Photoinduced intramolecular charge transfer (ICT) reaction in trans-methyl p-(dimethylamino) cinnamate: A combined fluorescence measurement and quantum chemical calculations

The photophysical behaviour of trans-methyl p-(dimethylamino) cinnamate (t-MDMAC) donor–acceptor system has been investigated by steady-state absorption and emission spectroscopy and quantum chemical calculations. The molecule t-MDMAC shows an emission from the locally excited state in non-polar solvents. In addition to weak local emission, a strong solvent dependent red shifted fluorescence in polar aprotic solvents is attributed to highly polar intramolecular charge transfer state. However, the formation of hydrogen-bonded clusters with polar protic solvents has been suggested from a linear correlation between the observed red shifted fluorescence band maxima with hydrogen bonding parameters (). Calculations by ab initio and density functional theory show that the lone pair electron at nitrogen center is out of plane of the benzene ring in the global minimum ground state structure. In the gas phase, a potential energy surface along the twist coordinate at the donor (–NMe₂) and acceptor (–CH = CHCOOMe) sites shows stabilization of S₁ state and destabilization S₂ and S₀ states. A similar potential energy calculation along the twist coordinate in acetonitrile solvent using non-equilibrium polarized continuum model also shows more stabilization of S₁ state relative to other states and supports solvent dependent red shifted emission properties. In all types of calculations it is found that the nitrogen lone pair is delocalized over the benzene ring in the global minimum ground state and is localized on the nitrogen centre at the 90° twisted configuration. The S₁ energy state stabilization along the twist coordinate at the donor site and localized nitrogen lone pair at the perpendicular configuration support well the observed dual fluorescence in terms of proposed twisted intramolecular charge transfer (TICT) model.     

Inversion of states in two conformers of 1,8-bis(dimethylamino) naphthalene—the proton sponge

The S₁↔S₀ transitions of two conformers of 1,8-bis(dimethylamino) naphthalene, the “proton sponge”, have been studied by semiempirical AM1 calculations. They reveal that “inversion of states” occurs in the asymmetric conformer DMAN-2, which in the gas phase may be emitted from the ¹L_a state in comparison to the ¹L_b state in symmetric DMAN-1. It was also concluded that because of the mixed character of the HOMO-1 orbital in both conformers, a certain CT contribution to the S₀↔S₁ transition has to be taken into account. The calculated maxima of absorption and emission have been compared to those experimentally obtained in supersonic expansion.     

Photobase-Driven Excited-State Intramolecular Proton Transfer (ESIPT) in a Strapped π-Electron System

We report a new design strategy for an excited-state intramolecular proton transfer (ESIPT) fluorophore that can be used in acidic media. A photobasic pyridine-centered donor-acceptor-donor-type fluorophore is combined with a basic trialkylamine “strap”. In the presence of an acid, protonation occurs predominantly at the amine moiety in the ground state. A single-crystal X-ray diffraction analysis confirmed the formation of a pre-organized intramolecular hydrogen-bonded structure between the resulting ammonium moiety and the pyridine ring. Upon excitation, the intramolecular charge-transfer transition increases the basicity of the pyridine moiety in the excited state, resulting in proton transfer from the amine to the pyridine moiety. Consequently, the fluorophore takes on a polymethine-dye character in the ESIPT state, which gives rise to significantly red-shifted emission with an increased fluorescence quantum yield.     

Acid-base Vapor Sensing Enabled by ESIPT-attributed Cd(II) Coordination Polymer with Switchable Luminescence

Fluorescence materials based on excited state intramolecular proton transfer(ESIPT) have attracted great attention due to the unique four-level energy states. Herein, we report the assembly of a Cd-L_F coordination polymer from purposely designed L_F(H₂hpi2cf) ligand, which can present switchable luminescence behavior by gain or loss protons originated in uncoordinated ESIPT sites and serve as acid-base vapor sensors. Fabricated into in-situ grown film or transparent ink by simple methods, Cd-L_F presents facile and portable amine sensor for food spoilage detection and fluorescent anti-counterfeiting ink applications.     

激发态分子内质子转移化合物的性能及作为荧光化学传感器的应用研究

本文简述了激发态分子内质子转移(ESIPT)化合物的理论研究进展,并对其作为荧光化学传感器的应用作了简要的综述,列举了一些代表性的工作,以期对该类化合物的后续研究工作有所帮助.     

