Exciplex Emission from a Boron Dipyrromethene (Bodipy) Dye Equipped with a Dicyanovinyl Appendage

The photophysical properties of a prototypic donor–acceptor dyad, featuring a conventional boron dipyrromethene (Bodipy) dye linked to a dicyanovinyl unit through a meso‐phenylene ring, have been recorded in weakly polar solvents. The absorption spectrum remains unperturbed relative to that of the parent Bodipy dye but the fluorescence is extensively quenched. At room temperature, the emission spectrum comprises roughly equal contributions from the regular π, π* excited‐singlet state and from an exciplex formed by partial charge transfer from Bodipy to the dicyanovinyl residue. This mixture moves progressively in favor of the locally excited π, π* state on cooling and the exciplex is no longer seen in frozen media; the overall emission quantum yield changes dramatically near the freezing point of the solvent. The exciplex, which has a lifetime of approximately 1 ns at room temperature, can also be seen by transient absorption spectroscopy, in which it decays to form the locally excited triplet state. Under applied pressure (P<170 MPa), formation of the exciplex is somewhat hindered by restricted rotation around the semirigid linkage and again the emission profile shifts in favor of the π, π* excited state. At higher pressure (170<P<550 MPa), the molecule undergoes reversible distortion that has a small effect on the yield of π, π* emission but severely quenches exciplex fluorescence. In the limiting case, this high‐pressure effect decreases the molar volume of the solute by approximately 25 cm³ and opens a new channel for nonradiative deactivation of the excited‐state manifold.     

Lifetimes of partial charge transfer exciplexes of 9-cyanophenanthrene and 9-cyanoanthracene

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.     

Dynamic study of excited state hydrogen-bonded complexes of harmane in cyclohexane-toluene mixtures

Photoinduced proton transfer reactions of harmane or 1-methyl-9H-pyrido[3,4-b]indole (HN) in the presence of the proton donor hexafluoroisopropanol (HFIP) in cyclohexane-toluene mixtures (CY-TL; 10% vol/vol of TL) have been studied. Three excited state species have been identified: a 1:2 hydrogen-bonded proton transfer complex (PTC), between the pyridinic nitrogen of the substrate and the proton donor, a hydrogen-bonded cation-like exciplex (CL*) with a stoichiometry of at least 1:3 and a zwitterionic exciplex (Z*). Time-resolved fluorescence measurements evidence that upon excitation of ground state PTC, an excited state equilibrium is established between PTC* and the cationlike exciplex, CL*, lambdaem approximately/= 390 nm. This excited state reaction is assisted by another proton donor molecule. Further reaction of CL* with an additional HFIP molecule produces the zwitterionic species, Z*, lambda(em) approximately/= 500 nm. From the analysis of the multiexponential decays, measured at different emission wavelengths and as a function of HFIP concentration, the mechanism of these excited state reactions has been established. Thus, three rate constants and three reciprocal lifetimes have been determined. The simultaneous study of 1,9-dimethyl-9H-pyrido[3,4-b]indole (MHN) under the same experimental conditions has helped to understand the excited state kinetics of these processes.     

Magnetic field effect on fluorescence in a mixture of N-ethylcarbazole and dimethyl terephthalate in a polymer film in the presence of electric fields

Magnetic field effects on the fluorescence spectrum and on the electrofluorescence spectrum (plots of the electric field-induced change in fluorescence intensity as a function of wavelength) have been examined in electron donor and acceptor pairs of N-ethylcarbazole (ECZ) and dimethyl terephthalate (DMTP) in polymer films at different ratios of donor/acceptor concentration. In the mixture having a high concentration of ECZ, electric field-induced quenching of the exciplex fluorescence originating from the photoinduced electron transfer becomes less efficient in the presence of a magnetic field. In the mixture having a low concentration of ECZ, on the other hand, no magnetic field effect was observed in the electrofluorescence spectrum, indicating that the hole carrier plays an important role in synergy effects of magnetic and electric field effects on exciplex fluorescence. In the absence of the applied electric field, the magnetic field does not affect either exciplex fluorescence with a peak at 450 nm or LE fluorescence emitted from the locally excited state of ECZ but enhances the broad emission with a peak at approximately 380 nm, probably assigned to the fluorescence of another type of exciplex between ECZ and DMTP. Thus, two kinds of magnetic field effects on fluorescence have been observed in a mixture of ECZ and DMTP in a polymer film.     

