Intersystem crossing and triplet excited state properties of thionated naphthalenediimide derivatives

Abstract

Thionated naphthalenediimide (NDI) derivatives with 2,6-dibromo and diaminoalkyl substituents are prepared by treating of 1,4,5,8-naphthalenetetracarboxylic dianhydride with Lawesson's reagent. Red shifted absorption was obtained for all thionated derivates as compared to the respective oxo-compounds. Thionated NDI derivative with 2,6-diamino substituents shows strong absorption at 674nm. Nanosecond time-resolved transient absorption spectroscopy confirmed the population of triplet excited state of the thionated compounds upon photoexcitation and the triplet state quantum yield was determined as ΦT = 58%. Thionated derivatives were used for singlet oxygen (1O2) photosensitizing, the compound shows high singlet oxygen quantum yield (ΦΔ = 56%) with excitation at 675nm. With femtosecond transient absorption spectroscopy, the intersystem crossing (ISC) of the thionated NDI was found to take 32ps. Based on TD-DFT computations, the ISC of these heavy atom-free NDI is attributed to the S1 → T2/T3 transition, which is in 1(π-π)* → 3(n-π)* feature, which obeys with El-Sayed rule. Moreover, the S1/T3 state energy gap is 0.02eV, much smaller than that of S1/T1 energy gap (0.86eV). These results are useful for the design of heavy-atom-free triplet photosensitizers.