A DFT/TD-DFT study of effect of different substituent on ESIPT fluorescence features of 2-(2'-hydroxyphenyl)-4-chloro- methylthiazole derivatives

Based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the effects of substituent on the excited-state intramolecular proton transfer (ESIPT) process and photophysical properties of 2-(2'-hydroxyphenyl)-4-chloromethylthiazole (HCT) are studied. The electron-donating group (CH₃, OH) and electron-withdrawing group (CF₃, CHO) are introduced to analyze the changes of intramolecular H-bond, the frontier molecular orbitals, the absorption/fluorescence spectra, and the energy barrier of ESIPT process. The calculation results indicate that electron-donating group strengthens the intramolecular H-bond in the S₁ state, and leads to an easier ESIPT process. The electron-withdrawing group weakens the corresponding H-bond and makes ESIPT process a little harder. Different substituents also affect the photophysical properties of HCT. The electron-withdrawing group (CF₃, CHO) has a little effect on electronic spectra. The electron-donating group (CH₃, OH) red-shifts both the absorption and fluorescence emission peaks of HCT, respectively, which causes the Stokes shift to increase.     

A role of solvent in vibrational energy relaxation of metalloporphyrins

The formation of a vibrationally excited photoproduct of metalloporphyrins upon (π, π*) excitation and its subsequent vibrational energy relaxation were monitored by picosecond time-resolved resonance Raman spectroscopy. Stokes Raman bands due to a photoproduct of nickel octaethylporphyrin (NiOEP) instantaneously appeared upon the photoexcitation. Their intensities decayed with a time constant of 300 ps, which indicates electronic relaxation from the (d, d) excited state (B_1g) to the ground state (A_1g), being consistent with the results of transient absorption measurements by Holten and coworkers. Anti-Stokes ν₄ and ν₇ bands for vibrationally excited (d, d) state of NiOEP decayed with time constants of 10 and 300 ps. The former is ascribed to vibrational relaxation, while the latter corresponds to the electronic relaxation from the (d, d) excited state to the electronic ground state. While the rise of anti-Stokes ν₄ intensity was instrument-limited, the rise of anti-Stokes ν₇ intensity was delayed by 2.6±0.5 ps, which indicates that intramolecular vibrational energy redistribution has not been completed in subpicosecond time regime. To study a mechanism of intermolecular energy transfer, solvent dependence of the time constants of anti-Stokes kinetics was investigated using various solvents. No significant solvent dependence of the rise and decay constants was observed for NiOEP. For an iron porphyrin, we observed two phases in intermolecular energy transfer. The fast phase was insensitive to solvent and the slow phase depended on solvents. A model of classical thermal diffusion qualitatively reproduced this behavior. For solute-solvent energy transfer process, low-frequency modes of proteins seem to be less important.     

Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer (ESDPT) in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol (HYDRAVH₂) ligand was studied by the density functional theory and time-dependent density functional theory method. The analysis of frontier molecular orbitals, infrared spectra, and non-covalent interactions have cross-validated that the asymmetric structure has an influence on the proton transfer, which makes the proton transfer ability of the two hydrogen protons different. The potential energy surfaces in both S₀ and S₁ states were scanned with varying O-H bond lengths. The results of potential energy surface analysis adequately proved that the HYDRAVH₂ can undergo the ESDPT process in the S₁ state and the double proton transfer process is a stepwise proton transfer mechanism. Our work can pave the way towards the design and synthesis of new molecules.     

Highly localized vibronic wavepackets in large reactive molecules

The ultrafast dynamics of the internal conversion of S₁ azulene and the excited-state intramolecular proton transfer in 2-(2^′-hydroxyphenyl)benzothiazole (HBT) were investigated by pump–probe spectroscopy with tunable pulses as short as 20 fs. In both cases we find very pronounced oscillatory contributions to the transients, which are due to vibrational wavepacket motion in the excited state. The damping times are on the order of 1 ps even for these large reactive molecules in solution at room temperature. For azulene only 2 out of 48 vibrational modes participate and in HBT only 4 out of 69. This high degree of localization of the wavepacket seems to be a general feature, and supports hopes that even for systems of chemical interest coherent control might be possible. Received: 24 January 2000 / Published online: 24 July 2000     

Photoassociation spectra of ultracold ~(85)Rb_2 molecule in 0_u~+ long range state near the 5S_(1/2)+ 5P_(1/2) asymptote

We investigate the high resolution photoassociation spectra of ~(85)Rb_2 molecules in 0~+_u long range state below the(5S_(1/2)+ 5P_(1/2)) asymptote. The ~(85)Rb atomic samples are trapped in a dark magneto–optical trap(MOT) and prepared in the dark state. With the help of trap loss technique, we obtain considerable photoassociation spectroscopy with rovibrational resolution, some of which have never been observed before. The observed spectrum is fitted by a rigid rotation model, and the rotational constants of ultracold ~(85)Rb_2 molecule in long range 0~+_u are obtained for different vibrational states. By applying the Le Roy–Bernstein method, we assign the vibrational quantum numbers and derive C_3 coefficient, which is used to obtain the potential energy curve.     