Intramolecular excited-state proton-transfer studies on flavones in different environments

The absorption and fluorescence spectra of some biologically active flavones have been studied as a function of the acidity (pH/H0) of the solution. Dissociation constants have been determined for the ground and first excited singlet states. The results are compared with those obtained from Forster-Weller calculations. The acidity constants obtained by fluorimetric titration method are in complete agreement (in most of the systems) with ground state data indicating a excited state deactivation prior to prototropic equilibration. Compared to umbelliferones, flavones are only weakly fluorescent in alkaline solution. This behaviour is explained by the small energy difference between the singlet excited state and triplet excited state giving rise to more efficient intersystem crossing. Most of the flavones studied here undergo adiabatic photodissociation in the singlet excited state indicating the formation of an exciplex or a phototautomer.     

EXCIPLEX FORMATION OF A POLYESTER WITH A TEREPHTHALATE MAIN CHAIN HAVING A PENDANT N, N-DIMETHYLAMINO GROUP

The fluorescence behaviours of a new polymer poly [oxy-2 (4-N, N-dimethylaminobenzyl) propane-1 , 3-diyloxy-terephthaloyl] (Ⅰ) in solution were studied. Inter-and intra -molecular exciplex is formed between the singlet excited state of N, N- dimethylanilino group (abbreviated to DMA) and the ground state of terephthalate group (abbreviated to TP). The intensity ratio of the long wavelength exciplex fluorescence to the short wavelength emission of DMA group is used as an index for the inter-and intra-molecular interaction of chromophores. The results are compared with model polymer polyoxy-2-(4-N ,N-dimethylaminobenzyl) propanc-1 , 3-diyloxyadipoyl (Ⅱ), model monomer compound 4-N, N-dimethylaminobenzylmethyl terephthalate(Ⅲ) and diethyl 2-( 4-N, N-dimethylaminobenzyl) malonate (Ⅳ). Polymer association derived by electron donor and acceptor interaction (EDA) is further verified as a key role in the interpolymer exciplex formation . The fluorescence decay time of (Ⅰ) and monomer (Ⅲ) are measured in solutions.     

Quenching of exciplex and charge-transfer complex fluorescence of poly(N-vinylcarbazole) – benzanthrone system

When benzanthrone (Bt), a weak electron acceptor, is doped into poly(N-vinylcarbazole) (PVCz) solution or film, an excited carbazole chromophore (D*) interacts with Bt to form a new exciplex state, which gives a broad fluorescence band (λ_max = 440 nm) in solution and a new state, which gives broad fluorescence (λ_max = 550 nm) in the film. In order to elucidate the origin of these new states, we have studied the results of experiments for absorption, concentration dependence of the excimer and exciplex fluorescence quenching, both in solution and in the film, and electric field-induced fluorescence quenching in the film. Taking into account that (i) the new state formation in the PVCz film containing small amounts of Bt enhances the photocurrent in the absorption region, where the photon energy is insufficient to excite polymer molecules directly into the conduction state, (ii) the 550 nm fluorescence of the PVCz - Bt system in film is only partly quenched by electric field, (iii) the appearance of structureless tail in the fluorescence excitation spectrum, the charge transfer interaction model of the PVCz - Bt system in film is proposed.     

