PHOTOPHYSICS OF N-ETHYLCARBAZOLE IN FLUID SOLUTION - EVIDENCE FOR SOLVENT DEPENDENCE AND TRIPLET EXCIMER FORMATION

The photoexcitation of N-ethylcarbazole (NEC) in fluid solutions of cyclohexane, N,N-dimethylformamide (DMF), ethanol, paraffin oil, and mixtures of glycerol with either ethanol or DMF has been carried out using 308-nm pulses from a XeCl excimer laser. Emission spectra recorded at 70 mus or longer after the excitation pulse depend upon the solvent. In ethanol and DMF red-shifted, structureless bands appear at these longer delay times which are attributed to delayed excimer fluorescence. No such bands are found using cyclohexane. In glycerol/ethanol or glycerol/DMF (90/ 10 (v/v)) monomeric delayed fluorescence is found at delay times of 70 mus or less, but at delay times from 700 mus to 3.8 ms a prominent band at 500 nm is observed which is attributed to emission from triplet excimers. Transient absorption spectra of NEC in cyclohexane recorded at delay times from 1 to 20 mus after the excitation pulse are entirely attributable to the carbazolyl triplet state. In ethanol and DMF, however, additional bands corresponding to radical cations and anions of NEC are also found. A broad band observed near 500 nm is provisionally assigned to the triplet excimer. Kinetic decays of the absorption signals due to triplets were fit to concurrent first-order and second-order processes whereas those due to the radical anion were fit to a biexponential decay. It is proposed that delayed excimer fluorescence arises from recombination of cation dimers and radical anions and that triplet excimers are formed by recombination of geminate ion pairs to form a monomeric triplet and a ground-state partner, followed by interaction of the triplet and the same partner. The primary mechanism for ion formation is thought to involve electron transfer from multiply excited molecules to ground-state species.