Low-Lying Excited States in Thiophene-Based Cyclic Molecule Suitable for Optoelectronics: A Density Matrix Renormalization Group Study.

The low-lying excited states of thiophene-based cyclic oligomer tetrathia[22]annulene[2,1,2,1] (TTA) are studied using the density matrix renormalization group technique within long-range Pariser-Parr-Pople (PPP) model Hamiltonian. The calculated lowest singlet dipole-allowed excited state S(1) (optical gap) is at 1.76 eV for TTA which is in very good agreement with experiment (1.6 eV), and therefore it is found to be promising in harvesting the visible to near-infrared region of the solar spectrum. The calculated diradical character of TTA and the lowest triplet to lowest singlet (T(1)/S(1)) ratio (0.12) in TTA obtained within the model PPP Hamiltonian suggest TTA to be a promising singlet fission material. Donor-acceptor (D-A) substitution in TTA also modulates the optical gap from 1.76 to 1.35 eV rendering (D-A)-substituted TTA applicable in organic light-emitting devices.