Radiative and radationless decay of exciplexes of 1,12-benzperylene

Many molecules in their excited states react with other species having suitable electron donor or acceptor properties to form complexes (exciplexes) stable only in the excited state. This letter reports a study of the modes of decay of a series of exciplexes in which the donor molecule (1,2-benzperylene) formed exciplexes with a series of dimethylaniline derivatives. By measuring the flourescence and intersystem crossing quantum yields, together with the fluorescence lifetimes of the exciplexes, it was possible to derive the rate constants for fluorescence, radationless decay to the ground state, and intersystem crossing. The first two decay processes were found to show a marked sensitivity to the exciplex energy, while the intersystem crossing rate constant was affected only by the presence of heavy atoms.     

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.     

Laser photolysis study of the spectral-kinetic characteristics of short-lived triplet exciplexes and the mechanism of atomic hydrogen transfer in the course of 2,6-diphenyl-1,4-benzoquinone triplet quenching with aromatic amines

The spectral-kinetic characteristics of short-lived triplet exciplexes arising in the quenching of 2,6-diphenyl-1,4-benzoquinone triplet with aromatic amines: N,N,N′,N′-tetramethyl-p-phenylenediamine, triphenylamine and diphenylamine have been studied by means of the nanosecond laser photolysis technique. The absorption spectra of triplet exciplexes exhibit distinct maxima characteristic of the absorption spectra of corresponding amine radical cations. The state with complete charge transfer gives the basic contribution to the exciplex structures. A detailed analysis is presented of the kinetic and thermodynamic deactivation characteristics of triplet exciplexes in low-polar solvents.     

Bonded exciplex formation: electronic and stereoelectronic effects

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes. Examples of competing reactions to form both conventional and bonded exciplexes are presented, which illustrate the delicate balance between these two processes when their reaction energetics are similar. Experimental and computational evidence is provided for the formation of a bonded exciplex in the reaction of the singlet excited state of 2,6,9,10-tetracyanoanthracene (TCA) with an oxygen-substituted donor, dioxane, thus expanding the scope of bonded exciplexes.     

Exciplex fluorescence emission from simple organic intramolecular constructs in non-polar and highly polar media as model systems for DNA-assembled exciplex detectors

Organic intramolecular exciplexes, N-(4-dimethylaminobenzyl)-N-(1-pyrenemethyl)amine (1) and N'-4-dimethylaminonaphthyl-N-(1-pyrenemethyl)amine (2), were used as model systems to reveal major factors affecting their exciplex fluorescence, and thus lay the basis for developing emissive target-assembled exciplexes for DNA-mounted systems in solution. These models with an aromatic pyrenyl hydrocarbon moiety as an electron acceptor appropriately connected to an aromatic dimethylamino electron donor component (N,N-dimethylaminophenyl or N,N-dimethylaminonaphthyl) showed strong intramolecular exciplex emission in both non-polar and highly polar solvents. The effect of dielectric constant on the maximum wavelength for exciplex emission was studied, and emission was observed for 1 and 2 over the full range of solvent from non-polar hydrocarbons up to N-methylformamide with a dielectric constant of 182. Quantum yields were determined for these intramolecular exciplexes in a range of solvents relative to that for Hoechst 33,258. Conformational analysis of 1 was performed both computationally and via qualitative 2D NMR using (1)H-NOESY experiments. The results obtained indicated the contribution of pre-folded conformation(s) to the ground state of 1 conducive to exciplex emission. This research provides the initial background for design of self-assembled, DNA-mounted exciplexes and underpins further development of exciplex-based hybridisation bioassays.     

The structure and decay dynamics of exciplexes derived from dibenzoyl-methanatoboron difluoride and alkylbenzenes in cyclohexane