The magnetic field effect in the presence of the electric field on fluorescence of a methylene-linked compound of pyrene and n,n-dimethylaniline doped in a polymer film

Recent experimental data by Mizoguchi and Ohta are analyzed under the assumption of fast equilibrium between the locally excited (LE), radical-ion-pair (RIP), and exciplex states. In the absence of the equilibrium, no magnetic field effect on the LE fluorescence would be observed. Owing to the equilibrium, the relative electric-field-induced changes of the quantum yields of the LE and exciplex emissions as functions of the magnetic field are linearly connected to each other. The electric field shifts the equilibrium from the LE state toward the RIP state. Predictions are made for the magnetic field effect on the fluorescence kinetics.     

Photopolymerization of N-substituted phenyl maleimide initiated with N,N-dimethyl-4-toluidine

Several N-phenyl maleimides with different p-substituents have been synthesized from the maleic anhydride and relevant aromatic amine. In the presence of N,N-dimethyl-4-toluidine (DMT), the N-substituted phenyl maleimide (4-XPhMIs) could be polymerized under UV irradiation. It was observed that a new absorption appeared on the UV-Vis spectrum of the mixture solution of 4-XPhMI and DMT, which reveals the formation of charge-transfer complex in the ground state. It was found that the fluorescence of DMT was quenched by 4-XPhMI and the quenching constant of 4-XPhMI, obtained from the Stern–Volmer plot, increases with the electron-deficiency of ethylene double bond of 4-XPhMI. The dynamic quenching of the fluorescence of DMT by 4-XPhMI suggests the formation of exciplex in excited state between DMT and 4-XPhMI. The radicals formed in the systems have been detected by spin-trapping techniques and electron paramagnetic resonance (EPR) spectrometer. Based on all of these results, it has been proposed that the initiation process of the polymerization involves the formation of exciplex and the initial radicals were produced by proton-transfer in the exciplex from DMT to 4-XPhMI. © 1997 John Wiley & Sons, Inc.     

EXCIPLEX STUDIES–V. ELECTRON EJECTION FROM INDOLE AND METHYL-INDOLE DERIVATIVES*

Abstract— Electron-scavenging experiments with N₂O as scavenger demonstrate at least two electron-producing reactions of the excited singlet states of the exciplex species formed by indole or 1 -methyl-indole with water. Most electrons reacting with N₂O result from collision of the scavenger with a metastable state formed from the initial exciplex state but finite electron yields from indole and 1-methyl-indole at limiting scavenger concentrations suggest that the intermediate states also eject electrons directly into the solvent. The formation of the first metastable state from the fluorescent exciplex state has an activation energy, E_M, estimated to be about 13 kcal/mole for both indole and 1 -methyl-indole water exciplexes. The E_M values for 1-methyl-indole from fluorescence and electron yields are the same, Indicating that at neutral and alkaline pH fluorescence quenching and electron extraction are both being controlled by the formation of the first metastable intermediate. Observed electron yields from indole-water and indole-methanol exciplexes are less than predicted using fluorescence data, although E_M values of 1 kcal/mole are obtained for the indole-methanol exciplex by both methods. At pH 12·0 and 28°C the total electron yields for indole-water and 1 -methyl-indole-water exciplexes are 0·30 and 0·25, respectively. The residual yields attributed to outright formation of hydrated electrons from the initial exciplex excited stateare 0·11 and 0·05, respectively. Electron yields from the indole-water exciplex are strongly pH dependent only near pH 1 where the fluorescence yields as well as the electron yields decrease rapidly with increasing acidity. The 1-methyl-indole-water exciplex shows an additional pH dependence which is first-order in hydrogen-ion activity and has an effective pK_a of about 11·5. Comparable yields for indole and 1-methyl-indole are found only above pH 12. High electron yields are found with indole in the exciplex-forming solvent dioxane and in the non-exciplex forming solvent cyclohexane. For the latter system electrons are probably derived only from the lowest excited state of indole on collision with N₂O.     

