Photoinduced energy-transfer and electron-transfer processes in molecules of tetrakis((E)-2-(50-hexyl-2,20-bithiophen-5-yl)vinyl)benzene and perylenediimide

Photoinduced energy- and electron-transfer processes of donor-σ-acceptor molecules composed of [1,2,4,5-tetrakis((E)-2-(50-hexyl- 2,20-bithiophen-5-yl)vinyl)benzene] (HPBT) with one, two and four entities of perylenediimide (PDI) forming HPBT-PDI_n (n = 1, 2 and 4) have been examined in this article by utilizing steady-state absorption and emission, computational, electrochemical and time-resolved transient absorption studies. The HPBT-PDI_n molecules are connected through long non-conjugated σ-bonds that may prevent the direct overlap of HPBT and PDI energy levels. Electrochemical studies suggest the exothermic photoinduced electron transfer processes when HPBT and PDI are selectively excited. Upon excitation the HPBT entity, the steady-state emission and femtosecond transient absorption measurements of HPBT-PDI_n revealed an efficient energy transfer from the singlet excited HPBT to PDI with time constants on the order of ∼10 ¹⁰ s^-1. The energy donor-acceptor distance, r = ∼22 , is calculated from the experimental energy transfer rates using Förster theory and from the MO calculations using ab initio B3LYP/6-311G method. By selective excitation the PDI entity, the electron-transfer processes take place from HPBT to the singlet excited PDI with time constants on the order of ∼10⁸ s^-1. The slow rates of electron transfer and energy transfer processes indicated that these molecules tend to take conformations with relatively long distance between HPBT and PDI entities.