Exploring Emission Ratios: Influence of Neighboring Groups on TSAL Core

The photophysics of tris(salicylidenealdimines) (TSALs) has been examined, as they offer the possibility of populating multiple emissive species through the excited-state intramolecular proton transfer (ESIPT) reaction, resulting in broad visible spectrum coverage, a desirable property for white organic light-emitting diodes (wOLEDs). In this contribution, the synthesis and photophysical characterization of a new TSAL derivative, C16, are reported. It displays a distinct emission profile compared to previously studied tris(salicylideneanilines), with an inversion of the ESIPT/LE emission intensity ratio, exhibiting almost exclusively tautomer emission upon excitation in less polar solvents. Ab initio calculations suggest that the absence of emission from the locally excited (LE) state in nonpolar solvents may be related to the competition between two processes: ESIPT and N-pyramidalization of the enaminic nitrogen. Conversely, in polar aprotic solvents, the planar LE conformation is stabilized, and dual emission is observed. In protic solvents, however, the ESIPT reaction is suppressed due to competition with intermolecular hydrogen bonding interactions.

Concerning for the solvent-polarity-dependent conformational equilibrium and ESIPT mechanism in Pz3HC system: A novel insight

In this report, we propose a new insight into the interaction between the solvent-polarity-dependent conformational equilibrium and excited state intramolecular proton transfer (ESIPT) behavior of Pz3HC system in four different polar solvents (polarity order: ACN > THF > TOL > CYC). Using quantum chemistry method, we first announce a coexistence mechanism between Pz3HC-1 and Pz3HC-3 in the ground state in four solvents based on the Boltzmann distribution. In particular, Pz3HC-1 is the principal configuration in non-polar solvent, but Pz3HC-3 is the principal configuration in polar solvent. In addition, the simulated fluorescence spectra interprets the negative solvatochromism effect of Pz3HC-1 and Pz3HC-3 in four solvents. The evidence from intramolecular hydrogen bonding (IHB) parameters and electronic perspective collectively confirms the light-induced IHB enhancement and intramolecular charge transfer (ICT) properties in Pz3HC-1 and Pz3HC-3, which raises the likelihood of the ESIPT process. Combining the calculation of potential energy curve (PEC) and intrinsic reaction coordinate (IRC), we demonstrate that the ESIPT ease of Pz3HC-1 in different polar solvents obeys the order of CYC > TOL > THF > ACN, while the order of ESIPT ease in Pz3HC-3 is opposite. Notably, the ESIPT process of Pz3HC-3 in CYC solvent is accompanied by the twisted intramolecular charge transfer (TICT) process. In addition, we also reveal that the enol* and keto* fluorescence peaks of Pz3HC-3 in CYC solvent are quenched by ISC and TICT process, respectively. Our work not only provides a satisfactory explanation of the novel dynamics mechanism for Pz3HC system, but also brings light to the design and application of new sensing molecules in the future.

Effects of solvent polarity on the novel excited-state intramolecular thiol proton transfer and photophysical property compared with the oxygen proton transfer

Recently, the dual-fluorescent phenomena of excited state intramolecular thiol proton transfer (ESIPT) for 3-thiolflavone derivative (3NTF) were reported by Chou and coworkers for the first time [J. Am. Chem. Soc. 143 (2021) 12715-12724], which opened a new chapter in the field of ESIPT. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT), the proton transfer processes of 3NTF in toluene, dichloromethane and acetonitrile were studied. By optimizing the structure of the ground (S0) state and first excited (S1) state of 3NTF in different solvents, the hydrogen-bond parameters and proton-transfer potential energy curves were calculated. It was shown that although photo-excitation enhanced the intramolecular hydrogen bonding strength and thus promoted the occurrence of ESIPT, the solvent polarities inhibited the enhancement of the hydrogen bond of S1 state, which was not conducive to ESIPT. The electron spectra analyses were consistent with experimental data, which confirmed the rationality of molecular configurations. The time-evolved excited state dynamics simulation was performed based on the optimized structure of 3NTF, indicating that the ESIPT was an ultrafast photochemical reaction less than 180 fs. Moreover, we compared the potential energy surfaces of ESIPT, electronic structures based on natural transition orbitals (NTOs) method and electron-hole isosurfaces for the 3NTF and the traditional flavone molecule (3NHF), concluded that the unusually large Stokes shift fluorescence of 3NTF was mainly caused by the coupling of ESIPT and twisting intramolecular charge transfer (TICT), and the 3NTF isomer had the more nπ* character in the electron transition process. The nπ* ICT significantly increased with the decrease of solvent polarities, affecting the molecular photophysical properties, this made it more widely used in biomedical, photochemical, materials science and other fields.