Structures of 1,2,3-trihydroxybenzene in the S0 and S1 states

In this paper we report on the structure and vibrations of gaseous pyrogallol (1,2,3-trihydroxybenzene) in the electronic ground state (S₀) and its first electronically excited state (S₁). Both ab initio CASSCF/CASMP2 calculations as well as R2PI spectroscopy have been performed. From the ab initio calculations three minimum energy structures are obtained and the vibrations of two structures are observed in the R2PI spectra. The minimum energy structures differ by their OH torsional angles. The full three-dimensional potential energy surface of the coupled torsional motions is investigated and the three-dimensional eigenvalues are calculated. The most stable structure of pyrogallol contains two intramolecular hydrogen bonds and turns out to be planar in the S₀ state. In the S₁ state the free OH group is rotated out of the plane of the aromatic ring by about 40°. The strong change in geometry of this structure is predicted by the CASSCF calculations and confirmed by the R2PI spectra of pyrogallol and its deuterated species. The low frequency region of the R2PI spectra can be explained by a torsional motion and the out of plane vibration 17b.     

Photophysics of inter- and intra-molecularly hydrogen-bonded systems: Computational studies on the pyrrole–pyridine complex and 2(2′-pyridyl)pyrrole

The role of electron and proton transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. We discuss generic mechanisms of the photophysics of a hydrogen-bonded aromatic pair (pyrrole–pyridine), as well as an intra-molecularly hydrogen-bonded π system composed of the same molecular sub-units (2(2′-pyridyl)pyrrole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections and the properties of frontier orbitals. A common feature of the photochemistry of these systems is the electron-driven proton transfer (EDPT) mechanism. In the hydrogen-bonded complex, a highly polar charge transfer state of ¹ππ^* character drives the proton transfer, which leads to a conical intersection of the S₁ and S₀ surfaces and thus ultrafast internal conversion. In 2(2′-pyridyl)pyrrole, out-of-plane torsion is additionally needed for barrierless access to the S₁–S₀ conical intersection. It is pointed out that the EDPT process plays an essential role in the fluorescence quenching in hydrogen-bonded aromatic complexes, the function of organic photostabilizers, and the photostability of biological molecules.     

Excited State Properties of Fucoxanthin Aggregates

The structure and excited state properties of the H- and J-aggregates of the marine carbonyl carotenoid, fucoxanthin(Fx), were studied by various spectroscopic methods, and compared with those of Fx monomers in polar organic solvents. The fluorescent analysis indicated tliat the higher vibronic states of S2 contribute more to populating the S1 state, from which fluorescent emission mainly originates. Resonance Raman and density functional theory calculations confirmed the ‘card-packed' and chead-to-taiF structures of the H- and J-aggregates of Fx, respectively. An fs time-resolved absorption study proved the coexistence of Si and intramolecular charge transfer relaxation pathways upon excitation to the S2 state for both tlie monomers and aggregates.     

1,3,6,8-Tetraethynylpyrene and 1,3,6,8-tetrakis (trimethylsilylethynyl) pyrene: Photophysical properties in homogeneous media

The photophysical properties of two new tetra substituted derivatives of pyrene: 1,3,6,8-tetraethynylpyrene (TEP) and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TEP-TMS) have been studied. Studies were done with respect to mirror image symmetry in the absorption and emission spectra and permissive or forbidden nature of S₀–S₁ transition, solvent sensitivity of the first and third vibronic bands and fluorescence anisotropy. Both the derivatives exhibited a strongly allowed S₀–S₁ transition, high fluorescence quantum yield, shorter fluorescence lifetime compared to pyrene and invariance of the vibronic band intensity ratio to solvent polarity. The behavior of the two pyrene derivatives validates the hypothesis “solvent polarity mediates vibronic coupling and therefore the emission band intensities, for forbidden S₀–S₁ transitions”. The trimethylsilyl derivative (TEP-TMS) was characterized by a strong fluorescence in solid state. The tetraethynyl derivative (TEP) showed high fluorescence anisotropy comparable to the well-known anisotropy probe DPH in glycerol at 0 °C. The fluorescence intensities of TEP and TEP-TMS did not show any significant change in the temperature ranger 0–40 °C for a low viscous solvent like ethanol and in the range 0–60 °C in glycerol. Unlike pyrene, no excimer emission was observed even up to 10⁻³ M for TEP and TEP-TMS.     

Enhancement of three-photon absorption cross-sections in a novel class of symmetrical charge transfer fluorene-based molecules

The three-photon absorption effect (3PA) of two novel symmetrical charge transfer fluorene-based molecules (abbreviated as BASF and BMOSF) has been determined by using a Q-switched Nd:YAG laser pumped with 38 ps pulses at 1064 nm in DMF. The measured 3PA cross-sections are 84 × 10⁻⁷⁸ and 114 × 10⁻⁷⁸ cm⁶ s², respectively. The geometries and electronic excitations of these two molecules are systematically studied by PM3 and ZINDO/S methods. The relationships between 3PA cross-sections and intramolecular charge transfer are discussed micromechanically. The experimental and theoretical results have shown that the larger intramolecular charge transfer, which was characterized by the charge density difference between the ground state (S₀) and the first excited state (S₁), the greater enhancement of the 3PA cross-sections.     

