Intersystem crossing and triplet excited state properties of thionated naphthalenediimide derivatives

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 674 nm. 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 Phi(T) = 58%. Thionated derivatives were used for singlet oxygen (102) photosensitizing, the compound shows high singlet oxygen quantum yield (Phi Delta=56%) with excitation at 675 nm. With femtosecond transient absorption spectroscopy, the intersystem crossing (ISC) of the thionated NDI was found to take 32 ps. Based on TD-DFT computations, the ISC of these heavy atom-free NDI is attributed to the S-1 -> T-2/T-3 transition, which is in 1(pi-pi)* > (3)(n-pi)* feature, which obeys with El-Sayed rule. Moreover, the S-1/T-3 state energy gap is 0.02 eV, much smaller than that of S-1/T-1 energy gap (0.86 eV). These results are useful for the design of heavy-atom-free triplet photosensitizers.