Linear and nonlinear optical properties of indeno[2,1-b]fluorene and its structural isomers.

Linear and non-linear optical properties of indeno[2,1-b]fluorene (1) and its structural isomers with 20 π-electrons have been studied using many body methods. As compared to other π electron systems of similar conjugation length, the experimentally measured optical gap (vertical excitation energy) and the singlet-triplet gap of 1 are quite small. The diradical character calculated using the ab initio density matrix renormalization group (DMRG) of 1 is the largest among its isomers, which explains its lowest singlet-triplet gap. The optical gaps calculated by the DMRG method, employing the long-range interacting Pariser-Parr-Pople model, show good agreement with the experimental values as compared to those calculated by DFT, MP2, and CASPT2 methods. Using correlated Hamiltonian and the DMRG approach, the third harmonic generation coefficient is found to correlate well with the diradical character.