Recent experimental advances on excited-state intramolecular proton coupled electron transfer reaction

Extended conjugation of ESIPT-type dopants in nematic liquid crystalline phase for enhancing fluorescence efficiency and anisotropy

Organic compounds capable of excited-state intramolecular proton transfer (ESIPT) show fluorescence with a large Stokes shift and serve as solid-state emitters, luminescent dopants, and fluorescence-based sensing materials. Fluorescence of ESIPT molecules is usually increased in the solid state, but is weak in solvents due to the accelerated non-radiative decays by rotational motions of a part of the molecular core in these environments. Here we report, using a representative ESIPT motif 2-(2-hydroxyphenyl)benzothiazole (HBT), the extended-conjugation strategy of keeping sufficient fluorescence efficiency both in the solid state and in organic media. The introduction of an alkyl-terminated phenylene-ethynylene group into the HBT molecule dramatically enhances the fluorescence quantum yield from 0.01 to 0.20 in toluene and from 0.07 to 0.32 in a representative room-temperature nematic liquid crystal, 4-pentyl-4'-cyano biphenyl (5CB). The newly-synthesized CnP-C[triple bond, length as m-dash]C-HBT (n = 5 or 8) serves as a fluorescent dopant in 5CB and exhibits anisotropic fluorescence with the order parameter of 0.48, where the luminescence is controlled by the applied electric-field. The enhanced emission efficiency is rationalized by the larger height of energy barrier for the ESIPT process due to the introduction of phenylene-ethynylene groups.

Triarylborane-Appended Anils and Boranils: Solid-State Emission, Mechanofluorochromism, and Phosphorescence

Herein, the design, synthesis, optical properties, and mechanofluorochromism characteristics of a series of conjugates having covalently linked triarylborane (TAB) and anil/boranil units (TAB-anil: 1 a-3 a and TAB-boranil: 1-3) are reported. The electronic interactions between TAB and anil/boranil in 1 a-3 a and 1-3 were fine-tuned by changing the boryl moiety's position on the phenyl spacer connecting the BMes₂ (Mes=mesityl) and anil/boranil units. A boryl moiety at the meta position (1 a) of the phenyl spacer stabilizes the enolic form (E-OH), whereas a boryl moiety at the para position (2 a and 3 a) stabilizes the keto form (Z-NH) in the solid state. However, in solution 1 a, 2 a, and 3 a exhibit keto-enol tautomerism in both ground and excited states. Compounds 1 a-3 a and 1-3 show red-shifted absorption compared with 4 a and 4, which are devoid of TAB moieties, which indicate effective participation of an empty p orbital on the boron center in 1 a-3 a and 1-3. Compounds 1 and 2 showed fluorescence variations in response to external stimuli such as mechanical grinding. Long phosphorescence lifetimes of 18-46 ms were observed for compounds 1-3. The observed optical properties of 1 a-3 a and 1-3 are rationalized in the context of quantum mechanical calculations.

Discovery of a size-record breaking green-emissive fluorophore: small, smaller, HINA

Astonishingly, 3-hydroxyisonicotinealdehyde (HINA) is despite its small size a green-emitting push–pull fluorophore in water (QY of 15%) and shows ratiometric emission response to biological relevant pH differences (pK_a2 ∼ 7.1). Moreover, HINA is the first small-molecule fluorophore reported that possesses three distinctly emissive protonation states. This fluorophore can be used in combination with metal complexes for fluorescent-based cysteine detection in aqueous media, and is readily taken up by cells. The theoretical description of HINA's photophysics remains challenging, even when computing Franck–Condon profiles via coupled-cluster calculations, making HINA an interesting model for future method development.

Astonishingly, 3-hydroxyisonicotinealdehyde (HINA) is despite its small size a green-emitting push–pull fluorophore in water (QY of 15%) and shows ratiometric emission response to biological relevant pH differences (pK_a2 ∼ 7.1).