Effect of conical intersection of benzene on non-adiabatic dynamics

The effect of conical intersection on the excited dynamics of benzene is studied by ab initio theory of electronic structure, which provides an important insight into photophysical and photochemical reactions. Based on the CASSCF(6,6)/6-31+G(d, p) method, the topological structures of conical intersections S₁/S and S₂/S₁ of benzene, as well as the optimal structures of the ground state (S) and excited states (S₁, S₂), are determined. The energy minima of the S₁ state and S₂ state are estimated at 4.608 eV and 6.889 eV, respectively. In addition, the energy values of the conical intersections of S₁/S and S₂/S₁ are predicted to be 5.600 eV and 6.774 eV. According to the topological structures and energy values of the S₂/S₁ and S₁/S conical intersections, the photophysical behavior of benzene excited to the S₂ state and the effects of the S₂/S₁ and S₁/S conical intersections are discussed in detail.     

Pinacyanol as a mode-locking saturable absorber for Rhodamine dye lasers

The absorption recovery time of the ground state of pinacyanol (S₁ → S₀) can be varied through the choice of solvent from 10 ps in methanol to 300 ps in glycerol. We show that mode-locking of Rhodamine 6G and Rhodamine B dye lasers can be achieved using methanol, DMSO or glycerol solutions.     

Assignment of the low frequency vibrations in electron donor-acceptor complexes by ultrafast coherent spectroscopy

Coherent oscillations in a spontaneous fluorescence of electron donor-acceptor (EDA) complexes in solution were observed with three different acceptors: tetracyanoethylene (TCNE), chloranil, and fluoranil with various electron donors. Different oscillatory frequencies were observed for the complexes with different acceptors. The similar frequencies are observed for the complexes with different donors and a common acceptor: 155 cm⁻¹ (period of 215 fs) for the complexes with TCNE, 178 cm⁻¹ (187 fs) for the complexes with chloranil, and 212 cm⁻¹ (157 fs) for the complex with fluoranil. It is concluded, that the out-of-plane vibrational mode of the acceptor, having b_3u symmetry, is responsible for the observed oscillations in all three cases (three acceptors). The time-dependent fluorescence spectrum for the TCNE - HMB complex is reconstructed and the fluorescence peak-shift correlation function is obtained. The ultrafast relaxation of the fluorescence peak position is superimposed with the oscillatory component. The main component of the spectral relaxation of 115 fs is attributed mainly to the intramolecular vibrational energy redistribution process in both donor and acceptor parts of the complex. The oscillatory behavior of the fluorescence peak position demonstrates the modulation of the transition frequency during the vibrations. The modulation of the mean transition moment is observed as well. Thus two mechanisms are observed to be responsible for the oscillations: the modulation of the transition frequency and the modulation of the mean transition moment by the vibrational motion.     

Femtosecond time-resolved difference absorption spectroscopy of all-trans-β-Apo-8′-carotenal

The femtosecond time-resolved difference absorption spectra of all-trans-βApo-8′-carotenal have been recorded and analyzed by the singular-value decomposition (SVD) method followed by global fitting using a sequential model for the excited-state energy relaxation. With this model, we have obtained the excited-state absorption spectra and the lifetimes of the corresponding excited states both in nonpolar solvent n-hexane and polar solvent methanol. Three excited states, namely S3(170fs), S2(2.32ps) and S1(26ps) in n-hexane, and two excited states S2 (190fs) and S1(9.4ps) in methanol have been observed. The excited-state absorption spectra of all-trans-β-Apo-8′-carotenal in methanol display a red shift and broadeness, while the lifetime of S1 state becomes shorter. It is proposed that these effects are related to the presence of a carbonyl functional group that leads to the solvent effect on the excited-state energy level. At the same time, it is shown that the SVD method is a useful tool in resolving the time-resolved absorption spectra.     

