidine (dThd) and oxy-uridine (Urd) are investigated in acetonitrile solutions by femtosecond broadband transient absorption spectroscopy. Varying the excitation wavelength leads to a significant difference in the branching of the excited state population at the Franck-Condon (FC) region, resulting higher fluorescence quantum yield with 285 nm pump but higher triplet state quantum yield under 267 nm excitation. Based on our results, a vibronic coupling regulated excited state relaxation mechanism is proposed. This mechanism information is important for understanding the formation of harmful photoproducts for DNA/RNA with different wavelength UV excitations.

N‐protonation for numerous fluorophores is widely known as an efficient switch for the fluorescence turn‐on/off in acidic conditions, which has been applied in various scenarios that involve pH monitoring. Yet the universal mechanism for fluorescence regulation through N‐protonation is still elusive. Herein, the excited state deactivation processes are systematically investigated for a series of nitrogen‐containing fluorescent probes through theoretical approaches. Two types of mechanisms for the complex fluorescent phenomena by N‐protonation are concluded: one is through the regulation for the transition to a ππ* twisted intramolecular charge transfer (TICT) state; the other one applies for the case when nonradiative decay pathway is predominant by a dark nπ* state, which is also accompanied by an evident structural twisting and can be regarded as another kind of TICT state. More generally, the formation of the TICT state is closely related to the conjugated π‐electrons on the single bond that links the acceptor and donor part of fluorophores, which provides a simple strategy for evaluating the occurrence of the TICT process. The current contributions can bring novel insights for the rational design of functional fluorophores that involve TICT process in the excited states. [ABSTRACT FROM AUTHOR]

Benzo[a]pyrene is a widespread environmental pollutant and a strong carcinogen. It is important to understand its bio-toxicity and degradation mechanism. Herein, we studied the excited state dynamics of benzo[a]pyrene by using time-resolved fluorescence and transient absorption spectroscopic techniques. For the first time, it is identified that benzo[a]pyrene in its singlet excited state could react with oxygen, resulting in fluorescence quenching. Additionally, effective intersystem crossing can occur from its singlet state to the triplet state. Furthermore, the interaction between the excited benzo[a]pyrene and ct-DNA can be observed directly and charge transfer between benzo[a]pyrene and ct-DNA may be the reason. These results lay a foundation for further understanding of the carcinogenic mechanism of benzo[a]pyrene and provide insight into the photo-degradation mechanism of this molecule. [ABSTRACT FROM AUTHOR]

Deriving diverse compound libraries from a single substrate in high yields remains to be a challenge in cycloparaphenylene chemistry. In here, a strategy for the late‐stage functionalization of shape‐persistent alkyne‐containing cycloparaphenylene has been explored using readily available azides. The copper‐free [3+2]azide‐alkyne cycloaddition provided high yields (>90 %) in a single reaction step. Systematic variation of the azides from electron‐rich to ‐deficient shines light on how peripheral substitution influences the characteristics of the resulting adducts. We find that among the most affected properties are the molecular shape, the oxidation potential, excited state features, and affinities towards different fullerenes. Joint experimental and theoretical results are presented including calculations with the state‐of‐the‐art, artificial intelligence‐enhanced quantum mechanical method 1 (AIQM1). [ABSTRACT FROM AUTHOR]

It is intriguing how a mixture of organic molecules survived the prebiotic UV fluxes and evolved into the actual genetic building blocks. Scientists are trying to shed light on this issue by synthesizing nucleic acid monomers and their analogues under prebiotic Era‐like conditions and by exploring their excited state dynamics. To further add to this important body of knowledge, this study discloses new insights into the photophysical properties of protonated isoguanine, an isomorph of guanine, using steady‐state and femtosecond broadband transient absorption spectroscopies, and quantum mechanical calculations. Protonated isoguanine decays in ultrafast time scales following 292 nm excitation, consistently with the barrierless paths connecting the bright S1 (ππ*) state with different internal conversion funnels. Complementary calculations for neutral isoguanine predict similar photophysical properties. These results demonstrate that protonated isoguanine can be considered photostable in contrast to protonated guanine, which exhibits 40‐fold longer excited state lifetimes. [ABSTRACT FROM AUTHOR]