咔唑及其衍生物对 9-氰基蒽的荧光猝灭机理的研究

本文研究咔唑及其衍生物对9-氰基蒽(9CNA)的荧光猝灭机理。结果表明, 猝灭过程有以下三种方式:(1)一系列N-烷基咔唑及1,4-二咔唑丁烷、反式1,2-二咔唑环丁烷、N-苄基咔唑等对9CNA的荧光猝灭是通过形成激基复合物。(2)咔唑对9CNA的荧光猝灭是通过形成氢键。(3)1,3-二咔唑丙烷及N-痖烯基咔唑对9CNA的荧光猝灭是属于一般碰撞猝灭过程。以上所有猝灭过程主要都是来自电荷转移相互作用。另外, 还讨论了空间位阻对形成激基复合物的影响。并由稳态和动态荧光实验结果,应用Ware关于激基复合物的形成和解离的动力学公式计算出一系列光物理速率常数。     

Intramolecular exciplex and intermolecular excimer formation of 1,8-naphthalimide-linker-phenothiazine dyads

Steady-state fluorescence spectra were measured for 1,8-naphthahlimide-linker-phenothiazine dyads (NI-L-PTZ, where L = octamethylenyl ((CH2)8) and 3,6,9-trioxaundecyl ((CH2CH2O)3C2H4)), NI-C8-PTZ and NI-O-PTZ, as well as the NI derivatives substituted on the nitrogen atom with various linker groups without PTZ as the reference NI molecule in n-hexane. Normal fluorescence peaks were observed at 367-369 nm in all NI molecules together with a broader emission around 470 nm, which is assigned to the excimer emission between the NI in the singlet excited state (1NI*) and the NI moiety of another NI molecule (1[NI/NI]*). In addition, a broad peak around 600 nm was observed only for NI-L-PTZ, which is assigned to an intramolecular exciplex emission between donor (PTZ) and acceptor (NI) moieties in the excited singlet state, 1[NI-L-NI]*. The formation of an intramolecular exciplex corresponds to the existence of a conformer with a weak face-to-face interaction between the NI and PTZ moieties in the excited state because of the long and flexible linkers. The excited-state dynamics of the NI molecules in n-hexane were established by means of time-resolved fluorescence spectroscopy.     

N,N′-双-β-萘甲基哌嗪分子内激发态复合物形成的混合溶剂效应

本文测定了N,N′-双-β-萘甲基哌嗪(DMNP)在苯与乙腈混合溶剂中的荧光光谱。在乙腈含量<5(mol·dm^-3)时,乙腈猝灭DMNP苯溶液的荧光符合Stern-Volmer方程,表明极性溶剂分子乙腈与DMNP分子内激基复合物存在着相互作用。随着乙腈含量的增加,DMNP分子内激基复合物(exci-plex)荧光的猝灭与红移以及分子内激基缔合物(excimer)的逐步形成则仅与体系的极性有关。文中还讨论了DMNP激发态复合物形成的机理。     

FLUORESCENCE QUENCHING OF INDOLES BY N,N-DIMETHYLFORMAMIDE

Abstract. The quenching of the excited state of indoles by small polar molecules is believed to occur via the formation of excited state complexes. The rate of excited state complex formation of indoles with dimethylformamide is shown to be a function of the group appended to the indole ring at the 3- or 5-position. Substituents which can strengthen the charge transfer character of the exciplex are observed to enhance the rate of fluorescence quenching. In contrast to other amides, dimethylformamide appears to be a moderately good quencher of indole fluorescence, and may be a useful probe for proteins.     