Magnifying the ESIPT process in tris(salicylideneanilines) via the steric effect - a pathway to the molecules with panchromatic fluorescence

Four tris(salicylideneanilines) (TSANs) with gradually increased steric interactions between the keto-enamine moiety and neighbouring phenyl substituent are presented. The steric interactions are induced by placing two alkyl groups at the ortho position in the N-aryl substituent. The impact of the steric effect over the radiative channels of deactivation of the excited state was evaluated through spectroscopic measurements and theoretical calculations using ab initio techniques. Our results show that the emission occurring after excited state intramolecular proton transfer (ESIPT) is favoured by placing the bulky groups in the ortho position of the N-phenyl ring of the TSAN. However, our TSANs seem to offer the opportunity to obtain a pronounced emission band at higher energy, significantly increasing the coverage of the visible spectrum, resulting in the enhancement of the dual emissive properties of tris(salicylideneanilines). Thus, TSANs may be promising molecules capable of white-like emission for use in organic electronic devices such as white OLEDs.

ESIPT in the pyrrol pyridine molecule: mechanism, timescale and yield revealed using dynamics simulations

Excited State Intramolecular Proton Transfer in pyrrol pyridine is theoretically investigated using non-adiabatic dynamics simulations. The photochemical process is completely characterised: the reaction time, the total yield and the accessibility of the conical intersection are evaluated. Finally, new mechanistic interpretation are extracted: the proton transfer reaction in this molecule is shown to be driven by two complementary mechanisms.

Effects of Twisted Intramolecular Charge Transfer Behavior on Excited-State Intramolecular Proton Transfer Reactions of Methyl Benzoate Derivatives in Water Solution

Many methyl benzoate derivatives were found to show intramolecular charge transfer (ICT), intramolecular proton transfer, and other properties, which have extensive applications in lasing media, metal ion sensors, active materials, and fluorescence probe fields. However, the intrinsic relationship and reaction mechanism between the excited-state intramolecular proton transfer (ESIPT) and ICT between methyl benzoate derivatives with different substituents have not been explained. In this paper, the density functional theory and time-dependent density functional theory methods were used to study the ESIPT and ICT behaviors of p-aminosalicylic acid methyl ester and p-dimethylaminosalicylic acid methyl ester in water and obtain the intrinsic interaction between the two behaviors. The bond parameters, infrared spectra, reduced density gradient scatter plots, and topological analyses of these two molecules in the ground state and excited state were analyzed to confirm the enhancement of the excited-state intramolecular hydrogen bonds (IHBs). The simulated absorption and fluorescence spectra of these molecules agreed well with the experimental values. Based on the optimized structure, we also plotted the natural transition orbitals, electron density difference maps, and frontier molecular orbitals (FMOs), which showed the changes of the charge distribution of these molecules intuitively upon photoexcitation. In addition, we also found that the degree of IHB enhancement with -N(CH₃)₂ substituents was less than that with -NH₂, reflecting an inhibition effect of twisted intramolecular charge transfer (TICT) on ESIPT reaction. This conclusion was confirmed by our calculated potential energy curves. This work may better deepen the comprehension of the intrinsic relationship between ESIPT and TICT behavior and sequentially provide better theoretical guidance for the synthesis of fluorescent molecules related to these two behaviors.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the ¹ππ*, ¹nπ*, ³ππ*, ³nπ*, and S₀ states, we have identified several feasible excited-state relaxation pathways for the initially populated S₂(¹ππ*) state to decay to the initial enol isomer' S₀ state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless ¹ππ* ESIPT process to generate the ¹ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S₁ → S₀ internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process. The minor ones are related to intersystem crossing (ISC) processes. At the S₂(¹ππ*) minimum, an S₂(¹ππ*)/S₁(¹nπ*)/T₂(³nπ*) three-state intersection region helps the S₂ system branch into the T₁ state through a S₂ → S₁ → T₁ or S₂ → T₂ → T₁ process. Once it has reached the T₁ state, the system may relax to the S₀ state via direct ISC or via subsequent nearly barrierless ³ππ* ESIPT to yield the T₁ keto tautomer and ISC. The resultant S₀ keto species significantly undergoes reverse GSIPT and only a small fraction yields the trans-keto form that relaxes back more slowly. However, due to small spin-orbit couplings at T₁/S₀ crossing points, the ISC to S₀ state occurs very slowly. The present work rationalizes not only the ultrafast excited-state decay dynamics of OB but also its phosphorescence emission at low temperature.