Developing conjugated small molecules (CSM) with intense NIR‐II (1000–1700 nm) absorption for phototheranostic is highly desirable but remains a tremendous challenge due to a lack of reliable design guidelines. This study reports a high‐performance NIR‐II CSM for phototheranostic by tailoring molecular planarity. A series of CSM show bathochromic absorption extended to the NIR‐II region upon the increasing thiophene number, but an excessive number of thiophene results in decreased NIR‐IIa (1300–1400 nm) brightness and photothermal effects. Further introduction of terminal nonconjugated alkyl chain can enhance NIR‐II absorption coefficient, NIR‐IIa brightness, and photothermal effects. Mechanism studies ascribe this overall enhancement to molecular planarity stemming from the collective contribution of donor/side‐chain engineering. This finding directs the design of NIR‐II CSM by rational manipulating molecular planarity to perform 1064 nm mediated phototheranostic at high efficiency. [ABSTRACT FROM AUTHOR]

Green‐to‐red photoconvertible fluorescent proteins (FPs) are vital biomimetic tools for powerful techniques such as super‐resolution imaging. A unique Kaede‐type FP named the least evolved ancestor (LEA) enables delineation of the evolutionary step to acquire photoconversion capability from the ancestral green fluorescent protein (GFP). A key residue, Ala69, was identified through several steady‐state and time‐resolved spectroscopic techniques that allows LEA to effectively photoswitch and enhance the green‐to‐red photoconversion. However, the inner workings of this functional protein have remained elusive due to practical challenges of capturing the photoexcited chromophore motions in real time. Here, we implemented femtosecond stimulated Raman spectroscopy and transient absorption on LEA‐A69T, aided by relevant crystal structures and control FPs, revealing that Thr69 promotes a stronger π–π stacking interaction between the chromophore phenolate (P‐)ring and His193 in FP mutants that cannot photoconvert or photoswitch. Characteristic time constants of ~60–67 ps are attributed to P‐ring twist as the onset for photoswitching in LEA (major) and LEA‐A69T (minor) with photoconversion capability, different from ~16/29 ps in correlation with the Gln62/His62 side‐chain twist in ALL‐GFP/ALL‐Q62H, indicative of the light‐induced conformational relaxation preferences in various local environments. A minor subpopulation of LEA‐A69T capable of positive photoswitching was revealed by time‐resolved electronic spectroscopies with targeted light irradiation wavelengths. The unveiled chromophore structure and dynamics inside engineered FPs in an aqueous buffer solution can be generalized to improve other green‐to‐red photoconvertible FPs from the bottom up for deeper biophysics with molecular biology insights and powerful bioimaging advances. [ABSTRACT FROM AUTHOR]

In this study, we investigate how modulating organic spacers in perovskites influences their X-ray detection performance and reveal the mechanism of low-dose detection with high sensitivity using femtosecond-transient absorption spectroscopy (fs-TAS). Particularly, we employ N,N,N',N'-tetramethyl-1,4-phenylenediammonium (TMPDA) and N,N-dimethylphenylene-p-diammonium (DPDA) as organic spacers to synthesize 2D perovskite single crystals (SCs). We find that DPDA-based SCs exhibit reduced interplanar spacing between inorganic layers, leading to increased lattice packing. Density functional theory (DFT) results indicate the reduced effective mass and lower lattice distortion in (DPDA)PbBr 4 suppressing the formation of self-trapped exciton (STEs) and electron-phonon coupling and enhancing carrier delocalization in these SCs. Further, X-ray detection measurements reveal that (DPDA)PbBr 4 demonstrates higher sensitivity than (TMPDA)PbBr 4 , attributed to its enhanced carrier delocalization, and higher mobility-lifetime product. The limit of detection (LoD) for (DPDA)PbBr 4 is determined to be 13 nGy/s, significantly lower than both commercial detectors and state-of-the-art perovskite-based X-ray detectors. Furthermore, fs-TAS study reveals that (DPDA)PbBr 4 crystals exhibit prolonged hot STE cooling and decay lifetimes, which directly correlate with their higher sensitivity. This study highlights the impact of organic spacers on X-ray detection performance, providing a framework for designing ultra-low LoD detectors essential for health and security applications., (© 2025 Wiley‐VCH GmbH.)

