Control of emission by intermolecular fluorescence resonance energy transfer and intermolecular charge transfer

Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermolecular charge transfer (ICT) is investigated with the quantum-chemistry method using two-dimensional (2D) and three-dimensional (3D) real space analysis methods. The work is based on the experiment of tunable emission from doped 1,3,5-triphenyl-2-pyrazoline (TPP) organic nanoparticles (Peng, A. D.; et al. Adv. Mater. 2005, 17, 2070). First, the excited-state properties of the molecules, which are studied (TPP and DCM) in that experiment, are investigated theoretically. The results of the 2D site representation reveal the electron-hole coherence and delocalization size on the excitation. The results of 3D cube representation analysis reveal the orientation and strength of the transition dipole moments and intramolecular or intermolecular charge transfer. Second, the photochemical quenching mechanism via IFRET is studied (here "resonance" means that the absorption spectrum of TPP overlaps with the fluorescence emission spectrum of DCM in the doping system) by comparing the orbital energies of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of DCM and TPP in absorption and fluorescence. Third, for the DCM-TPP complex, the nonphotochemical quenching mechanism via ICT is investigated. The theoretical results show that the energetically lowest ICT state corresponds to a pure HOMO-LUMO transition, where the densities of the HOMO and LUMO are strictly located on the DCM and TPP moieties, respectively. Thus, the lowest ICT state corresponds to an excitation of an electron from the HOMO of DCM to the LUMO of TPP.     

Planarized Intramolecular Charge Transfer: A Concept for Fluorophores with both Large Stokes Shifts and High Fluorescence Quantum Yields

Fluorophores were successfully used in several areas of chemistry and biochemistry. For many purposes, however, it is necessary that the fluorescence compound features a high fluorescence quantum yield as well as a large Stokes shift. The latter is, for example, achieved by the use of a twisted intramolecular charge‐transfer (TICT) compound, which shows a twisted geometry in the excited state. However, the higher the twisting is, the lower becomes in general the fluorescence quantum yield as the resulting emission from the twisted state is forbidden. In order to escape this dilemma, we propose the model of planarized intramolecular charge‐transfer (PLICT) states. These compounds are completely twisted in the ground states and planar in the excited states. By means of quantum chemical calculations (time‐dependent (TD)‐B3LYP and CC2) and experimental studies, we could demonstrate that 1‐aminoindole and its derivatives form photoinduced PLICT states. They show both very large Stokes shifts ( =9000–13 500 cm^?1, i.e., λ=100–150 nm) and high fluorescence quantum yields. These characteristics and their easy availability starting from the corresponding indoles, make them very attractive for the use as optical switches in various fields of chemistry as well as biological probes.     

基于分子内荧光能量转移的碘聚合膜荧光传感器

合成了基于分子内荧光能量转移的蒽（An）-四苯基卟啉（TPP）双发色团碘荧光探针1．由于An的荧光光谱与TPP的S吸收带具有较好的重叠,供体An与受体TPP之间可以发生有效的分子内荧光能量转移,以An的最大吸收波长作为激发波长时,由于分子内荧光能量转移,受体TPP发出荧光．当碘与探针分子中的识别基团An作用时,导致探针分子的荧光转导基团TPP荧光淬灭．与An、TPP和An＋TPP混合物作敏感材料相比,将探针1固定在PVC膜中制备的敏感膜对碘选择性高、灵敏度好．另外,敏感膜具有很好的重现性、可逆性和稳定性,响应时间小于60S．除Cr2O7^2-和MnO4^-外,食品中常见的无机离子和可能存在的干扰物质不影响碘的测定．在最优条件下,传感器的线性范围为2．04×10^-6-2．36×10^-2mol／L,检出限为3．30×10^-8mol／L．本方法应用于加碘食盐中碘含量的测定,结果满意．     

Wavelength-dependent electron and energy transfer pathways in a side-to-face ruthenium porphyrin/perylene bisimide assembly