Dual fluorescence of 2-(2'-hydroxyphenyl) benzoxazole derivatives via the branched decays from the upper excited-state

As a special fluorescence phenomenon, double fluorescence has been widely developed and applied in various fields. Nevertheless, most of the research on fluorescence emission channels focuses on the first excited state, while the research on how to control the fluorescence emission channel through the upper excited state is relatively under-explored. Here, we use the time-dependent density functional theory method and consider the 2-(2'-hydroxyphenyl) benzoxazole (HBO) derivative system as an example to study the effect of upper excited states on double fluorescence. According to the calculation results, a new mechanism for the dual fluorescence was proposed, which involved the different decay pathways from the upper excited-state, the internal conversion through vibrational relaxation, and conical intersection, respectively. This research has potential value and can help in determining how to control the fluorescence emission channel through the upper excited state.

Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview

The excited-state intramolecular proton transfer (ESIPT) phenomenon is nowadays widely acknowledged to play a crucial role in many photobiological and photochemical processes. It is an extremely fast transformation, often taking place at sub-100 fs timescales. While its experimental characterization can be highly challenging, a rich manifold of theoretical approaches at different levels is nowadays available to support and guide experimental investigations. In this perspective, we summarize the state-of-the-art quantum-chemical methods, as well as molecular- and quantum-dynamics tools successfully applied in ESIPT process studies, focusing on a critical comparison of their specific properties.

Polarized Helical Coumarins: [1,5] Sigmatropic Rearrangement and Excited-State Intramolecular Proton Transfer

The tandem process of phenol addition to a cyclic α,β-unsaturated ester followed by intramolecular transesterification and [1,5] sigmatropic rearrangement affords a series of helical coumarins based upon a previously unknown 3-amino-7-hydroxybenzo[3,4]cyclohepta[1,2-c]chromen-6-one core. These novel polarized coumarins, possessing a β-ketoester moiety, have been employed to synthesize more rigid and helical coumarin–pyrazolones, which display green fluorescence. The enhanced emission of coumarin–pyrazolones in polar solvents depends on the nature of the S₁ state. The coumarin–pyrazolones are predicted to have two vertical states close in energy: a weakly absorbing S₁ (¹LE) followed by a bright S₂ state (¹CT). In polar solvents, the ¹CT can be stabilized below the ¹LE and may become the fluorescent state. Solvatochromism of the fluorescence spectra confirms this theoretical prediction. The presence of an N—H···O=C intramolecular hydrogen bond in these coumarin–pyrazolone hybrids facilitates excited-state intramolecular proton transfer (ESIPT). This process leads to a barrierless conical intersection with the ground electronic state and opens a radiationless deactivation channel effectively competing with fluorescence. Solvent stabilization of the CT state increases the barrier for ESIPT and decreases the efficiency of the nonradiative channel. This results in the observed correlation between solvatochromism and an increase of fluorescence intensity in polar solvents.