Different ESIPT Mechanisms for Angular‐Shaped Quinacridone in Toluene and Dimethyl Formamide (DMF) Solvents: A Theoretical Study

Adopting density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we investigat and present two different excited‐state intramolecular proton transfer (ESIPT) mechanisms of angular‐quinacridone (a‐QD) in both toluene and DMF,theoretically. Comparing the primary structural variations of a‐QD involved in the intramolecular hydrogen bond, we conclude that N1–H2⋯O3 should be strengthened in the S₁ state, which may facilitate the ESIPT process. Particularly, in toluene, the S₁‐state‐stable a‐QD enol* could not be located because of the non‐barrier ESIPT process. Concomitantly, infrared vibrational spectral analysis further verified the stability of the hydrogen bond. In addition, the role of charge–transfer interaction has been addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. The potential energy curves according to variational N1–H2 coordinate demonstrates that the proton transfer process should occur spontaneously in toluene; however, in DMF, a low potential energy barrier of 0.493 kcal/mol is needed to complete the ESIPT reaction. Although this barrier of 0.493 kcal/mol is too low to make an important impact on the ESIPT reaction, just because of the existence of barrier, ESIPT mechanisms in toluene and DMF are different.     

A density functional theory-time-dependent density functional theory investigation of photo-induced hydrogen bond and proton transfer for 2-(3,5-dichloro-2,6-dihydroxy-phenyl)-benzoxazole-6-carboxylicacid

Given the paramount importance of excited-state relaxation in the photochemical process, excited-state hydrogen bonding interactions and excited-state intramolecular proton transfer (ESIPT) are always hot topics. In this work, we theoretically explore the excited-state dynamical behaviors for a novel 2-(3,5-dichloro-2,6-dihydroxy-phenyl)-benzoxazole-6-carboxylicacid (DDPBC) system. As two intramolecular hydrogen bonds (O1 H2⋯N3 and O4 H5⋯O6) exist in the DDPBC structure, we first check if the double proton transfer form cannot be formed in the S1 state. Then, we explore the changes of geometrical parameters involved in hydrogen bonds, based on which we confirm that the dual intramolecular hydrogen bonds are strengthened on photo-excitation. The O1 H2⋯N3 hydrogen bond particularly plays a more important role in excited state. When it comes to the photo-induced excitation, we find charge transfer and electronic density redistribution around O1 H2 and N3 atom moieties. We verify the ESIPT tendency arising from the O1 H2⋯N3 hydrogen bond. In the analysis of the potential energy curves, along with O1 H2⋯N3 and O4 H5⋯O6, we demonstrate that the ESIPT reaction should occur along with O1 H2⋯N3 rather than O4 H5⋯O6. This work not only clarifies the specific ESIPT mechanism for DDPBC system but also paves the way for further novel applications based on DDPBC structure in the future.     

Excited state proton transfer based two color fluorescence: Perspectives and some biophysical applications

Photoinduced excited state intramolecular proton transfer (ESIPT) reactions comprise an important and extensively explored class of reactions in photochemistry. Till date, plant flavonols are one of the most widely known class of naturally abundant organic molecules exemplifying ESIPT and ‘two color’ fluorescence. From a bio-medical perspective, flavonols and related polyphenols, which are powerful antioxidants, have attracted significant interest as novel drugs (of high potency and low cyto-toxicity) for the prophylaxis and therapy of free radical induced and other important diseases. This article presents perspectives on proton transfer in photoexcited organic molecules from a historical context, emphasizing ESIPT reactions, in particular. Highlights of representative research findings are discussed, exemplifying the promising potential of plant flavonols as their own ESIPT based ‘fluorescence sensors’ for exploring their interactions with proteins, DNA (possessing duplex as well as higher order structures), and biomembranes, which represent the targets underlying the various pharmacological actions of flavonols. The usefulness of such approach for studying the confinement of intrinsically fluorescent flavonols in nano-vehicles for drug encapsulation, is also demonstrated.     