A new side-to-face supramolecular array of chromophores, where a pyridyl-substituted perylene bisimide dye axially binds to two ruthenium porphyrin fragments, has been prepared by self-assembly. The array is formulated as DPyPBI[Ru(TPP)(CO)](2), where DPyPBI = N,N'-di(4-pyridyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic acid bisimide and TPP = 5,10,15,20-tetraphenylporphyrin. The photophysical behavior of DPyPBI[Ru(TPP)(CO)](2) has been studied by fast (nanoseconds) and ultrafast (femtoseconds) time-resolved techniques. The observed behavior sharply changes with excitation wavelength, depending on whether the DPyPBI or Ru(TPP)(CO) units are excited. After DPyPBI excitation, the strong fluorescence typical of this unit is completely quenched, and time-resolved spectroscopy reveals the occurrence of photoinduced electron transfer from the ruthenium porphyrin to the perylene bisimide dye (tau = 5.6 ps) followed by charge recombination (tau = 270 ps). Upon excitation of the Ru(TPP)(CO) fragments, on the other hand, ultrafast (tau < 1 ps) intersystem crossing is followed by triplet energy transfer from the ruthenium porphyrin to the perylene bisimide dye (tau = 720 ps). The perylene-based triplet state decays to the ground state on a longer time scale (tau = 9.8 micros). The photophysics of this supramolecular array provides remarkable examples of (i) wavelength-dependent behavior (a small change in excitation wavelength causes a sharp switch from electron to energy transfer) and (ii) intramolecular sensitization (the triplet state of the perylene bisimide, inaccessible in the free dye, is efficiently populated in the array).     

氧鎓盐分子在受限制体系中的电荷转移和发光行为

2,4,6-三芳基氧盐是一类类似于三苯甲烷染料的化合物^[1].其结构中存在两种跃迁矩,分别对应于分子的X-轴方向(2,6-位取代基)和Y-轴方向(4-位取代基).因此,2,4,6-三芳基氧盐的吸收光谱中存在着两个较具特征的吸收峰^[2].     

正己烷中对二烷基氨基苯甲酸的分子内电荷转移双重荧光

在非极性溶剂正己烷中,观察到了系列对二烷基氨基苯甲酸的双重荧光.皮秒激光诱导时间分辨荧光和溶剂极性效应研究证实:该双重荧光系由于激发态分子内电荷转移过程形成的电荷转移态和局部激发态所发射.超快反应动力学研究表明:在非极性溶剂中,对二烷基氨基苯甲酸分子内电荷转移过程是由较低的反应活化能所致.     

Photophysical properties of a rhodium tetraphenylporphyrin-tin corrole dyad. The first example of a through metal-metal bond energy transfer

The luminescence spectroscopy study and the determination of the photophysical parameters for the M-M'-bonded rhodium meso-tetraphenylporphyrin-tin(2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole) complex, (TPP)Rh-Sn(Me6Et2Cor) 1, was investigated. The emission bands as well as the lifetimes (tau(e)) and the quantum yields (Phi(e); at 77 K using 2MeTHF as solvent) were compared with those of (TPP)RhI 2 (TPP = tetraphenylporphyrin) and (Me6Et2Cor)SnCl 3 (Me6Et2Cor = 2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole) which are the two chemical precursors of 1. The energy diagram has been established from the absorption, fluorescence and phosphorescence spectra. The Rh(TPP) and Sn(Me6Et2Cor) chromophores are the energy donor (D) and acceptor (A), respectively. The total absence of fluorescence in 1 (while fluorescence is observed in the tin derivative 3) indicates efficient excited state deactivation, presumably due to heavy atom effect and intramolecular energy transfer (ET). The large decreases in tau(P) and Phi(P) of the Rh(TPP) chromophore going from 2 to 1 indicate a significant intramolecular ET in the triplet states of 1 with an estimated rate ranging between 10(6) and 10(8) s(-1). Based on the comparison of transfer rates with other related dyads that exhibit similar D-A separations and no M-M' bond, and for which slower through space ET processes (10(2)-10(3) s(-1)) operate, a through M-M' bond ET has been unambiguously assigned to 1.     