Theoretical investigation on the excited‐state intramolecular proton transfer mechanism of 2‐(2′‐benzofuryl)‐3‐hydroxychromone

Spectroscopic studies on excited‐state proton transfer of a new chromophore 2‐(2′‐benzofuryl)‐3‐hydroxychromone (BFHC) have been reported recently. In the present work, based on the time‐dependent density functional theory (TD‐DFT), the excited‐state intramolecular proton transfer (ESIPT) of BFHC is investigated theoretically. The calculated primary bond lengths and angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared (IR) vibrational spectra as well as the calculated hydrogen bonding energies. Further, hydrogen bonding strengthening manifests the tendency of excited state proton transfer. Our calculated results reproduced absorbance and fluorescence emission spectra of experiment, which verifies that the TD‐DFT theory we used is reasonable and effective. The calculated Frontier Molecular Orbitals (MOs) further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O―H coordinate, the potential energy barrier of about 14.5 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 5.4 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

苯乙炔分子电子激发态超快动力学研究

采用飞秒时间分辨质谱技术结合飞秒时间分辨光电子影像技术研究了苯乙炔分子电子激发态超快非绝热弛豫动力学.用235 nm光作为泵浦光,将苯乙炔分子激发到第二激发态S2,用400 nm光探测激发态的演化过程.时间分辨的母体离子的变化曲线用指数和高斯函数卷积得到不同的两个组分,一个是超快衰减组分,时间常数为116 fs,一个是慢速组分,时间常数为106 ps.通过分析时间分辨的光电子影像得到光电子动能分布,结合时间分辨光电子能谱数据发现,时间常数为116 fs的快速组分反映了S2态向S1态的内转换过程.实验还表明S1态通过内转换被布局后向T1态的系间窜跃过程为重要的衰减通道.本工作为苯乙炔分子S2态非绝热弛豫动力学提供了较清晰的物理图像.     

Theoretical verification of intermolecular hydrogen bond induced thermally activated delayed fluorescence in SOBF-Ome

Thermally activated delayed fluorescence (TADF) molecules have attracted great attention as high efficient luminescent materials. Most of TADF molecules possess small energy gap between the first singlet excited state (S₁) and the first triplet excited state (T₁) to favor the up-conversion from T₁ to S₁. In this paper, a new TADF generation mechanism is revealed based on theoretical simulation. By systematic study of the light-emitting properties of SOBF-OMe in both toluene and in aggregation state, we find that the single SOBF-OMe could not realize TADF emission due to large energy gap as well as small up-conversion rates between S₁ and T₁. Through analysis of dimers, we find that dimers with intermolecular hydrogen bond (H-bond) are responsible for the generation of TADF, since smaller energy gap between S₁ and T₁ is found and the emission wavelength is in good agreement with experimental counterpart. The emission properties of SOBF-H are also studied for comparison, which reflect the important role of H-bond. Our theoretical results agree ith experimental results well and confirm the mechanism of H-bond induced TADF.     

Mechanisms of metastable N2 and Kr electron stimulated desorption from N2---Kr double layer and mixed films

Desorption of metastable particles from layered and mixed films, composed of N₂ and Kr, is induced by the impact of 6–50 eV monoenergetic electrons. From yield functions and time-of-flight analysis of the metastable particles emanating from these films, N*₂ and Kr* are identified as the desorbing species. Basic mechanisms responsible for their desorption are discussed. It is suggested that the desorption of Kr* arises from dissociation of transitory [Kr·N₂]* excimers. The desorption of N*₂ can arise from cavity expulsion, intramolecular vibrational energy transfer (with or without prior electronic excitation energy transfer from Kr excitions to N₂) and the dissociation of [Kr·N₂]* excimers.     

超冷铯分子0g-长程态的振转光谱研究

通过超冷铯原子光缔合制备激发态的超冷铯分子. 利用冷原子荧光频率调制技术获得了超冷铯分子第一激发态6S_1/2+6P_3/2离解限0_g^-长程态高分辨振转光谱, 计算得到0_g^- 长程态振动量子数从0到51的不同振动能级的转动常数, 和Daniel研究小组的理论结果符合得很好. 关键词： 超冷铯分子 光缔合 振转光谱     

Vibrational Spectra and Potential Energy Surface for Electronic Ground State of Jet-Cooled Molecule S2O

The vibration states of transition molecule S₂O, including both bending and stretching vibrations, are studied in the framework of dynamical symmetry groups U₁(4)\otimes U₂(4). We get all the vibration spectra of S₂O by fitting 22 spectra data with 10 parameters. The fitting rms of the Hamiltonian is 2.12 cm^-1. With the parameters and Lie algebraic theory, we give the analytical expression of the potential energy surface, which helps us to calculate the dissociation energy and force constants of S₂O in the electronic ground state.     