Impact of benzannulation on ESIPT in 2-(2'-hydroxyphenyl)-oxazoles: a unified perspective in terms of excited-state aromaticity and intramolecular charge transfer

Hydroxyphenyl-azoles are among the most popular ESIPT (Excited State Intramolecular Proton Transfer) scaffolds and as such, they have been thoroughly studied. Nevertheless, some aspects regarding the interplay between the emissive properties of these fluorophores and the size of their π-conjugated framework remain controversial. Previous studies have demonstrated that benzannulation of 2′-hydroxyphenyl-oxazole at the phenol group of the molecule can lead to either red- or blue-shifted fluorescence emission, depending on the site where it occurs. In this report, benzannulation at the heterocyclic unit (the oxazole site) is analysed in order to get the whole picture. The extension of π-conjugation does not significantly affect the ESIPT emission wavelength, but it leads instead to higher energy barriers for proton transfer in the first excited singlet state, as a consequence of dramatic changes in the charge transfer character of excitation caused by successive benzannulation. Theoretical calculations revealed an interesting connection between intramolecular charge transfer and excited-state aromaticity in the S₁ state. The theoretical approach presented herein allows the behaviour of hydroxyphenyl-oxazoles in the excited state to be rationalized and, more generally, a deeper understanding of the factors governing the ESIPT process to be obtained, a crucial point in the design of new and efficient fluorophores.

Benzannulation of a typical fluorophore reveals the interplay between ESIPT, excited-state aromaticity and intramolecular charge transfer.

Excited-state intramolecular proton transfer in the kinetic-control regime

A new series of molecules bearing a 2,11-dihydro-1H-cyclopenta[de]indeno[1,2-b]quinoline (CPIQ) chromophore with the N-HN type of intramolecular hydrogen bond are strategically designed and synthesized, among which CPIQ-OH, CPIQ-NHAc and CPIQ-NHTs in solution exhibit a single emission band with an anomalously large Stokes shift, whereas CPIQ-NH2 and CPIQ-NHMe show apparent dual-emission property. This, in combination with time-resolved spectroscopy and the computational approach, leads us to conclude that CPIQ-OH, CPIQ-NHAc and CPIQ-NHTs undergo ultrafast, highly exergonic excited-state intramolecular proton transfer (ESIPT), while a finite rate of ESIPT is observed for CPIQ-NH2 and CPIQ-NHMe with a time constant of 117 ps and 39 ps, respectively, in acetonitrile at room-temperature. Further temperature-dependent studies deduce an appreciable ESIPT barrier for CPIQ-NH2 and CPIQ-NHMe. Different from most of the barrier associated ESIPT molecules that are commonly in the thermodynamic-control regime, i.e. found in the thermal pre-equilibrium between excited normal and proton-transfer tautomer states, CPIQ-NH2 and CPIQ-NHMe cases are in the kinetic-control regime where ESIPT is irreversible with a significant barrier. The barrier is able to be tuned by the electronic properties of the -R group in the NR-H proton donor site, resulting in ratiometric fluorescence for normal versus tautomer emission.

Dual Emission: Classes, Mechanisms, and Conditions

There has been much interest in dual-emission materials in the past few years for materials and life science applications; however, a systematic overview of the underlying processes is so-far missing. We resolve this issue herein by classifying dual-emission (DE) phenomena as relying on one emitter with two emitting states (DE1), two independent emitters (DE2), or two correlated emitters (DE3). Relevant DE mechanisms for materials science are then briefly described together with the electronic and/or geometrical conditions under which they occur. For further reading, references are given that offer detailed insight into the complex mechanistic aspects of the various DE processes or provide overviews on materials families or their applications. By avoiding ambiguities and misinterpretations, this systematic, insightful Review might inspire future targeted designs of DE materials.

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

Excited-state proton transfer (ESPT) to solvent is often explained according to the two-step Eigen-Weller model including a contact ion pair (CIP*) as an intermediate, but general applicability of the model has not been thoroughly examined. Furthermore, examples of the spectral identification of CIP* are scarce. Here, we report on a detailed investigation of ESPT to protic (H₂O, D₂O, MeOH and EtOH) and aprotic (DMSO) solvents utilizing a broadband fluorescence technique with sub-200 fs time resolution. The time-resolved spectra are decomposed into contributions from the protonated and deprotonated species and a clear signature of CIP* is identified in DMSO and MeOH. Interestingly, the CIP* intermediate is not observable in aqueous environment although the dynamics in all solvents are multi-exponential. Global analysis based on the Eigen-Weller model is satisfactory in all solvents, but the marked mechanistic differences between aqueous and organic solvents cast doubt on the physical validity of the rate constants obtained.