A theoretical investigation on the excited state intramolecular single or double proton transfer mechanism of a salicyladazine system

Given the tremendous potential applications of excited state intramolecular proton transfer (ESIPT) systems, ESIPT molecules have received widespread attention. In this work, based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we theoretically study the excited state dynamical behaviors of salicyladazine (SA) molecules. Our simulated results show that the double intramolecular hydrogen bonds of SA are strengthened in the S₁ state via exploring bond distances, bond angles, and infrared (IR) vibrational spectra. Exploring the frontier molecular orbitals (MOs), we confirm that charge redistributions indeed have effects on excited state dynamical behaviors. The increased electronic densities on N atoms and the decreased electronic densities on O atoms imply that charge redistribution may trigger the ESPT process. Analyzing the constructed S₀‐state and S₁‐state potential energy surfaces (PESs), we confirm that only the excited state single proton transfer reaction can occur although SA possesses two intramolecular hydrogen bonds. In this work, we clarify the specific ESIPT mechanism, which may facilitate developing novel applications based on the SA system in future.     

A fluorescent sensing membrane for iodine based on intramolecular excitation energy transfer of anthryl appended porphyrin

A single anthryl appended meso-tetraphenylporphyrin (TPP) dyad has been synthesized and applied in fluorescence sensing of iodine based on the intramolecular excitation energy transfer. The molecular recognition of the sensor is based on the interaction of iodine with inner anthracene moiety of the dyad, while the signal reporter for the recognition process is the TPP fluorescence quenching. Because the emission spectrum of anthracene is largely overlapped with the Soret band absorption of TPP, intramolecular excitation energy transfer interaction occurs between the donor, anthracene and acceptor, TPP. This energy transfer leads to TPP fluorescence emission by excitation of anthracene. The sensor was constructed by immobilizing the dyad in a plasticized poly(vinyl chloride) (PVC) membrane. The sensing membrane shows higher sensitivity compared to the sensors by using anthracene, TPP, or a mixture of anthracene and TPP as sensing materials. Under the optimum conditions, iodine in a sample solution can be determined from 2.04 to 23.6 mmol·L⁻¹ with a detection limit of 33 nmol·L⁻¹. The sensing membrane shows satisfactory response characteristics including good reproducibility, reversibility and stability, as well as the short response time of less than 60 s. Except for Cr₂O₇²⁻ and MnO₄⁻, other common metal ions and anions in foodstuff do not interfere with iodine determination. The proposed method was applied in the determination of iodine in table salt samples. The results agree well with those obtained by other methods. Supported by the National Outstanding Youth Science Foundation of China (Grant No. 20525518), the National Natural Science Foundation of China (Grant No. 20775005), and the National Natural Science Foundation of Hunan province (Grant No. JJ076021)     

The electronic origin of the dual fluorescence in donor-acceptor substituted benzene derivatives

The origin of the dual fluorescence of DMABN (dimethylaminobenzonitrile) and other benzene derivatives is explained by a charge transfer model based on the properties of the benzene anion radical. It is shown that, in general, three low-lying electronically excited states are expected for these molecules, two of which are of charge transfer (CT) character, whereas the third is a locally excited (LE) state. Dual fluorescence may arise from any two of these states, as each has a different geometry at which it attains a minimum. The Jahn-Teller induced distortion of the benzene anion radical ground state helps to classify the CT states as having quinoid (Q) and antiquinoid (AQ) forms. The intramolecular charge transfer (ICT) state is formed by the transfer of an electron from a covalently linked donor group to an anti-bonding orbital of the pi-electron system of benzene. The change in charge distribution of the molecule in the CT states leads to the most significant geometry change undergone by the molecule which is the distortion of the benzene ring to a Q or AQ structure. As the dipole moment is larger in the perpendicular geometry than in the planar one, this geometry is preferred in polar solvents, supporting the twisted intramolecular charge transfer (TICT) model. However, in many cases the planar conformation of CT excited states is lower in energy than that of the LE state, and dual fluorescence can be observed also from planar structures.     

