Electron paramagnetic resonanace of the photoexcited triplet state of electron—donor—acceptor complex crystals: Naphthalene—tetrachlorophthalic anhydrides

The EPR spectra and kinetics of the photoexcited triplet state of naphthalene—tetrachlorophthalic anhydride (N—TCPA) complex crystals are reported. The phosphorescent state of N—TCPA is naphthalene-like with 10% charge-transfer character at 4.2 K. Detailed temperature and orientational studies show that donors and acceptors reorientate along the stack axis at very low temperatures (20 K and below) to gain a maximum overlapping (X-trap). The activation energy of the detrapping process is 60 ± 10 cm^?1. At higher temperatures (50 K and above), the state of the system is best described as an excitation jumping between a localized state and a thermally accessible higher delocalized state (exciton). The activation energy of the excitation jumping is 150 ± 10 cm^?1. The upper limit of the average time spent in the exciton state is 8 × 10^?10 s. The transient studies yield the triplet population and decay rate constants of N—TCPA complexes which differ from those of uncomplexed naphthalenes. The difference is attributed to the excitation to CT singlet state followed by intersystem crossing in different pathways.