Iridium-Catalyzed Direct C-H Amidation Producing Multicolor Fluorescent Molecules Emitting Blue-to-Red Light and White Light

We report a powerful strategy, iridium-catalyzed direct C-H amidation (DCA) for synthesizing various fluorescent sulfonamides that emit light over the entire visible spectrum with excellent efficiency (up to 99% yields). By controlling electronic characters of the resulting sulfonamides, a wide range of blue-to-red emissions was predictably obtained via an excited-state intramolecular proton-transfer process. Furthermore, we even succeeded in a white-light generation, highlighting that this DCA is an excellent synthetic method to prepare a library of fluorophores.

Multiple-State Emissions from Neat, Single-Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)-3H,3'H-[1,1'-biisobenzofuranylidene]-3,3'-dione, (E)-3-(3-oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one, and (E)-3H,3'H-[1,1'-bibenzo[c] thiophenylidene]-3,3'-dione, are found to fluoresce in their neat solid phases, from upper (S₂ ) and lowest (S₁ ) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3-9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by "suppressing" Kasha's rule.

Exciplex formation between a pair of synthesized AIEgens leads to white light generation: a spectroscopic exploration

Compounds BIMP and ECPA show aggregation promoted ESIPT and AIE emissions, respectively. The BIMP–ECPA ensemble behaves as a white light emitter through exciplex formation.

Boosting Circularly Polarized Luminescence of Organic Conjugated Systems via Twisted Intramolecular Charge Transfer

Realizing a high luminescence dissymmetry factor (g _lum) is a paramount yet challenging issue in the research field of circularly polarized luminescence (CPL). Here, we reported a novel set of organic conjugated systems with twisted intramolecular charge transfer (TICT) characteristics based on conjugated o-carborane-binaphthyl dyads composing of binaphthyl units as chiral electron donors and o-carborane units as achiral electron acceptors, demonstrating intense CPL with large g _lum values. Interestingly, single-crystalline o-1 exhibited a high-level brightness and a large g _lum factor as high as +0.13, whereas single-crystalline o-2 processed a relatively low brightness with a decreased g _lum value to -0.04. The significant diversity of CPL-active properties was triggered by the selective introduction of o-carborane units onto the binaphthyl units. Benefiting from the large magnetic dipole transition moments in TICT states, the CPL activity of TICT o-carborane-based materials exhibited amplified circular polarization. This study provides an efficient molecular engineering strategy for the rational design and development of highly efficient CPL-active materials.

An HBT-based fluorescent probe for nitroreductase determination and its application in Escherichia coli cell imaging

HBTPN exhibited excellent performance such as rapid response time, large Stokes shift, good selectivity and sensitivity, and long-wavelength emission.

Unveiling coupled electronic and vibrational motions of chromophores in condensed phases

The quest for capturing molecular movies of functional systems has motivated scientists and engineers for decades. A fundamental understanding of electronic and nuclear motions, two principal components of the molecular Schrödinger equation, has the potential to enable the de novo rational design for targeted functionalities of molecular machines. We discuss the development and application of a relatively new structural dynamics technique, femtosecond stimulated Raman spectroscopy with broadly tunable laser pulses from the UV to near-IR region, in tracking the coupled electronic and vibrational motions of organic chromophores in solution and protein environments. Such light-sensitive moieties hold broad interest and significance in gaining fundamental knowledge about the intramolecular and intermolecular Hamiltonian and developing effective strategies to control macroscopic properties. Inspired by recent experimental and theoretical advances, we focus on the in situ characterization and spectroscopy-guided tuning of photoacidity, excited state proton transfer pathways, emission color, and internal conversion via a conical intersection.

Investigation of the intramolecular hydrogen bonding interactions and excited state proton transfer mechanism for both Br-BTN and CN-BTN systems

In the present work, two novel Br-BTN and CN-BTN compounds have been investigated theoretically. We in-depth explore the excited state hydrogen bonding interactions and the excited state intramolecular proton transfer (ESIPT) behaviors for the Br-BTN and CN-BTN system. We firstly verify the formation of hydrogen bond effects of O–H⋯N based on reduced density gradient (RDG) versus sign(λ₂)ρ. The simulated primary bond lengths and bond angles as well as infrared (IR) vibrational spectra reveal that the hydrogen bond O–H⋯N should be strengthened in the excited state. Combining the frontier molecular orbital (MO) investigations, we infer that the charge transfer phenomenon (from HOMO to LUMO) around hydrogen bonding moieties reveals the tendency of ESIPT reaction. Particularly, the increased electronic densities around proton acceptor atoms facilitate attracting a hydrogen proton, which plays a decisive role in opening the ESIPT reaction. Via constructing potential energy curves in both S₀ and S₁ states, the ultrafast ESIPT process can be verified which explains previous experimental characteristics. Furthermore, via searching the transition state (TS) structure and constructing the intrinsic reaction coordinate (IRC) reaction path, we check and confirm the ESIPT mechanism for both Br-BTN and CN-BTN systems. We sincerely hope that our theoretical work could guide novel applications based on Br-BTN and CN-BTN compounds in future.

