Effects of site-specific substitution to hexacene and its effect towards singlet fission.

The singlet fission (SF) process, which is involved in organic solar cell, is a spin allowed and extremely fast internal conversion process by which a photo-excited singlet exciton produces two triplets. To accelerate this fission and to increase the efficiency of solar cell, designing of new materials/molecules is an interesting area of research and our current interest. Several alternate hydrocarbons of the acene series and their derivatives with structural variety are desirable for this purpose. Therefore, we have theoretically modeled and investigated different substituted hexacenes in detail. Different electron donating groups (EDGs) as well as electron withdrawing groups (EWGs) and few groups of other varieties are chosen for site-specific monosubstitution to the hexacene ring for manipulation of their excited singlet-triplet energy levels and thereby to match the SF driving force (Δ) and triplet-triplet annihilation (TTA) deactivation force (Ω). The geometries, electronic structures, frontier molecular orbital (FMO) energies, optimization of excited state and calculation of Δ and Ω of the substituted hexacenes are investigated with Time Dependent Density functional theory (TDDFT) method using B3LYP/6-31G∗ basis set. After a detail theoretical analysis on sixty five (65) hexacene derivatives, the δ-triisopropylsilylethynyl (-TIPS) hexacene and β-NO2 hexacene are predicted to exhibit good SF characteristics.