Triplet States of Thioflavin T in Fluorescent Molecular Rotor Model

Abstract

Quantum-chemical INDO/S calculations of Thiofl avin T have been carried out taking account of variation of the angle φ between the planes of the benzothiazole (BTZ) and dimethylaniline (DMA) rings. It was found that when the angle φ changes from 40° to 90° the energy of the triplet state increases by 4000 cm−1, whereas the energy of the singlet state S 1 decreases by 1900 cm−1 and reaches a minimum. The function $$ {E}_{T_1}\left(\upvarphi \right) $$ has a minimum at φ = 30°, which is 300 cm−1 less than at φ = 40°. The calculated T 1 S 0 interval at φ = 30–40° amounts to 15,600–16,000 cm−1, which agrees well with the phosphorescence data (17,100–17,400 cm−1). For φ = 80–90° the T 1, T 2 , and T 3 levels (T 1 and T 2 are lower than S 1 ) are close to the S 1 level. The S 1 and T 3 levels intersect at φ ~ 85°, and at φ = 90° the interval $$ \Delta {E}_{S_1{T}_3} $$ amounts to only 100 cm−1 due to the small value of the exchange integral corresponding to electron transfer DMA → BTZ.