In the present work, two novel Br-BTN and CN-BTN compounds have been investigated theoretically.

Unraveling the Mechanisms of the Excited-State Intermolecular Proton Transfer (ESPT) for a D-π-A Molecular Architecture

Precise revealing the mechanisms of excited-state intermolecular proton transfer (ESPT) and the corresponding geometrical relaxation upon photoexcitation and photoionization remains a formidable challenge. In this work, the compound (E)-4-(((4H-1,2,4-triazol-4-yl)imino)methyl)-2,6-dimethoxyphenol (TIMDP) adopting a D-π-A molecular architecture featuring a significant intramolecular charge transfer (ICT) effect has been designed. With the presence of perchloric acid (35 %), TIMDP can be dissolved through the formation of a HClO₄ -H₂ O-OH(TIMDP)-N(TIMDP) hydrogen-bonding bridge. At the ground state, the ICT effect is dominant, giving birth to crystals of TIMDP. Upon external stimuli (e.g., UV light irradiation, electro field), the excited state is achieved, which weakens the ICT effect, and significantly promotes the ESPT effect along the hydrogen-bonding bridge, resulting in crystals of [HTIMDP]⁺ ⋅[H₂ O]⋅[ClO₄ ]^- . As a consequence, the mechanisms of the ESPT can be investigated, which distorted the D-π-A molecular architecture, tuned the emission color with the largest Stokes shift of 242 nm, and finally, high photoluminescence quantum yields (12 %) and long fluorescence lifetimes (8.6 μs) have achieved. These results not only provide new insight into ESPT mechanisms, but also open a new avenue for the design of efficient ESPT emitters.

Adiabatic deprotonation as an important competing pathway to ESIPT in photoacidic 2-phenylphenols

ESIPT (Excited State Intramolecular Proton Transfer) to C atom in 2-phenylphenol is known to be an intrinsically inefficient process. However, to the best of our knowledge, a structure-ESIPT efficiency relationship has not been elucidated yet. Here, we show that there exists a competitive interplay between photoacidity and ESIPT efficiency for the 2-phenylphenol system. The attachment of electron withdrawing groups to the phenol moiety promotes adiabatic deprotonation in the excited state and diminishes the charge transfer character of the excitations, and both these factors contribute in decreasing the ESIPT reaction yield. On the other hand, unfavorable conformational distribution in the ground state also appears as another important aspect responsible for the low ESIPT extent of 2-phenylphenol. A new derivative bearing electron donating, bulky substituents at ortho and para positions of the phenol ring shows an outstanding ESIPT performance, which demonstrates that the efficiency of the process can be significantly enhanced by modifying the substitution pattern. We anticipate that our results will help to guide the molecular designing of new compounds with high ESIPT efficiency.

Photochemical mechanism of 1,5-benzodiazepin-2-one: electronic structure calculations and nonadiabatic surface-hopping dynamics simulations

Due to the significant applications in bioimaging, sensing, optoelectronics etc., photoluminescent materials have attracted more and more attention in recent years. 1,5-Benzodiazepin-2-one and its derivatives have been used as fluorogenic probes for the detection of biothiols. However, their photochemical and photophysical properties have remained ambiguous until now. In this work, we have adopted combined static electronic structure calculations and nonadiabatic surface-hopping dynamics simulations to study the photochemical mechanism of 1,5-benzodiazepin-2-one. Firstly, we optimized minima and conical intersections in S0 and S1 states; then, we proposed three nonadiabatic decay pathways that efficiently populate the ground state from the Franck-Condon region based on computed electronic structure information and dynamics simulations. In the first pathway, upon photoexcitation to the S1 state, the system proceeds with an ultrafast excited-state intramolecular proton transfer (ESIPT) process. Then, the molecule tends to rotate around the C-C bond until it encounters keto conical intersections, from which the system can easily decay to the ground state. The other two pathways involve the enol channels in which the S1 system hops to the ground state via two enol S1/S0 conical intersections, respectively. These three energetically allowed S1 excited-state deactivation pathways are responsible for the decrease of fluorescence quantum yield. The present work will provide detailed mechanistic information of similar systems.

