Effect of donor and acceptor on optoelectronic properties of benzo[1,2-b:4,5-b′]dithiophene

A series of acceptor and donor groups anchored to benzo[1,2-b:4,5-b ']dithiophene (BDT) molecule have been systematically investigated at the density functional theory (DFT) and time-dependent density functional theory (TDDFT) level to reveal structure-property relationships, charge transfer, and fluorescence lifetimes. The DFT optimization shows that the hetero atom in the ring induces the polarity from central ring to both ends of the thiophene ring, participating in the conjugation. The donor and acceptor groups were anchored at the terminals of the BDT at two different positions to fine-tune the properties according to the requirement and study the push-pull effect. All the models studied in this work retain their aromaticity as estimated from NICS(0) and NICS(1) aromaticity index in ground and excited states. The results show that the hardness, softness, HOMO-LUMO gaps, ionization potentials (IP), and electron affinities (EA) of the BDTs are significantly affected by the electron-withdrawing and electron-donating groups. The H-1 and C-13 NMR chemical shift values have been computed to quantify the push-pull effect. Further, the charge transfer properties in these BDTs were explored based on reorganization energies and diagnostic descriptors derived from hole-electron theory that present different electron excitation behavior. The relationship between the computed variables such as highest occupied molecular orbital, lowest unoccupied molecular orbital, oscillator strength, dipole moment, absorption, and fluorescence energy correlates the system with one another and also to extend the possible applications of the system in optical devices. Structure-property relationship of various BDTs reveal that, upon optical excitation, the resonance effect plays an important role changing the bonding character between the substituent and BDT unit, enabling efficient electron delocalization. The examination of TDDFT results indicates that among the various models studied in this work, nitro-substituted model is better candidate for optoelectronic properties with relatively large absorption wavelength and long fluorescence lifetime.