Excimer Fluorescence of Benzene and Its Alkyl Derivatives—Concentration and Temperature Dependence

Abstract

Fluorescence spectra and quantum yields have been obtained for methylcyclohexane solutions of benzene, toluene, ethylbenzene, cumene, p-, m-, o-xylene, and 1,3,5-, 1,2,3-, 1,2,4-trimethylbenzene as a function of aromatic concentration over the temperature range from 25 to −100°C. At low temperatures distinct excimer emissions were observed for all compounds studied. The intrinsic emission quantum yields of monomer φm and of excimer φe have been determined by a simple technique which requires no assumptions regarding the details of the monomer–excimer kinetics. With decreasing temperature, φe is observed to decrease contrary to the behavior of φm, suggesting that the rate constant for the excimer radiative transition decreases strongly as the temperature is lowered. Such temperature dependence is explained as arising from the existence of a substantial vibronic component in the transition moment that is induced by thermal excitation of upper-state vibrational motions (e.g., torsional, tilting, etc.) of one monomer with respect to the other. From analysis of the temperature dependence of the fluorescence, lower bounds on the excimer binding energies Eb have been determined. The difference between this lower bound and Eb is approximately equal to the activation energy for radiative decay of the excimer. An estimate of this activation energy indicates that, for the case of benzene, Eb > 0.36 eV. The probability for association of excited monomer to form excimer and the probability for dissociation of excimer to an excited and unexcited monomer have been determined for benzene at 25 and −78°C from an appropriate analysis of the fluorescence quenching effect of CCl4. Additionally it has been demonstrated that the observed increase in CCl4 quenching efficiency at high benzene concentrations is predominantly due to an energy migration process. The probability per encounter for formation of excimer has been determined for benzene to be ≈1.0 and to decrease with alkyl substitution in a manner consistent with the steric requirements of sandwich-type excimer configurations.

Multireference Approaches for Excited States of Molecules

“…984,981 The counterpoise-corrected CASPT2 binding energy of 0.43 eV is in good agreement with the experimental values of 0.34 -0.36 eV. 990,991 This accuracy is reached using the single-reference linear-response coupled cluster method with triple excitations (CCSDR(3)) included. 984 Concerning suitability of the TDDFT approach, 992,981 the TD-B3LYP binding energy is slightly overestimated, 975 still close to the results of the CASPT2 method.…”

Temperature and Exciton Concentration Induced Excimer Emission of 4,4′-Bis(4′′-Triphenylsilyl) Phenyl-1,1′-Binaphthalene and Application for Sunlight-Like White Organic Light-Emitting Diodes

“…Exciplex emission usually occurs at the interface between emissive layer and HTL or ETL as the excited emissive molecule interacting with the inhomogeneous ground-state molecule, while it has no relationship with the doping concentration. In our case, doping-concentration dependence was clearly observed; the emission should thus be originated from excimer [ 24 ].…”

Fluorescence in the Heavy Ion Radiolysis of Benzene

“…31 For simplicity, the total decomposition rate of the monomer to products other than the excimer is given as k m () k f + k ic + k is ) and the corresponding term for the excimer as k e () k f′ + k ic′ + k is′ ). As will be seen below, the values of k a [C 6 H 6 ] and k d are much greater than k m and k e , respectively.…”