A photochemical and theoretical study of the triplet reactivity of furano- and pyrano-1,4-naphthoquionones towards tyrosine and tryptophan derivatives

The photochemical reactivity of the triplet state of pyrano- and furano-1,4-naphthoquinone derivatives (1 and 2) has been examined employing nanosecond laser flash photolysis. The quinone triplets were efficiently quenched by l-tryptophan methyl ester hydrochloride, l-tyrosine methyl ester hydrochloride, N-acetyl-l-tryptophan methyl ester and N-acetyl-l-tyrosine methyl ester, substituted phenols and indole (k q ∼109 L mol-1 s-1). For all these quenchers new transients were formed in the quenching process. These were assigned to the corresponding radical pairs that resulted from a coupled electron/proton transfer from the phenols, indole, amino acids, or their esters, to the excited state of the quinone. The proton coupled electron transfer (PCET) mechanism is supported by experimental rate constants, isotopic effects and theoretical calculations. The calculations revealed differences between the hydrogen abstraction reactions of phenol and indole substrates. For the latter, the calculations indicate that electron transfer and proton transfer occur as discrete steps.

Theoretical study on excited-state intramolecular proton transfer process of cyanide group substituted 2-(2-hydroxyphenyl)benzothiazole

In this work, the influence of cyanide group substituted on the excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzothiazole (HBT) has been theoretical studied. It is found that the intramolecular hydrogen bonds of HBT and its cyanide substituted derivatives are significantly strengthened in the first excited (S₁) state, while this strengthening tends to decrease with the substitution of cyano group. The natural population analysis (NPA) is performed to interpret this phenomenon and the calculated results show that the introduction of cyano groups induces intramolecular charge transfer, leading to the decrease of electrostatic interaction of intramolecular hydrogen bond in the S₁ state compared to isolated HBT. Therefore, proton transfer of HBT and its derivatives become difficult with the introduction of cyano groups, which is consistent with the calculated potential barriers (1.95, 2.71 and 2.81 kcal/mol respectively) of the three molecules in the S₁ state. This work interprets the hindrance of cyanide group on the proton transfer of HBT and its derivatives in the S₁ state, which will bring new insight into the study of analogous molecular system.

Investigation of Different Prototropic Forms of Biologically Active Flavin Lumichrome in the Presence of Liposome

Herein, we reported the photophysical behavior of lumichrome (LC), one of the biologically active flavin molecules, in the presence of small unilamellar vesicle of anionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). With the help of different spectroscopic techniques, we have proposed that anionic DMPC liposome interacts with the cationic form LC in ground state and in excited state and modulate the spectral properties of LC. Photophysical study reveals that different prototropic forms of LC are present in DMPC liposome medium. In the presence of DMPC liposome, fluorescence emission properties of LC vary with change in excitation and emission wavelengths. This indicates switch over between different structural forms of LC. From fluorescence lifetime measurements and fluorescence lifetime imaging (FLIM) study, it was revealed that emission decay profile of LC was fitted biexponentially in the presence of liposome. It suggests that in the presence of liposome, more than one form of LC is present. We have constructed the time-resolved area-normalized emission spectra (TRANES) of LC in the liposome and found one isoemissive point. This confirmed that two emissive species of LC are present in liposome. FLIM study and FE-SEM study give an idea about the structural feature of the complex between LC and liposome.

Intramolecular electron-induced proton transfer and its correlation with excited-state intramolecular proton transfer

Electron-induced proton transfer depicts the proton motion coupled with the attachment of a low-energy electron to a molecule, which helps to understand copious fundamental chemical processes. Intramolecular electron-induced proton transfer is a similar process that occurs within a single molecule. To date, there is only one known intramolecular example, to the best of our knowledge. By studying the 10-hydroxybenzo[h]quinoline and 8-hydroxyquinoline molecules using anion photoelectron spectroscopy and density functional theory, and by theoretical screening of six other molecules, here we show the intramolecular electron-induced proton transfer capability of a long list of molecules that meanwhile have the excited-state intramolecular proton transfer property. Careful examination of the intrinsic electronic signatures of these molecules reveals that these two distinct processes should occur to the same category of molecules. Intramolecular electron-induced proton transfer could have potential applications such as molecular devices that are responsive to electrons or current.

