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Laser flash photolysis was used to measure rates of triplet energy transfer from carbazole (Cz) to naphthalene (Np) in rigid solutions of C Z -( C H~)~-N~, where n = 4,5,6, or 8. The results are interpreted in terms of Dexter's equation and a model which allows for a distribution of donor-acceptor distances for a given n. The distribution was calculated using an arithmetic mean of a center-to-center and closest edge distance between the donor and acceptor chromophores. This model provided the transfer rate for all the compounds tested, and the results indicated a through-space mechanism.

The photoexcitation of N-ethylcarbazole ( N E C ) in fluid solutions of cyclohexane, N,N-dimethylformamide (DMF), ethanol, paraffin oil, and mixtures of glycerol with either ethanol or D M F has been carried out using 308-nm pulses from a XeCl excimer laser. Emission spectra recorded a t 70 p s or longer after the excitation pulse depend upon the solvent. In ethanol and D M F red-shifted, structureless bands appear a t 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 p s or less, but a t delay times from 700 p s to 3.8 ms a prominent band a t 500 nm is observed which is attributed to emission from triplet excimers. Transient absorption spectra of NEC in cyclohexane recorded a t delay times from 1 to 20 ps 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.

Triplet energy migration in chromophoric polymer films has been investigated by .phosphorescence spectroscopy. Poly[ (2-naphthylalkyl methacrylate)-co-(methyl methacrylate)] and poly[(9-phenanthrylmethyl methacrylate)-co-(methyl methacrylate)] films show phosphorescence spectra from 'triplet trap sites". The emission is slightly red-shifted from the monomer phosphorescence with a vibrational structure which is different from that of the triplet excimer. The energy migration rate in these systems is complex since the trap sites have various energy levels depending on the amplitude of interaction energies. The kinetic analysis was made on the basis of Btissler's model in which the site energies have a Gaussian distribution. The simulations are in good agreement with experimental data in a temperature range of 115-165 K. The resulta provide strong evidence for the hopping mechanism of trapped excitons among a distribution of energy sites for a particular energy level.

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