Exciplex intermediates in photoinduced electron transfer of porphyrin-fullerene dyads

The photoinduced electron transfer in differently linked zinc porphyrin-fullerene dyads and their free-base porphyrin analogues was studied in polar and nonpolar solvents with femto- to nanosecond absorption and emission spectroscopies. A new intermediate state, different from the locally excited (LE) chromophores and the complete charge-separated (CCS) state, was observed. It was identified as an exciplex. The exciplex preceded the CCS state in polar benzonitrile and the excited singlet state of fullerene in nonpolar toluene. The behavior of the dyads was modeled by using a common kinetic scheme involving equilibria between the exciplex and LE chromophores. The scheme is suitable for all the studied porphyrin-fullerene compounds. The rates of reaction steps depended on the type of linkage between the moieties. The scheme and Marcus theory were applied to calculate electronic couplings for sequential reactions, and consistent results were obtained.     

聚酯聚醚嵌段共聚物与1,4-二咔唑环丁烷在激发态及基态的相互作用

<正> 聚酯聚醚嵌段共聚物具有十分良好的物理和机械性能,能加工成橡胶、纤维、塑料胶粘剂等,近年来也有作为医用高分子材料的报道。所以对聚酯聚醚嵌段共聚物的研究十分活跃。然而,有关其光物理性质的研究还很少见。本文报道了混合聚醚-聚酯嵌段共聚物(MPEE)与反式1,4-二咔唑环丁烷(1,4-DCC)在激发态和基态下的相互络合作用。结果表明,MPEE可以猝灭1,4-DCC的荧光,同时形成激基复合物。在分散     

带不同取代基氧鎓盐化合物电子光谱的研究

本工作对几种不同取代氧鎓盐化合物的荧光光谱和荧光猝灭进行研究,发现在液氮温度下,激发的氧鎓盐分子能和作为猝灭剂的电子给体分子在光谱的长波方向发出激基复合物的荧光。在基态条件下,氧鎓盐能和电子给体生成稳定的电荷转移络合物(CTC)。在冻结的条件下,由于激发的络合物不容易解离或减少了某些无辐射衰减的途径,有可能明显地观察到激发的CTC的发光。     

芳香叔胺引发丙烯腈光聚合的引发机理

芳香叔胺引发丙烯腈(AN)光聚合是通过形成激基复合物(exciPlex)进行的。紫外光谱和荧光光谱表明,芳香叔胺在基态可以和AN形成电荷转移复合物(CTC),而在激发态可和AN形成exciplc(称定域激发)。CTC经光照亦可激发(称CTC激发)。 定域激发引起光聚合速率为CH_3C_6H_4N(CH_3)_2>C_6H_5N(CH_3)_2>HOCH_2·C_6H_4N(CH_3)_2>CH_3C_6H_4N(CH_2CH_2OH)_2,与芳胺荧光被AN淬灭的Stern-Vo-lmer常数顺序一致。CTC激发引起的光聚合顺序为:CH_3C_6H_4N(CH_3)_2>CH_3C_6H_4N(CH_2CH_2OH)_2>HOCH_2C_6H_4N(CH_3)_2>C_6H_5N(CH_3)_2,与芳胺上取代基推电子能力一致。端基分析表明聚合物有芳胺端基。     

Exciplex formation in aromatic copolymers and photoinduced electron transfer from exciplex to acceptor

The exciplex formation in 9-vinylphenanthrene-p-N,N-dimethylaminostyrene copolymers, its characteristics, and the electron transfer process in polar solvents were studied. The copolymer exhibited a more intense intramolecular exciplex fluorescence than the low-molecular-weight model system, phenanthrene-N,N-dimethylaniline, in which the intermolecular exciplex formation occurred. Intensities of the exciplex fluorescence, which were unchanged regardless of the copolymer composition, led us to speculate that the efficient energy migration takes place from an excited phenanthrene unit to an exciplex forming site on the polymer chain. The electron transfer in the copolymer-p-dicyanobenzene system was studied in polar media. The formation of p-dicyanobenzene anion radical was measured by flash photolysis and electron spin resonance (ESR). p-Dicyanobenzene anion radical was generated by the electron transfer process via exciplex and the direct electron transfer process from the excited phenanthrene unit in the copolymer.