Theoretical insights into excited-state intramolecular and multiple intermolecular hydrogen bonds in 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone

The photophysical properties and photochemistry reactions of 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ) system in different solutions are studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Our theoretical investigation explores that an ultrafast barrier-free excited state intramolecular proton transfer (ESIPT) process occurs and the configuration twisting is found in the electronic excited state. In the polar protic methanol solution, the hydrogen-bonded complex composed by HPQ and two methanol molecules (HPQ-2M) could exist stably in the ground state. Upon photoexcitation the isolated HPQ is initially excited to the first excited state, while the HPQ-2M system is firstly excited to the S₃ state and undergoes internal conversion (IC) to the S₁ state. The intermolecular hydrogen bonds are strengthened in the excited state. The simulated electronic spectra agree well with the experimental results. The strengthening of the intermolecular hydrogen bonds is also confirmed by the calculated vibrational spectra. In addition, the intramolecular charge transfer happens in both HPQ and HPQ-2M systems from the frontier molecular orbital analysis.

Excited state proton transfer based fluorescent molecular probes and their application in studying lipid bilayer membranes

The distribution and prototropic equilibria of 1-naphthol (NpOH) in lipid bilayer membrane.

Solvent-dependent transition from concerted electron-proton to proton transfer in photoinduced reactions between phenols and polypyridine Ru complexes with proton-accepting sites

The bimolecular rate coefficients (kobsq) for quenching the metal-to-ligand charge transfer excited states of two Ru polypyridine complexes containing H-bond accepting sites by six p-substituted phenols exhibit abrupt deviations from the expected linear correlations of log kobsq with phenol's Hammett σp constant. This pattern is attributed to a transition of the quenching mechanism from a concerted electron-proton transfer (EPT) to a proton transfer (PT); the latter becomes predominant for the most acidic phenols in acetonitrile, but not in dichloromethane. This assertion is supported by a detailed thermochemical analysis, which also excludes the quenching pathways involving electron transfer from phenols with or without deprotonation of phenols to the solvent, either concerted or sequential. The transition from EPT to PT upon the σp increase is consistent/supported by the magnitudes of the measured and computed PhOH/OD kinetic isotope effects and by the observed reduction of the EPT product yields upon replacing the low σp methoxyphenol by the high σp nitrophenol. In addition to modulating the relative contribution of the EPT and PT quenching pathways, the solvent strongly affects the bimolecular rate coefficients for the EPT quenching proper. Unlike with H-atom transfer reactions, this kinetic solvent effect could not be quantitatively accounted for by the phenol-solvent H-bonding alone, which suggests a solvent effect on the H-bonding constants in the phenol-Ru complex precursor exciplexes and/or on the unimolecular EPT rate coefficients within these exciplexes.

Experimental and theoretical insights into the influence of electronic density on proton-transfer reactions

We report on the excited-state behavior of proton-transfer phenanthroimidazole derivatives, such as HPPI and NMHPPI, in solutions using steady-state and femto- to nanosecond time-resolved fluorescence spectroscopies. Experimental observations are supported by theoretical calculations (TDDFT). In dichloromethane (DCM) and acetonitrile (ACN), two different paths are found for the excited-state intramolecular proton-transfer (ESIPT) reactions following two different channels. A fast and direct channel (ESIPT1) in 1-2.5 ps and a slower one (ESIPT2) in 12-15 ps, the latter being more influenced by the solvent viscosity (30 ps for HPPI and 20 ps for NMHPPI in triacetin (TAC) solutions). The slowing down of the ESIPT2 reaction is explained in terms of an intramolecular charge transfer (ICT) reaction coupled to a twisting motion to reach a more suitable conformation of the involved parts in the proton-transfer motion. The absence of OH/OD exchange effects in the ultrafast and slow proton-transfer dynamics suggests that the ESIPT reactions, which involve intramolecular and solvent coordinates, do not occur via tunneling. These results reveal new insights into the photobehavior of proton-transfer dyes, which might help in designing photosensors or lighting devices.