Dibenzomethanatoboron difluoride (DBMBF2) interacted with alkylbenzenes from its singlet excited state to form exciplexes ranging from weak polarity up to contact radical ion pairs (CRIP); this exciplex series shows the characteristics in the Marcus "normal" region. In cyclohexane these exciplexes gave intense fluorescence spectra and high quantum yields (phi(infinity)ex). The dipole moment of these exciplexes calculated from the solvatochromic shift of the fluorescence maximums (vmax) was used to estimate the coefficient ("a" and "c") of the CT and LE terms in the exciplex wavefunction. On the basis of the measured lifetimes and phi(infinity)ex of these exciplexes, the radiative (k(ex)f) and nonradiative (k(ex)NR) rate constants were calculated. The former k(ex)f were also computed from a semi-empirical approach based on the assumption that the exciplex wavefunction could be adequately described by CT and LE states, and that *DBMBF2 primarily contributes to the probability of exciplex emission. Two results agree with each other with small systematic deviations for those less polar exciplexes. The plots of k(ex)r and k(ex)NR (or their logarithmic value) against the LE contribution (c2) and transition energy gaps (hvmax) afford better correlation than those against -deltaG(-et). This indicates the role played by the LE contribution in generating the stabilization energy (U(s)) in these exciplexes through the A-D+)<==> *AD) resonance interaction; U(s), in turn, modifies -deltaG(-et) to afford the decay driving force hvmax. Also, those plots from k(ex)f values (being determined directly from experiments) show better correlation than those from k(ex)NR. In contrast to the CRIP type exciplexes in the Marcus "inverted" region, these k(ex)f and k(ex)NR increase in the common trend with increasing transition energy gaps. The k(ex)NR plots show less steep slopes and attains more quickly a minimum toward the CRIP region; the latter is identified as the turning point from the "normal" to "inverted" region. Both the attenuation and reversal of the k(ex)NR value with increasing polarity are believed to be generated by the emerging contribution of the intersystem crossing process as an additional nonradiative process, which is induced by the increased spin-orbit coupling in highly polar exciplexes.     

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.     

Electronic relaxation of molecular exciplexes in the vapor phase

Interaction of electronically excited TCNB (tetracyanobenzone) and ground-state MB (methylated benzenes) leads to the formation of fluorescent exciplexes in the vapor phase. Studies of exciplex fluorescence as a function of excess vibrational energy in TCNB and of temperature and pressure of added buffer gas, lead to the conclusion that the low-frequency inter-molecular vibrations play a very important role in the radiationless deactivation of exciplexes. The results also indicate that collisional relaxation of low-frequency intermolecular vibrations proceeds much more rapidly than that of high-frequency intramolecular vibrations.     

Salt-induced Charge Separation in Photoinduced Electron Transfer Reactions. The Effect of Ion Size*

The effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/electron donor (D) systems was studied by the technique of laser flash photolysis. We investigate the exciplex-forming pyrene/p-dicyanobenzene (Py/DCB) and pyrene/N,N-dimethylaniline (Py/DMA) systems in ethyl acetate. The salts selected for this study are tetrabutylammonium chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenyl-borate. In most cases, the salts quench the emission of the exciplexes with rate constants near the diffusional rate limit in ethyl acetate. An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentration is also generally observed. Laser flash photolysis experiments showed that in the absence of the salts both A/D systems yield exclusively the triplet excited state of the polyaromatic. However, in the presence of many of the electrolytes studied, induced free radical ion formation is observed. The experimental efficiencies for induced charge separation (n) depend on the A/D system and on the nature of the salt. The measured n values vary between 0 and 0.5. The most striking variation corresponds to the lithium and sodium perchlorates. These salts are almost totally inefficient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the soft/hard character of the interacting ions.     

Photophysical properties of β-(1-pyrenyl)ethyl p-cyanobenzoate in binary solvents of isooctane-ethyl acetate and ethyl acetate-acetonitrile

The effects of solvent polarity on the fluorescence spectra and fluorescence decays of β-(1-pyrenyl)ethyl p-cyanobenzoate (P2CN) were investigated in detail using binary solvents consisting of various mixing ratios of isooctane-ethyl acetate or ethyl acetate-acetonitrile (dielectric constants ()=1.94–36.2). Whereas both the intensity and wavelength maxima of an intramolecular exciplex emission (EX) are dependent on the solvent polarity, only the intensity of an emission from the locally excited pyrene (LE) is dependent on the solvents used. When monitored at 377 nm, the picosecond SPC (single photon counting) measurements reveal a slow decay (>150 ns) in addition to a fast decay (<1 ns) of the locally excited P2CN. There are also two decays for the EX which vary the intensity ratios by the monitored wavelength. The decay rate constants, k_EX1 and k_EX2, have a good linear correlation with the dielectric constants of the solvents, indicating that there exist two kinds of exciplexes. It is suggested that the decays of the locally excited-state of P2CN are so fast due to result of the efficient electron transfer that the two kinds of intramolecular exciplexes are formed from the two discrete conformers in the ground state.     

Exciplexes of pyrene with indole and diethylaniline in liposomes and biomembranes

The formation of exciplexes of pyrene with indole and diethylaniline incorporated into the lipid bilayer was observed in phospholipid liposomes and membranes of sarcoplasmic reticulum from rabbit muscles. The intensity and lifetime of pyrene luminescence were found to decrease and the structureless emission of the exciplexes appeared. Exciplex emission in the membrane has a low quantum yield (compared with exciplexes formed in hexane solutions). This is probably due to the presence of polar groups in the membranes. It was shown that the formation of exciplexes is markedly dependent on the physical state of membranes. It is suggested also that pyrene can form an exciplex with tryptophan residues of membrane proteins.     