Insights into the Excited-State Processes in 1-Hydroxy-2-acetonaphthone at ADC(2) and CASSCF Levels

1-Hydroxy-2-acetonaphthone (HAN) has been extensively studied both experimentally and computationally to ascertain the existence of the excited-state proton transfer process. However, the process of full photocycle including the nonradiative relaxation pathways is yet to be proposed. Therefore, in the present study, we aim at providing a comprehensive picture of the excited-state processes in HAN including the proton transfer and relaxation processes through electronic structure calculations at second-order algebraic diagrammatic construction (ADC(2)) and complete active space second-order perturbation theory (CASPT2)//complete active space self-consistent field (CASSCF) and dynamics simulations at ADC(2) levels. Our studies show that the proton transfer process in the S₁ state is barrierless and produces a stable keto form, which is in accordance with previous experimental and computational studies. Adiabatic dynamics simulations at the ADC(2) level confirmed the ultrafast process with an average proton transfer time of 43 fs. The resultant keto conformer then undergoes torsional rotation, leading to a conical intersection that mediates the internal conversion process to the ground state. Our dynamics simulation predicted that this deactivation process occurs at a time scale beyond 600 fs of simulation time. We also explored nonradiative relaxation from the enol Franck-Condon region, and this process was found to be improbable from the static point of view at both the ADC(2) and CASPT2 levels of theory due to a high energy barrier along the torsional coordinate.

Substituent Effect on the Photophysics and ESIPT Mechanism of N,N'-Bis(salicylidene)-p-phenylenediamine: A DFT/TD-DFT Analysis

Excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) processes are widely exploited in the designing of organic materials for multifarious applications. This work explores the aftereffects of combining both ESIPT and ICT events in a single molecule, namely, N,N'-bis(salicylidene)-p-phenylenediamine (BSP) exploiting DFT and TD-DFT formalisms. The PBE0 functional employed in the present study is found to yield results with better accuracy for excited-state calculations. The results reveal that introduction of electron donor (-NH₂) and electron acceptor (-NO₂) substituents on BSP produces a strikingly red-shifted emission with respect to the corresponding emission from the unsubstituted analogue in polar solvents. This red-shifted emission originated due to the coupled effect of ESIPT and planar-ICT (PICT) processes from the coplanar geometry adopted by the substituted molecule (s-BSP). Based on the computed potential energy curves, the ground-state intramolecular proton transfer (GSIPT) was found to take place more favorably in s-BSP than in BSP under all solvent conditions. In the case of ESIPT, the barrier and relative energies of the phototautomers of s-BSP were slightly higher than BSP, which shows that simultaneous substitution of -NH₂ and -NO₂ groups causes slight perturbation to the ESIPT process. Overall, the computed results show that simultaneous substitution of suitable electron donor and acceptor substituents provides profitable changes in the photophysical properties of ESIPT molecules like BSP. These molecular-level insights will pave way for designing better materials for diverse applications.

Substituent effects on the photophysical properties of tris(salicylideneanilines)

The role of electron acceptor/donor group substitution on the photophysical properties of tris(salicylideneanilines) (TSANs) was investigated. These compounds were synthesised and characterised through spectroscopic techniques including steady state absorption and emission spectroscopies. Their photochemical reaction mechanisms and properties were explored with the aid of ab initio methods of quantum chemistry. The obtained results allow us to verify the dependence of multiple emission bands on the substitution of electron donating and accepting groups to the tris(salicylideneaniline) core. The results also stress the differences in phosphorescence behaviour of TSANs for which this type of emission has not been reported so far.

TD-DFT and CC2 insights into the dual-emissive behaviour of 2-(2'-hydroxyphenyl)oxazoles core and their derivatives

Two efficient excited state intramolecular proton transfer (ESIPT) dyes based on the hydroxyphenyl-oxazole core and containing one or two triphenylamine donor groups are explored with theoretical tools. These compounds are known to show clear experimental dual emission behaviour, leading to nearly pure white-light emission for one derivative. To probe the excited state properties, we use both Time Dependent Density Functional Theory (TD-DFT) and post Hartree-Fock methods [ADC(2) and CC2] coupled to different solvent models to describe polarisation effects. After validating our theoretical protocol on the two known systems, we design 14 new derivatives with different substitution patterns to quantify the impact of electron accepting and donating groups on the fluorescence spectrum and the ESIPT mechanism. We show that the selected protocol delivers accurate spectroscopic values for the two experimentally-characterised structures, and more importantly, that the relative stabilisation of the keto tautomer depends on the substitution side. Adding donor or acceptor groups to the ESIPT donor moiety favours the formation of the keto form, whereas when placed on the ESIPT accepting side, they tend to preclude ESIPT. Moreover, combining two donor or acceptor substituents generally results in similar ESIPT behaviour as single substitution on one of the two sides: simple additive rules do not apply.