Time-Resolved Fluorescence Study on the Pressure-Induced Viscosity Dependence of Exciplex Formation in Liquid Solution

Abstract

The contribution of diffusion to the exciplex formation of the 1,2-benzanthracene (BZAN)/dimethylaniline (DMA) and pyrene (PY)/DMA systems in methycyclohexane at pressures up to 400 MPa was investigated. The rate constant, k 3 , for the exciplex formation, which was determined by the analysis of the rise and decay curve of the exciplex emission, decreased significantly with increasing pressure. The apparent activation volume for k 3 , ∆V 3 * , was 14.9 and 18.1 cm 3 /mol for BZAN/DMA and PY/DMA, respectively. For comparison, the rate constant, k q , for the fluorescence quenching by carbon tetrabromide (CBr 4 ) of BZAN and PY that is believed to be fully diffusion-controlled was also measured as a function of pressure. The apparent activation volume for k q , ∆V q * , was 17.9 and 20.1 cm 3 /mol for BZAN/CBr 4 and PY/CBr 4 , respectively. These results are interpreted by assuming an encounter complex formed between the donor and acceptor pairs, and it was concluded that the exciplex formation reaction studied in this work is not fully diffusion controlled but competes with a diffusion process of the donor and acceptor molecules that is expressed by a modified Debye equation. It was also concluded that the quenching by CBr 4 is not fully diffusion controlled at the lower pressure range. The bimolecular rate constant, k bim () k c k diff /k -diff ), for the exciplex formation was estimated to be 1.5 × 10 10 M -1 s -1 for BZAN/DMA and 1.9 × 10 10 M -1 s -1 for PY/DMA. The activation volumes for the various rate processes also are discussed.

Modular Synthesis of Pentagonal and Hexagonal Ring-Fused NBN-Phenalenes Leading to an Excited-State Aromatization-Induced Structural Planarization Molecular Library

“…However, the TA-predicted excimer formation (τ 3 = 244 ps) is too fast to be observed by our TRFL setup due to the limitation of temporal resolution (∼500 ps). Considering that excimer formation is highly dependent on the diffusion process, − we attempted TRFL experiments of 3a and 4a in a high-viscosity system, i.e. in a glycerol/ethanol (v/v = 4/1) mixture with η ≈ 171 cP at room temperature, − in which excimer formation can be dramatically slowed. − The global fitting of the TRFL results of 3a shows a clear red shift with a time constant of t 1 = 0.96 ns (SAS1 → SAS2, see Figure a), followed by further population decay (SAS2 →, t 2 = 3.02 ns), which is consistent with the slow population decay (X →, τ 4 > 2 ns) observed in the TA experiment, as such a population decay is much less sensitive to liquid viscosity in comparison to excimer formation.…”

Contribution of local density augmentation in the vicinity of 9-cyanoanthracene and dynamic fluorescence quenching by oxygen in liquid and supercritical carbon dioxide

“…In our previous publications, 25,26,29 we showed that the fluorescence quenching of some aromatic molecules, 1 M*, by quencher, Q, such as carbon tetrabromide and polybromoethanes in liquid solution is not fully but nearly diffusioncontrolled. The contribution of diffusion to the observed quenching rate constant, k q , was interpreted satisfactorily by the kinetic model that involves the encounter, (M*Q) en , and the exciplex, (MQ)*, formed between 1 M* and Q in the solvent cage.…”

“…24 However, for meso-substituted anthracence derivatives with electron withdrawing substituents, such as 9,10-dicyanoanthracene (DCNA), the rate constant for quenching, k q , is smaller by about one order of magnitude than that for diffusion (k q $ 10 9 M À1 s À1 ). 22,23 In previous publications, 25,26 the pressure effect on the fluorescence quenching of pyrene by polybromoethanes, Q, with wide quenching ability in liquid solutions has been studied and successfully interpreted on the basis of a kinetic model, which involves the exciplex, (MQ)*, via the encounter complex, 1 (M*Q) en , formed between the lowest excited singlet state, 1 M*, and Q. This model was successfully applied to the fluorescence quenching of DMEA by oxygen (8.5-40.0 MPa) and CBr 4 (8.5-60.0 MPa) 27 and of DCNA by oxygen (8.0-60.0 MPa) 28 in SCF CO 2 at 35 C as well as in liquid CO 2 (10.0-60.0 MPa) at 25 C, and the fluorescence quenching for these systems in SCF CO 2 was shown to be interpreted in the same framework as that in liquid CO 2 , indicating no contribution of local composition enhancement to the quenching in the pressure range examined.…”

“…In eqn. (3), the Debye equation predicts that a is 2000 and 3000 for the slip and stick boundary limits, respectively, but in our previously reported work, [25][26][27][28][29] a was taken as a parameter to be determined experimentally. According to reaction (I), together with eqn.…”

Metal-enhanced excimer (P-type) fluorescence

“…[11] The excimer E* is a relatively stable complex between one excited molecule of pyrene and another in its ground state, with a binding energy of 38-42 kJ mol −1 in non-viscous solvents. The formation process, K 1 , K 2 are related to the diffusion-controlled rate of the donor/acceptor system [11]. The fluorescence spectra of highly concentrated solutions of pyrene show two distinct emission bands.…”