This work employs femtosecond transient absorption spectroscopy to investigate the ultrafast carrier dynamics of bound states in In0.14Ga0.86N/GaN quantum wells. The ground state (GS) dynamics usually dominate these characteristics, appearing as a prominent peak in the absorption spectra. It is observed that the excited state also contributes to the overall dynamics, with its signature showing up later. The contributions of both the ground and excited states in the absorption spectra and time-resolved dynamics are decoupled in this work. The carrier density in the GS first increases and then decays with time. The carriers populate the excited state only at a delayed time. The dynamics are studied considering the Quantum-Confined Stark Effect-induced wavelength shift in the absorption. The relevant microscopic optoelectronic processes are understood phenomenologically, and their time constants are extracted. An accurate study of these dynamics provides fundamentally essential insights into the time-resolved dynamics in quantum-confined heterostructures and can facilitate the development of efficient light sources using GaN heterostructures. [ABSTRACT FROM AUTHOR]

The photochemical reaction of potassium ferrocyanide (K4Fe(CN)6) exhibits excitation wavelength dependence and non-Kasha rule behavior. In this study, the excited-state dynamics of K4Fe(CN)6 were studied by transient absorption spectroscopy. Excited state electron detachment (ESED) and photoaquation reactions were clarified by comparing the results of 260, 320, 340, and 350 nm excitations. ESED is the path to generate a hydrated electron ( e aq − ). ESED energy barrier varies with the excited state, and it occurs even at the first singlet excited state (1T1g). The 1T1g state shows ∼0.2 ps lifetime and converts into triplet [Fe(CN)6]4− by intersystem crossing. Subsequently, 3[Fe(CN)5]3− appears after one CN− ligand is ejected. In sequence, H2O attacks [Fe(CN)5]3− to generate [Fe(CN)5H2O]3− with a time constant of approximately 20 ps. The 1T1g state and e aq − exhibit strong reducing power. The addition of uridine 5′-monophosphate (UMP) to the K4Fe(CN)6 solution decrease the yield of e aq − and reduce the lifetimes of the e aq − and 1T1g state. The obtained reaction rate constant of 1T1g state and UMP is 1.7×1014 (mol/L)−1·s−1, and the e aq − attachment to UMP is ∼8×109 (mol/L)−1·s−1. Our results indicate that the reductive damage of K4Fe(CN)6 solution to nucleic acids under ultraviolet irradiation cannot be neglected. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) is an upconversion photophysical process based on the population of a bright excited singlet state from an excited triplet via reverse intersystem crossing (rISC), which might be employed to overcome the statistical limitation of electron‐hole recombination in organic light‐emitting diodes. In this work, we describe the intricacies of TADF dynamics by means of a quantum master equation and through perturbative approaches to rate constants of internal conversion and ISC. Simulation of the time evolution from the lowest excited triplet in systems with small energy gaps allows to disentangle the role of different electronic states mediating the population of the singlet manifold and the importance of various rISC mechanisms. Energy gaps between charge transfer (CT) and local excitons (LE), and interstate (hyperfine, spin‐orbit and vibronic) couplings are key factors tuning the feasibility of rISC. Since in molecules LE/CT spin‐orbit coupling is, in general, larger than hyperfine interactions, the rISC rate is largely increased in those situations where either the direct or mediated triplet‐singlet interconversion between states of different character is energetically available. Interestingly, temperature dependence of rISC rate is stronger for the spin‐vibronic coupling mechanism than via hyperfine interaction. [ABSTRACT FROM AUTHOR]

