Theoretical Determination of Rate Constants from Excited States: Application to Benzophenone

Molecular Insights into the Quenching Mechanism of the Triplet Excited State of Rose Bengal through Oxidative and Reductive Organic Compounds

In oxygenated aquatic environments, the predominant scavenging of the triplet excited state of chromophoric dissolved organic matter (3CDOM*) involves dissolved ground-state oxygen, diverting attention away from the scavenging mechanisms of 3CDOM* mediated through specific organic compounds. Previous studies demonstrated that model 3CDOM* exhibited quantum yields (i.e., 1-56%) in the formation of radical ions, resulting from the competition between physical and chemical quenching through a common exciplex intermediate. Physical quenching was rationalized through the reverse intersystem crossing of the exciplex, followed by back electron transfer, yielding ground-state reactants. Despite this, direct experimental evidence for exciplex involvement has been elusive, owing to detection challenges. Herein, employing density functional theory (DFT) and time-dependent DFT specifically for excited state surrogate CDOM and organic scavengers, we unveil, for the first time, the underlying mechanisms responsible for the quenching of Rose Bengal through oxidative and reductive scavengers. Our computational findings provide evidence for the involvement of exciplexes during the quenching process of the excited triplet state of Rose Bengal, highlighting the impact of electronic coupling between Rose Bengal and quenchers on the quantum yield for radical ion formation.

Shedding light on thermally-activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a hot research topic in view of its impressive applications in a wide variety of fields from organic LEDs to photodynamic therapy and metal-free photocatalysis. TADF is a rare and fragile phenomenon that requires a delicate equilibrium between tiny singlet-triplet gaps, sizable spin-orbit couplings, conformational flexibility and a balanced contribution of charge transfer and local excited states. To make the picture more complex, this precarious equilibrium is non-trivially affected by the interaction of the TADF dye with its local environment. The concurrent optimization of the dye and of the embedding medium is therefore of paramount importance to boost practical applications of TADF. Towards this aim, refined theoretical and computational approaches must be cleverly exploited, paying attention to the reliability of adopted approximations. In this perspective, we will address some of the most important issues in the field. Specifically, we will critically review theoretical and computational approaches to TADF rates, highlighting the limits of widespread approaches. Environmental effects on the TADF photophysics are discussed in detail, focusing on the major role played by dielectric and conformational disorder in liquid solutions and amorphous matrices.

Ultrafast Excited-State Proton Transfer in 4-Nitrocatechol: Implications for the Photochemistry of Nitrophenols

Nitrophenols are a class of environmental contaminants that exhibit strong absorption at atmospherically relevant wavelengths, prompting many studies of their photochemical degradation rates and mechanisms. Despite the importance of photochemical reactions of nitrophenols in the environment, the ultrafast processes in electronically excited nitrophenols are not well understood. Here, we present an experimental study of ultrafast electron dynamics in 4-nitrocatechol (4NC), a common product of biomass burning and fossil fuel combustion. The experiments are accompanied by time-dependent quantum mechanical calculations to help assign the observed transitions in static and transient absorption spectra and to estimate the rates of singlet-to-triplet intersystem crossing. Our results suggest that electronic triplet states are not efficiently populated upon 340 nm excitation, as efficient proton transfer occurs in the excited state on a time scale of a few picoseconds in water and tens of picoseconds in 2-propanol. This suggests that triplet states do not play a significant role in the photochemical reactions of 4NC in the environment and, by extension, in nitrophenols in general. Instead, consideration should be given to the idea that this class of molecules may serve as strong photoacids.

A narrow-band deep-blue MRTADF-type organic afterglow emitter

We report a multi-resonant thermally activated delayed fluorescent (MRTADF) afterglow emitter with unprecedented long emission lifetime > 100 ms, full-width at half-maximum < 40 nm, and deep-blue emission color of CIEy at 0.048. Such emitters remain rarely achieved and would show potential applications in multiplexed bioimaging and high-density information encryption.

ESIPT-induced spin-orbit coupling enhancement leads to tautomer fluorescence quenching of the 10-HHBF molecule

We present novel insights into the interplay between excited state intramolecular proton transfer (ESIPT) and spin-orbit coupling (SOC) in the 10-hydroxy-11H-benzo[b]fluoren-11-one (10-HHBF) molecule, utilizing the time-dependent density functional theory approach and femtosecond transient absorption spectroscopy. Our discoveries entail a reassessment of the luminescence mechanism for 10-HHBF, characterizing it as an ESIPT fluorophore. Additionally, we demonstrate that the molecule undergoes intersystem crossing (ISC) following proton transfer, which quenches the fluorescence of the proton-transferred state, thus resulting in the absence of dual emission and a limited spectral range of fluorescence. Furthermore, our investigation reveals that 10-HHBF displays an SOC enhancement feature induced by ESIPT, which facilitates the ISC process. This trait serves as a barrier to the application of 10-HHBF in single-molecule white light emitters (SMWLEs). Our findings underscore the notable influence of the ESIPT-induced spin-orbit interaction enhancement on luminescent properties, which necessitates consideration in the design of SMWLEs.