A detailed DFT/TDDFT study on excited‐state intramolecular hydrogen bonding dynamics and proton‐transfer mechanism of 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol

In this present work, using density functional theory and time‐dependent density functional theory methods, we theoretically study the excited‐state hydrogen bonding dynamics and the excited state intramolecular proton transfer mechanism of a new 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol (2PYP) system. Via exploring the reduced density gradient versus sign(λ₂(r))ρ(r), we affirm that the intramolecular hydrogen bond O1‐H2?N3 is formed in the ground state. Based on photoexcitation, comparing bond lengths, bond angles, and infrared vibrational spectra involved in hydrogen bond, we confirm that the hydrogen bond O1‐H2?N3 of 2PYP should be strengthened in the S₁ state. Analyses about frontier molecular orbitals prove that charge redistribution of 2PYP facilitates excited state intramolecular proton transfer process. Via constructing potential energy curves and searching transition state structure, we clarify the excited state intramolecular proton transfer mechanism of 2PYP in detail, which may make contributions for the applications of such kinds of system in future.     

Rydberg原子nS1/2→(n+1)S1/2双光子激发EIT-AT光谱

主要研究了热原子蒸气池中铯Rydberg原子nS_1/2→(n+1)S_1/2微波耦合的双光子光谱.铯原子基态(6S_1/2)、第一激发态(6P_3/2)、Rydberg态(69S_1/2)形成阶梯型三能级系统,弱探测光作用于基态到激发态6S_1/2→6P_3/2的跃迁,强耦合光则作用于6P_3/2→69S_1/2的Rydberg跃迁形成电磁感应透明(EIT)效应,实现对Rydberg原子的光学探测.频率fMW=11.735 GHz的微波场耦合69S_1/2→70S_1/2的Rydberg跃迁,形成微波双光子光谱.利用EIT-AT分裂光谱研究微波电场强度对双光子光谱的影响.研究表明:在强微波场作用时,EIT-AT分裂与微波场功率成正比,而弱微波场时的EIT-AT分裂与微波场功率成非线性依赖关系,理论计算与实验测量结果相一致.本文的研究对微波电场的精密测量具有一定的指导意义.     

The ESIPT mechanism of dibenzimidazolo diimine sensor: a detailed TDDFT study

Spectroscopic investigations on excited state proton transfer of a new dibenzimidazolo diimine sensor (DDS) were reported by Goswami et al. recently. In our present work, based on the time‐dependent density functional theory (TDDFT), the excited‐state intramolecular proton transfer (ESIPT) mechanism of DDS is studied theoretically. Our calculated results reproduced absorption and fluorescence emission spectra of the previous experiment, which verifies that the TDDFT method we adopted is reasonable and effective. The calculated dominating bond lengths and bond angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared vibrational spectra. Further, hydrogen bonding strengthening manifests the tendency of ESIPT process. The calculated frontier molecular orbitals further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O–H coordinate, the potential energy barrier of about 5.02 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 0.195 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical investigation of excited-state proton transfer (ESPT) for 2,5-bis(2-benzothiazolyl)hydroquinone: single or double?

ABSTRACT

In this work, the single and double proton transfer dynamic process in the first excited state of 2,5-bis(2-benzothiazolyl)hydroquinone (BBTHQ) were theoretically investigated based on the time-dependent density functional theory (TD-DFT) method. The calculations of primary bond lengths, bond angles, IR vibrational spectra and NCI analysis reveal that two intramolecular hydrogen bonds of BBTHQ are strengthened in the first excited state, which provides a driving force for excited state proton transfer. The frontier molecular orbitals (FMOs) indicate the nature of intramolecular charge transfer. In addition, the potential energy surfaces (PESs) of the ground state and first excite state were also constructed to further elucidate the mechanism of intramolecular proton transfer of BBTHQ.     

Scanning the energy dissipation process of energetic materials based on excited state relaxation and vibration–vibration coupling

The energy dissipation mechanism of energetic materials(EMs) is very important for keeping safety. We choose nitrobenzene as a model of EM and employ transient absorption(TA) spectroscopy and time-resolved coherent anti-stokes Raman scattering(CARS) to clarify its energy dissipation mechanism. The TA data confirms that the excited nitrobenzene spends about 16 ps finishing the twist intramolecular charge transfer from benzene to nitro group, and dissipates its energy through the rapid vibration relaxation in the initial excited state. And then the dynamics of vibrational modes(VMs) in the ground state of nitrobenzene, which are located at 682 cm~(-1)(v_1), 854 cm~(-1)(v_2), 1006 cm~(-1)(v_3), and 1023 cm~(-1)(v_4),is scanned by CARS. It exhibits that the excess energy of nitrobenzene on the ground state would further dissipate through intramolecular vibrational redistribution based on the vibrational cooling of vi and v_2 modes, v_1 and v_4 modes, and v_3 and v_4 modes. Moreover, the vibration-vibration coupling depends not only on the energy levels of VMs, but also on the spatial position of chemical bonds relative to the VM.