生色团连接的苯骈三氮唑衍生物的激发态分子内质子转移

用从头算和密度泛函理论研究了对硝基二苯乙烯作为生色团连接的2-(2-羟基-苯基)-苯骈三氮唑的衍生物2-羟基-5-[对硝基-二苯乙烯基-氧亚甲基]-苯基-(2H-苯骈三氮唑)(C1)和4′-硝基-3,4-二[2-羟基-(2H-苯骈三氮唑)-苄氧基]-二苯乙烯(C2)发生激发态分子内质子转移(ESIPT)的可能性.系统研究了C1和C2发生ESIPT的互变异构体的基态与激发态的性质变化,包括相关的键长、键角等结构参数,Mulliken电荷和偶极矩,前线轨道以及势能曲线.计算结果表明,对于C1来讲,酮式(keto)的基态(K)不存在稳定结构,因此发生基态分子内质子转移(GSIPT)可能性很小.酮式的激发态(K*)的氢键强度要远强于烯醇式(enol)的激发态(E*)的氢键强度.分子在光致激发后,质子供体所带负电荷减小而质子受体所带负电荷增加.在K*,HOMO→LUMO的电子跃迁导致电子密度从"酚环"向质子化杂环转移.E*→K*跃迁只需要克服较小的能垒(约41 kJ.mol-1).计算结果表明C1发生ESIPT的可能性很大.C2由于具有高能量,其具有基态的单质子转移特征的异构体EK(同时含烯醇E与酮K结构)、具有基态的双质子转移特征的异构体2K(含有双酮结构),以及具有双酮结构特征的激发态2K*均无法获得它们的稳定结构,因此,基态分子内单或双质子转移和激发态分子内双重质子转移发生的可能性极小.然而,由于双烯醇式的激发态(2E*)和EK的激发态(EK*)存在稳定结构,且2E*→EK*跃迁具有低能垒,因此C2有可能发生激发态分子内单重质子转移.本文进一步计算了两个分子的紫外-可见吸收光谱与荧光发射光谱,获得了具有较大斯托克位移的ESIPT的荧光发射峰.     

Time resolved spectroscopy of 2-(dimethylamine)fluorene. Solvent effects and photophysical behavior

The effect of different solvents on the fluorescent properties of 2-(dimethylamine)fluorene (DAF) were studied. In aprotic solvents we detected a strongly emissive intramolecular charge transfer (ICT) state that decayed by intersystem crossing to triplet. In proton-accepting solvents DAF exhibits in the excited state an intramolecular proton transfer. An ionized species is postulated, which simultaneously twists to a rotated conformation in the excited state. Thus, the specific solvent interactions supplement but do not replace the twist mechanism and accompany the charge transfer accepted as the prerequisite for twisted intramolecular charged transfer (TICT) state formation.     

4-[2-(9,10-二氰基蒽)乙烯基] -N,N-二甲基苯胺的三荧光发射

从分子水平进行电子转移,电荷分离的研究是十分重要的,它不仅是自然界光合作用的基本过程,也是现代高新技术中的一个关键问题。近年来分子内含电子给体与电子受体的D-A化合物一直引起人们的极大兴趣。这些化合物能发生光致分子内电子转移,使其激发态分子的偶极矩远大于基态,它们的发射光谱对介质的粘度及极性十分敏感,随分子结构的变化而变化,展现出特有的光电性质,可利用作为非线性光学材料、光电转换材料以及荧光探针等。     

Insights into the excited state intramolecular proton transfer process and mechanism of the novel 3‐hydroxythioflavone system: A theoretical study

It is well known that the molecular excited state dynamical process plays important roles in designing and developing novel applications. In this work, based on density functional theory and time‐dependent density functional theory methods, we theoretically explored a novel 3‐hydroxythioflavone (3HTF). Through calculating the electrostatic potential surface of the 3HTF structure, we confirm the formation of intramolecular hydrogen bonding O2‐H3···O4. Our theoretically obtained dominating bond lengths and bond angles involved in hydrogen bonds demonstrate that the intramolecular hydrogen bonds should be strengthened in the S₁ state. Coupling with the simulated infrared vibrational spectra, we further verify the enhanced hydrogen bonding O2‐H3···O4 in the S₁ state. Upon photoexcitation, we found that the charge transfer characteristics around hydrogen bonding moieties play important roles in facilitating the excited state intramolecular proton transfer (ESIPT) process. Via constructing potential energy curves in both S₀ and S₁ states, we confirm the almost nonbarrier ESIPT reaction should be an ultrafast process that further explains the previous experimental phenomenon. At last, we search the S₁‐state transition state (TS) structure along with ESIPT path, based on which we simulate the intrinsic reaction coordinate path that further confirms the ESIPT mechanism. We hope that our theoretical work could guide novel applications based on the 3HTF system in future.     