Accurate tuning of the electronic and photophysical properties of quantum dots is required to maximize the light conversion efficiencies in semiconductor-assisted processes. Herein, we report a facile synthetic procedure for AgIn(S x Se 1- x ) 2 quantum dots with S content ( x ) ranging from 1 to 0. This simple approach allowed us to tune the bandgap (2.6-1.9 eV) and extend the absorption of AgIn(S x Se 1- x ) 2 quantum dots to lower photon energies (near-IR) while maintaining a small QD size (∼5 nm). Ultraviolet spectroscopy studies revealed that the change in the bandgap is modulated by the electronic shifts in both the valence band and the conduction band positions. The negative overall charge of the as-synthesized quantum dots enabled us to make films of quantum dots on mesoscopic TiO 2 . Excited state studies of the AgIn(S x Se 1- x ) 2 quantum dot films demonstrated a fast charge injection to TiO 2 , and the electron transfer rate constant was found to be 1.5-3.5 × 10 11 s -1 . The results of this work present AgIn(S x Se 1- x ) 2 quantum dots synthesized by the one-step method as a potential candidate for designing light-harvesting assemblies.

Transient absorption spectra (TAS) of lead halide perovskites can provide important insights into the nature of the photoexcited state dynamics of this prototypical class of materials. Here, we perform ground and excited state molecular dynamics (MD) simulations within a restricted open shell Kohn‐Sham (ROKS) approach in order to interpret the characteristic features of the TAS of CsPbBr3. Our results reveal that properties such as the finite temperature band gap, the Stokes shift, and therefore, also the TAS are strongly size‐dependent. Our TAS simulations show an early positive red‐shifted feature on the fs scale that can be explained by geometric relaxation in the excited state. As excited‐state processes can crucially affect the electronic properties of this class of photoactive materials, our observations are an important ingredient for further optimization of lead halide based optoelectronic devices. [ABSTRACT FROM AUTHOR]

Transient absorption spectra (TAS) of lead halide perovskites can provide important insights into the nature of the photoexcited state dynamics of this prototypical class of materials. Here, we perform ground and excited state molecular dynamics (MD) simulations within a restricted open shell Kohn‐Sham (ROKS) approach in order to interpret the characteristic features of the TAS of CsPbBr3. Our results reveal that properties such as the finite temperature band gap, the Stokes shift, and therefore, also the TAS are strongly size‐dependent. Our TAS simulations show an early positive red‐shifted feature on the fs scale that can be explained by geometric relaxation in the excited state. As excited‐state processes can crucially affect the electronic properties of this class of photoactive materials, our observations are an important ingredient for further optimization of lead halide based optoelectronic devices. [ABSTRACT FROM AUTHOR]

An intersystem crossing (ISC) rate constant of 1.0×1011 s−1 was previously registered with a spiro‐bis‐benzophenone scaffold. Triplet generation efficiency could be further enhanced by stabilizing the spiro‐charge‐transfer (CT) state and rationally designing spiro‐compounds (SCTs) that consist of electron‐rich diphenyl ether as the spiro‐CT donor and electron‐deficient dinaphthyl ketone as the spiro‐CT acceptor. Through fine‐tuning of the energy level between the CT and high energy triplet states, near‐unity triplet generation quantum yield was achieved and the underlying ISC mechanism is revealed by using ultrafast spectroscopy and quantum chemical calculations. Potential triplet sensitizing application was demonstrated in SCTs. Our findings suggest that a spiro‐bichromophoric molecular system with an enhanced spiro‐charge transfer warrants efficient triplet generation and is a powerful strategy of heavy‐atom‐free triplet sensitizers with predictable ISC properties. [ABSTRACT FROM AUTHOR]

Artificial biomimetic chromophore‐protein complexes inspired by natural visual pigments can feature color tunability across the full visible spectrum. However, control of excited state dynamics of the retinal chromophore, which is of paramount importance for technological applications, is lacking due to its complex and subtle photophysics/photochemistry. Here, ultrafast transient absorption spectroscopy and quantum mechanics/molecular mechanics simulations are combined for the study of highly tunable rhodopsin mimics, as compared to retinal chromophores in solution. Conical intersections and transient fluorescent intermediates are identified with atomistic resolution, providing unambiguous assignment of their ultrafast excited state absorption features. The results point out that the electrostatic environment of the chromophore, modified by protein point mutations, affects its excited state properties allowing control of its photophysics with same power of chemical modifications of the chromophore. The complex nature of such fine control is a fundamental knowledge for the design of bio‐mimetic opto‐electronic and photonic devices. [ABSTRACT FROM AUTHOR]

