Singlet and Triplet Excited State Energy Ordering in Cyclopenta-Fused Polycyclic Aromatic Hydrocarbons (CP-PAHs) Suitable for Energy Harvesting: An Exact Model and TDDFT Study.

We calculated the ground and low-lying excited states of cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) using exact diagonalization in full configuration interaction (CI) within the model Pariser-Parr-Pople Hamiltonian as well as a time-dependent density functional theory technique. The CP-PAHs include acenapthylene, isomers of pyracylene, cycloocta-pentalene, and three isomers of dicyclo-pentacyclo-octenes (DCPCO). We used the inherent symmetries of these systems to calculate the energy ordering of the lowest singlet (S) and lowest triplet excited (T) states with respect to the ground state (S). The calculation shows that the lowest dipole allowed singlet absorption varies from 0.43 to 1.42 eV for most of these systems. Such an optical absorption range is very promising in harvesting solar radiation ranging from the visible to near-infrared region improving the efficiency of photovoltaic device application. The calculated optical gaps for pyracylene, acenapthylene, and two isomers of DCPCO are in very good agreement with experimental results reported in the literature. The calculated S-T energy gaps (Δ) in cycloocta-pentalene and in the DCPCO isomers are very small ranging from 0.01 to 0.2 eV, which is highly desirable in improving their electroluminescence efficiency in light-emitting device applications.