π堆积的腺嘌呤-胸腺嘧啶体系激发态电荷转移和无辐射失活的半经典动力学模拟

采用半经典电子-辐射-离子动力学(SERID)模型模拟了π堆积的腺嘌呤-胸腺嘧啶(A-T)体系激发态的光物理失活过程. 设置激光脉冲仅作用于T, 模拟发现电子由A转移到T, 形成(A⁺T^-)^*激基缔合物(exciplex). 当分子间距缩短至0.300 nm时, 由于轨道离域效应产生电荷重组, 体系恢复电中性; 当A分子的C4'－C5'扭曲程度最大时, 体系通过避免交叉点衰减至基态. Exciplex 的失活途径由分子间距离和A分子的变形程序两个因素决定. 由于A分子的C4'、C5'原子位阻较大, 难以达到失活所必需的强烈扭曲, 因此(A-T)^*的寿命比胸腺嘧啶堆积体系(T-T)^*显著增长.     

Emission from aromatic hydrocarbon-diene and -olefin exciplexes

Luminescence from aromatic hydrocarbon-olefin and -diene exciplexes, providing strong evidence for their intermediary in singlet quenching processes, is reported. Solvent dependence of the emission maximum gives a value of 10.8D for the dipole moment of the 1-cyanonaphthalene-1,2-dimethylcyclopentene exciplex while the temperature dependence affords a value of 6.7 kcal/M for the heat of formation. Linear-free-energy correlations of rate constants for 1-cyanonaphthalene and naphthalene fluorescence quenching by dienes and olefins and strained hydrocarbons with the adiabatic ionization potentials of the quenchers are consistent with major contributions from charge-resonance in the exciplex formation process.     

Laser photolysis study of chloranil triplet exciplexes with stilbene

Absorption spectra and decay kinetics of the polar triplet exciplexes (contact radical-ion pairs) formed during quenching of the chloranil triplet state by trans- or cis-stilbenes in benzene with added acetonitrile and methanol, have been studied by laser flash photolysis. The exciplexes include cation-radicals of stilbene dimers, which are deactivated by reverse electron transfer within 10–50 nsec. The dynamics of the intercombination electron transfer and the exciplex dissociation into ion-radicals were determined. The isomerization of stilbene via triplet exciplex formation was not observed.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 117977. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 572–576, March, 1992.     

The Importance of Excited‐State Energy Alignment for Efficient Exciplex Systems Based on a Study of Phenylpyridinato Boron Derivatives

Understanding excited‐state dynamics is critical for improving the photoluminescence (PL) efficiency of exciplexes. A series of exciplexes based on conventional hole‐transporting materials as donor and newly developed phenylpyridinato boron derivatives as acceptor were investigated. High PL efficiencies were achieved in only some combinations, and a large difference in performance among combinations provided insight into nonradiative processes in exciplex systems. Furthermore, the triplet local excited states (³LE) of each donor and acceptor were found play an important role in triplet exciplex harvesting. Significant contributions from triplets were clearly observed when the charge‐transfer excited states (¹CT and ³CT) and ³LE were ideally aligned. We also demonstrated fine control of relative energy alignment via the concentration to improve the PL efficiency.     

The use of doped crystals as a medium for the study of excited state bimolecular interactions

A molecular crystal is an attractive medium for the study of excited state intenmolecular interactions since the ground state positions of interacting molecules may be accurately determined. Besides excimer interactions in pure crystals it is possible to study (in doped crystals) exciplex interactions between host and guest molecules, and even excimer formation between two adjacent guest molecules. This approach has been utilized to observe excimer formation by several anthracene derivatives introduced as dopants in pyrene crystals. Photochemical reactions of some of these exciplexes and excimers are discussed. Excimer forming crystals are considered to be good candidates for excited state structure determination by means of pulsed x-ray diffraction.     

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.     

吡唑啉衍生物有机电致发光器件中激基复合物的发射

以两种吡唑啉衍生物为空穴传输材料(HTM)和BBOT为电子传输材料组成双层器件, 获得了相对于组成材料的荧光光谱红移和宽化的电致发光. 双层器件和HTM∶BBOT等摩尔混蒸薄膜的光致发光及电致发光测量表明, 该谱带来自HTM/BBOT界面激基复合物的发射, 根据器件的能级图, 激基复合物的类型为BBOT的激发态BBOT⁽与基态的HTM相互作用的复合物. 用HTM∶BBOT混合发光层增加器件中激基复合物的形成界面, 提高了器件的发射性能.