A direct observation of up-converted room-temperature phosphorescence in an anti-Kasha dopant-matrix system

It is common sense that emission maxima of phosphorescence spectra (λ_P) are longer than those of fluorescence spectra (λ_F). Here we report a serendipitous finding of up-converted room-temperature phosphorescence (RTP) with λ_P < λ_F and phosphorescence lifetime > 0.1 s upon doping benzophenone-containing difluoroboron β-diketonate (BPBF₂) into phenyl benzoate matrices. The up-converted RTP is originated from BPBF₂'s T_n (n ≥ 2) states which show typical ³n-π* characters from benzophenone moieties. Detailed studies reveal that, upon intersystem crossing from BPBF₂'s S₁ states of charge transfer characters, the resultant T₁ and T_n states build T₁-to-T_n equilibrium. Because of their ³n-π* characters, the T_n states possess large phosphorescence rates that can strongly compete RTP(T₁) to directly emit RTP(T_n) which violates Kasha's rule. The direct observation of up-converted RTP provides deep understanding of triplet excited state dynamics and opens an intriguing pathway to devise visible-light-excitable deep-blue afterglow emitters, as well as stimuli-responsive afterglow materials.

Calculation of Excited State Internal Conversion Rate Constant Using the One-Effective Mode Marcus-Jortner-Levich Theory

In this article, the one-effective mode Marcus-Jortner-Levich (MJL) theory and the classical Marcus theory for electron transfer were applied to estimate the internal conversion rate constant, k_IC, of organic molecules and a Ru-based complex, all belonging to the Marcus inverted region. For this, the reorganization energy was calculated using the minimum energy conical intersection point to account for more vibrational levels, correcting the density of states. The results showed good agreement with experimental and theoretically determined k_IC, with a small overestimation by the Marcus theory. Also, molecules less dependent on the solvent effects, like benzophenone, presented better results than molecules with an expressive dependence, like 1-aminonaphthalene. Moreover, the results suggest that each molecule possesses unique normal modes leading to the excited state deactivation that does not necessarily match the X-H bond stretching, as previously suggested.

Analytical derivative couplings within the framework of time-dependent density functional theory coupled with conductor-like polarizable continuum model: Formalism, implementation, and applications

The nonadiabatic phenomena, which are characterized by a strong coupling between electronic and nuclear motions, are ubiquitous. The nonadiabatic effect of the studied system can be significantly affected by the surrounding environment, such as solvents, in which such nonadiabatic process takes place. It is essential to develop the theoretical models to simulate these processes while accurately modeling the solvent environment. The time-dependent density functional theory (TDDFT) is currently the most efficient approach to describe the electronic structures and dynamics of complex systems, while the polarizable continuum model (PCM) represents one of the most successful examples among continuum solvation models. Here, we formulate the first-order derivative couplings (DCs) between the ground and excited states as well as between two excited states by utilizing time-independent equation of motion formalism within the framework of both linear response and spin flip formulations of TDDFT/CPCM (the conductor-like PCM), and implement the analytical DCs into the Q-CHEM electronic structure software package. The analytic implementation is validated by the comparison of the analytical and finite-difference results, and reproducing geometric phase effect in the protonated formaldimine test case. Taking 4-(N,N-dimethylamino)benzonitrile and uracil in the gas phase and solution as an example, we demonstrate that the solvent effect is essential not only for the excitation energies of the low-lying excited-states but also for the DCs between these states. Finally, we calculate the internal conversion rate of benzophenone in a solvent with DC being used. The current implementation of analytical DCs together with the existing analytical gradient and Hessian of TDDFT/PCM excited states allows one to study the nonadiabatic effects of relatively large systems in solutions with low computational cost.

Promoting Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence via the Heavy-Atom Effect

Thermally activated delayed fluorescence (TADF) molecules are promising for realizing durable organic light-emitting diodes in all color regions. Fast reverse intersystem crossing (RISC) is a way of improving the device lifetime of TADF-based organic light-emitting diodes. To date, RISC rate constants (k_RISC) of 10⁸ s^-1 have been reported for metal-free TADF molecules. Here, we report the heavy-atom effect on TADF and a molecular design for further promoting RISC. First, we reproduced all the relevant rate constants of a sulfur-containing TADF molecule (with k_RISC of 10⁸ s^-1) via density functional theory. The role of the heavy-atom effect on the rapid RISC process was clarified. Our calculations also predicted that much larger k_RISC (>10¹⁰ s^-1) will be obtained for selenium- and tellurium-containing TADF molecules. However, a polonium-containing molecule promotes phosphorescence without exhibiting TADF, indicating that a too strong heavy-atom effect is unfavorable for achieving both rapid RISC and efficient TADF.

Comprehensive understanding of multiple resonance thermally activated delayed fluorescence through quantum chemistry calculations

Molecules that exhibit multiple resonance (MR) type thermally activated delayed fluorescence (TADF) are highly efficient electroluminescent materials with narrow emission spectra. Despite their importance in various applications, the emission mechanism is still controversial. Here, a comprehensive understanding of the mechanism for a representative MR-TADF molecule (5,9-diphenyl-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracene, DABNA-1) is presented. Using the equation-of-motion coupled-cluster singles and doubles method and Fermi's golden rule, we quantitatively reproduced all rate constants relevant to the emission mechanism; prompt and delayed fluorescence, internal conversion (IC), intersystem crossing, and reverse intersystem crossing (RISC). In addition, the photoluminescence quantum yield and its prompt and delayed contributions were quantified by calculating the population kinetics of excited states and the transient photoluminescence decay curve. The calculations also revealed that TADF occurred via a stepwise process of 1) thermally activated IC from the electronically excited lowest triplet state T1 to the second-lowest triplet state T2, 2) RISC from T2 to the lowest excited singlet state S1, and 3) fluorescence from S1.

Molecular Ruby: Exploring the Excited State Landscape

The discovery of the highly NIR-luminescent Molecular Ruby [Cr(ddpd)2]3+ 13+ (ddpd = N,N’-dimethyl-N,N’-dipyridine-2-ylpyridine-2,6-diamine) has been a milestone in the development of earth-abundant luminophors and has led to important new impulses...