Exploration of high‐efficiency agents for near‐infrared‐II fluorescence imaging (NIR‐II FI) promotes the development of NIR‐II FI in life science. Despite the extensive use of organic semiconducting nanomaterials for NIR‐II FI, the fluorescence efficiency is barely satisfying, and the molecular guideline to improve the imaging quality has not been clarified yet. This contribution designs self‐brightened organic semiconducting polymers (OSPs) for improved NIR‐II phototheranostics of cancer. The amplification of NIR‐II brightness is realized by incorporating a weak electron‐donating unit (5,5′‐dibromo‐4,4′‐didodecyl‐2,2′‐bithiophene, DDB) into the semiconducting backbone with strong electron donor–acceptor alternated structure, which exhibits 6.3‐fold and 25‐fold fluorescence enhancement compared with the counterpart OSP at the same optical concentration and mass concentration, respectively. The broadband femtosecond transient absorption spectra experimentally elucidate the DDB doping‐induced suppression of vibrational relaxation as the underlying reason for the NIR‐II fluorescence amplification. Biocompatible nanoparticles fabricated from the optimal OSP12 exhibit excellent NIR‐II phototheranostic performance both in vitro and in vivo. Our research not only reveals the mechanistic insights for fluorescence enhancement of the designed OSPs from the essential view but also highlights an effective molecular methodology to guide the rational design of imaging agents with enhanced NIR‐II brightness for improved phototheranostics in living subjects. [ABSTRACT FROM AUTHOR]

Epigenetic modification of nucleic acid plays a vital role in realizing the epigenetic functions in living organisms. It can participate the regulation of cell differentiation, gene expression and other important physiological processes. However, epigenetic modification may undermine the photostability of nucleic acid, which may turn the corresponding nucleobases into important mutation sites for diseases such as skin cancer. Therefore, it is of great significance to study the effect of epigenetic modification on the photophysical and photochemical properties of nucleobases. In this paper, we reviewed recent research progress on the excited state dynamics of a series of epigenetic modified nucleobases. With the help of femtosecond time-resolved spectroscopy and high level quantum chemistry calculations, we demonstrated that the effect of epigenetic modification on the excited state properties of nucleobases are mainly in three aspects: significantly increasing the lifetime of the bright ππ∗ 1 states, introducing intramolecular charge transfer states and effectively promoting intersystem crossing from singlet to triplet states. [ABSTRACT FROM AUTHOR]

After binding to human serum albumin, bilirubin could undergo photoisomerization and photo-induced cyclization process. The latter process would result the formation of a product, named as lumirubin. These photo induced behaviors are the fundamental of clinical therapy for neonatal jaundice. Previous studies have reported that the addition of long chain fatty acids is beneficial to the generation of lumirubin, yet no kinetic study has revealed the mechanism behind. In this study, how palmitic acid affects the photochemical reaction process of bilirubin in Human serum albumin (HSA) is studied by using femtosecond transient absorption and fluorescence up-conversion techniques. With the addition of palmitic acid, the excited population of bilirubin prefers to return to its hot ground state (S0) through a 4 ps decay channel rather than the intrinsic ultrafast decay pathways (<1 ps). This effect prompts the Z-Z to E-Z isomerization at the S0 state and then further increases the production yield of lumirubin. This is the first time to characterize the promoting effect of long chain fatty acid in the process of phototherapy with femtosecond time resolution spectroscopy and the results can provide useful information to benefit the relevant clinical study. [ABSTRACT FROM AUTHOR]