黄芩素激发态分子内质予转移耦合电荷转移反应的实验和理论研究

通过稳态光谱实验和量子化学计算相结合,研究了黄芩素激发态质子转移耦合电荷转移的反应. 实验和计算中S1态吸收峰的缺失表明S1态是暗态. S1暗态导致在实验中观察不到黄芩素在乙醇溶液中的荧光峰,且固体的荧光峰很弱. 黄芩素分子的前线分子轨道和电荷差异密度表明S1态是电荷转移态,然而S2态是局域激发态. 计算的黄芩素分子的势能曲线在激发态只有一个稳定点,这表明了黄芩素激发态分子内质子转移的过程是一个无能垒的过程.     

A DFT/TDDFT Investigation of Excited State Dynamical Mechanism of (E)‐1‐((2,2‐Diphenylhydrazono)methyl)naphthalen‐2‐ol

The excited‐state intramolecular proton transfer (ESIPT) mechanism of a new compound (E )‐1‐((2,2‐diphenylhydrazono)methyl)naphthalen‐2‐ol ( EDMN ) sensor, reported and synthesized by Mukherjee et al . [Sensors Actuat. B‐Chem . 2014, 202 , 1190], is investigated in detail theoretically. The calculations on primary bond lengths, bond angles, and the corresponding infrared (IR) vibrational spectra and hydrogen‐bond energy involved in intramolecular hydrogen bond between the S₀ and S₁ states confirm that the intramolecular hydrogen bond is strengthened in the S₁ state, which reveals the tendency of ESIPT reaction. The fact that the experimental absorption and emission spectra are well reproduced demonstrates the rationality and effectiveness of the time‐dependent density functional theory (TDDFT) level of theory we adopt here. Furthermore, intramolecular charge transfer based on the frontier molecular orbitals (MOs) gives indication of the ESIPT reaction. The constructed potential energy curves of both the S₀ and S₁ states while keeping the O─H distance of EDMN fixed at a series of values are used to illustrate the ESIPT process. The lower barrier of ~3.934 kcal/mol in the S₁ state potential energy curve (lower than the 8.254 kcal/mol in the S₀ state) provides the transfer mechanism.     

4-硝基咪唑A-带激发态结构动力学

通过共振拉曼光谱实验和量子化学计算的方法研究了4-硝基咪唑(4NI)A-带激发态衰变动力学.对4NI的振动光谱、紫外电子吸收光谱、荧光光谱和共振拉曼光谱进行了指认.在全活化空间自洽场法(CASSCF)/6-31G(d)计算水平下获得了单重激发态S1(nOπ*)和S2(ππ*)和势能面交叉点S1(nOπ*)/S2(ππ*)的优化几何结构和能量,分析了A-带共振拉曼光谱的强度模式特征,获得了短时结构动力学,并结合全活化空间自洽场法(CASSCF)理论计算结果确定了4NI在S2(ππ*)态衰变通道主要是S2,FC→S2,min(ππ*)→S0辐射弛豫.     

Internal Conversion with N,N-substituted 1-aminonaphthalenes:Photophysics and Applications

The solvent polarity dependence of the fluorescence quantum yield and lifetime of 1-dimethylaminonaphthalene (DMAN) was fund different from that of normal case in that the two parameters increase with increasing solvent polarity, in spite of the fact that the emissive state of DMAN was also of ICT character. Steady state and time-resolved fluorescence studies have currently indicated that a thermally activated internal conversion (IC) occurred with DMAN^[1,2]. The IC was assumed to be the consequence, of the vibronic coupling of the emissive S₁ state and S₂ state with the activation energy of the IC process depending on the energy gap between S₁ and S₂ states. It was hence put, forward that with increasing solvent, polarity the energy of the S₁ state would be lowered more than that of the S₂ state, leading to higher energy gap between the S₁ and S₂ states and therefore suppressed IC. As a consequence, increased fluorescence quantum yield and lengthened lifetime were observed.     

REINVESTIGATION OF THE PHOTOPHYSICS OF 2-(2'-HYDROXY-4'-DIETHYLAMINOPHENYL)BENZOTHIAZOLE

Abstract— The photophysical properties of 2-(2'-hydroxy-4'-diethylaminophenyl) benzothiazole (HABT) have been investigated by steady-state and time-resolved spectroscopies. In n-heptane HABT exhibits both normal and tautomer emissions with ∼equal fluorescence intensity at room temperature, in contrast to a previous report in which negligible tautomer emission was observed. The normal/tautomer (400/500 nm) ratio of emission intensity increases as the temperature decreases. Two possible excited-state intramolecular proton transfer (ESIPT) mechanisms are proposed, which cannot be resolved at the present stage. One proposed mechanism incorporates state mixing between -OH and -N(C₂H₅)₂ charge transfer states, resulting in a significant energy barrier for ESIPT. An alternative mechanism is also proposed in which fast proton tunneling may take place between enol and keto forms, which are in equilibrium in the excited singlet state.