Perylene Bisimide and Naphthyl‐Based Molecular Dyads: Hydrogen Bonds Driving Co‐planarization and Anomalous Temperature‐Response Fluorescence

The origin of the positive temperature effect in fluorescence emission of a newly designed perylene bisimide (PBI) derivative with two naphthyl units containing ortho‐methoxy group (NM) at its bay positions (PBI‐2NM) was elucidated. A key point is the finding of a weak hydrogen bond (<5.0 kcal mol^?1) between the methoxy group of the NM unit and a nearby hydrogen atom of the PBI core. It is the bonding that drives co‐planarization of the different aromatic units, resulting in delocalization of the π‐electrons of the compound as synthesized, inducing fluorescence quenching via intramolecular charge transfer (ICT). With increasing temperature, the co‐planar structure could be distorted in part, resulting in a decreased degree of ICT, and hence leading to enhanced fluorescence emission. The unique positive temperature effect in emission induced by H‐bond‐driven co‐planarization may pave a new avenue in designing functional molecular systems complementary to conventional methods.     

气相水杨酸激发态分子内质子转移特性分析

采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)研究了气相水杨酸(SA)分子的激发态氢键动力学过程。通过对水杨酸分子基态和激发态结构的优化,以及对其稳态吸收和发射光谱特性、前线分子轨道、红外振动光谱和势能曲线的计算分析,阐明水杨酸分子内质子转移可在激发态下自发地发生,导致其激发态可存在烯醇式和酮式两种异构体结构,并揭示了这种质子转移源于分子内电荷转移的激发态氢键的加强机制。     

Solid‐State Emission of the Anthracene‐o‐Carborane Dyad from the Twisted‐Intramolecular Charge Transfer in the Crystalline State

The emission process of the o ‐carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene‐o ‐carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation‐ and crystallization‐induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual‐emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o ‐carborane would allow for rotation even in the condensed state.     

Photophysical studies of dipolar organic dyes that feature a 1,3-cyclohexadiene conjugated linkage: the implication of a twisted intramolecular charge-transfer state on the efficiency of dye-sensitized solar cells

A detailed study of the synthesis and photophysical properties of a new series of dipolar organic photosensitizers that feature a 1,3‐cyclohexadiene moiety integrated into the π‐conjugated structural backbone has been carried out. Dye‐sensitized solar cells (DSSCs) based on these structurally simple dyes have shown appreciable photo‐to‐electrical energy conversion efficiency, with the highest one up to 4.03 %. Solvent‐dependent fluorescence studies along with the observation of dual emission on dye 4 b and single emission on dyes 4 a and 32 suggest that dye 4 b possesses a highly polar emissive excited state located at a lower‐energy position than at the normal emissive excited state. A detailed photophysical investigation in conjunction with computational studies confirmed the twisted intramolecular charge‐transfer (TICT) state to be the lowest emissive excited state for dye 4 b in polar solvents. The relaxation from higher‐charge‐injection excited states to the lowest TICT state renders the back‐electron transfer process a forbidden one and significantly retards the charge recombination to boost the photocurrent. The electrochemical impedance under illumination and transient photovoltage decay studies showed smaller charge resistance and longer electron lifetime in 4 b ‐based DSSC compared to the DSSCs with reference dyes 4 a and 32 , which further illustrates the positive influence of the TICT state on the performance of DSSCs.     

First Principles Probing of Photo-Generated Intermolecular Charge Transfer State in Conjugated Oligomers

We perform density functional theory calculations to investigate the polaron pair (charge transfer state) photo-generation in donor-acceptor oligomer methyl-capped (4, 7-benzo[2, 1, 3]thiadiazole-2, 6-(4, 4-bis (2-ethylhexyl)-4H-cyclopenta[1, 2-b; 3, 4-b']dithiophene-4, 7-benzo[2, 1, 3]thiadiazole)(CPDTBT).Results show that effective photo-generation of charge transfer state can happen in CPDTBT dimer when the group 4, 7-benzo[2, 1, 3]thiadiazole (BT) in one monomer deviates against the conjugated plane (onset torsion angle is about 20°).The lower excitation energy (530 nm) can only generate the intramolecular excitonic state, while the higher excitation energy (370 nm) can generate the intermolecular charge transfer state, in good agreement with the experiment.Moreover, the mechanism of charge separation in CPDTBT oligomers is discussed.