Photolabile protecting groups offer synthetic routes with facile deprotection by photolysis, providing higher yields with less workup. Protecting groups producing only gaseous byproducts upon removal are particularly desirable as sustainable reagents. Cheletropic reactions provide just this combination of photocleavable groups with gaseous byproducts. However, only few protecting groups for cheletropic reactions have been described and little is known about their photochemistry and associated stereochemistry. Here we show that the sulfolene protecting group, used for diene protection and functionalization, can be photochemically removed. First, using TD‐DFT, we conclude that the photochemical ring‐opening occurs conrotatorily in a concerted process, consistent with the Woodward‐Hoffmann rules. Experimentally, we demonstrate that this process can be photoinitiated by radiation between 180 and 300 nm and confirm the proposed stereochemistry in solution. Using time‐resolved photoelectron spectroscopy, we determine that the first steps in the photochemical ring‐opening of sulfolene occur on ultrafast timescales (≤84 fs), consistent with a fully concerted process. Our findings expand applications of the sulfolene protecting group and promise an easy strategy for control of cis‐trans isomerization. [ABSTRACT FROM AUTHOR]

Fluorescence spectroscopy and time-resolved study have successfully explored the impact of supramolecular assembly, cucurbit[6]uril (CB6) on the dynamics of proton transfer in 2-(4'-pyridyl) benzimidazole (4-PBI). It has been observed that the guest molecules are capable of binding in three different states of protonation: cation C, tautomer T, and neutral N. While the acid–base equilibria is altered by the host molecule in the ground and excited states. Binding modes of the dye molecule with cucurbit[6]uril have been investigated by proton NMR spectroscopy. The binding of the guest with the supramolecular host is further confirmed by Quantum chemical calculations. The current study successfully recognises how the convoluted acid–base equilibria of dye molecule (4-PBI) are affected by cucurbit[6]uril (CB6) supramolecular assembly upon complexation. [ABSTRACT FROM AUTHOR]

Cyanobacterial photosynthetic systems efficiently capture sunlight using the pigment-protein megacomplexes, phycobilisome (PBS). The energy is subsequently transferred to photosystem I (PSI) and II (PSII), to produce electrochemical potentials. In the present study, we performed picosecond (ps) time-resolved fluorescence and femtosecond (fs) pump-probe spectroscopies on the intact PBS from a thermophilic cyanobacterium, Thermosynechococcus vulcanus, to reveal excitation energy transfer dynamics in PBS. The photophysical properties of the intact PBS were well characterized by spectroscopic measurements covering wide temporal range from femtoseconds to nanoseconds. The ps fluorescence measurements excited at 570 nm, corresponding to the higher energy of the phycocyanin (PC) absorption band, demonstrated the excitation energy transfer from the PC rods to the allophycocyanin (APC) core complex as well as the energy transfer in the APC core complex. Then, the fs pump-probe measurements revealed the detailed energy transfer dynamics in the PC rods taking place in an ultrafast time scale. The results obtained in this study provide the full picture of the funnel-type excitation energy transfer with rate constants of (0.57 ps)−1 → (7.3 ps)−1 → (53 ps)−1 → (180 ps)−1 → (1800 ps)−1. [ABSTRACT FROM AUTHOR]

Fewest-switches surface hopping (FSSH) is employed in order to investigate the nonadiabatic excited-state dynamics of thiophene and related compounds and hence to establish a connection between the electronic system, the critical points in configuration space and the deactivation dynamics. The potential-energy surfaces of the studied molecules were calculated with complete active space self-consistent field and time-dependent density-functional theory. They are analyzed thoroughly to locate and optimize minimum-energy conical intersections, which are essential to the dynamics of the system. The influence of decoherence on the dynamics is examined by employing different decoherence schemes. We find that irrespective of the employed decoherence algorithm, the population dynamics of thiophene give results which are sound with the expectations grounded on the analysis of the potential-energy surface. A more detailed look at single trajectories as well as on the excited-state lifetimes, however, reveals a substantial dependence on how decoherence is accounted for. In order to connect these findings, we describe how ensemble averaging cures some of the overcoherence problems of uncorrected FSSH. Eventually, we identify carbon–sulfur bond cleavage as a common feature accompanying electronic transitions between different states in the simulations of all thiophene-related compounds studied in this work, which is of interest due to their relevance in organic photovoltaics. [ABSTRACT FROM AUTHOR]