DFT investigations of electro-optical and charge transport properties of two photochromic isomers 3,6-bis-((E)-2-(benzo[b]thiophen-5(6)-yl)vinyl)phenanthrene (BTVP) for OLED and NLO applications

Density functional theory (DFT) studies were conducted in the gas phase and in solvent media to assess the intrinsic properties of two photochromic isomers namely: 3,6-bis-((E)-2-(benzo[b]thiophen-5-yl)vinyl)phenanthrene and 3,6-bis-((E)-2-(benzo[b]thiophen-6-yl)vinyl)phenanthrene, referred to as exo-BTVP and endo-BTVP, respectively. The stability of both molecules resulting from conjugative interactions and charge delocalization was investigated using natural bond orbital (NBO) analysis. Both BTVP isomers have their HOMO level at − 5.47 and − 5.43 eV, values compatible with those of air-stable organic semiconductors. Likewise, the low LUMO levels, on the order of − 2.1 eV, facilitate efficient electron injection, essential for an n-type organic semiconductors and also contribute to their air-stability. The adiabatic ionization potential (IPa) value of both BTVP isomers falls within the range of 5.680–6.786 eV of the IPa values for air-stable p-type OFET materials. The reorganization energies of the holes are 0.177 and 0.166 eV for exo- and endo-BTVP, respectively, whereas those of the electrons are 0.209 and 0.189 eV. These values are close, demonstrating the ambipolar nature of both molecules. Thus, both BTVP isomers exhibit charge transport rates on the order of 1014 s−1 and charge mobilities of at least 7 cm2 V−1 s−1, which give them excellent charge transport properties and can be used as good materials for OLED-based applications. Absorption spectra analysis revealed that both molecules mainly absorb in the ultraviolet and exhibit light-harvesting efficiency (LHE) values close to 1; making them potential candidates for photocurrent enhancement in organic solar cell devices. The emission fluorescence consists of two prominent peaks, that occur in the UV-A1 band and deep blue. The radiative lifetime ranges from 1 to 2 ns, making both molecules good emitters for first-generation OLEDs. They exhibit a medium Stokes shift (from 53 to 56 nm) when emitting in the UV-A1 band, against a large Stokes shift (from 84 to 91 nm) when emitting in the deep blue. Linear and nonlinear optical (NLO) response findings revealed that, despite their relatively low dipole moments, both BTVP isomers exhibit excellent 2nd and 3rd order NLO responses, compared with those of reference NLO materials. Hyper–Rayleigh Scattering values (βHRS) of target compounds were predicted, and depolarization ratio values indicate the dipolar nature of exo- and endo-BTVP at 800 and 1064 nm, against an octupolar nature at 532 nm. The nonlinear refractive index values of exo- and endo-BTVP at 1064 nm are respectively 9.8 and 12.1 times higher than that of fused silica. Both BTVP isomers can therefore be used for potential NLO applications and in the design of devices requiring good NLO properties.