Nonlinear Optical Response and Photodynamics of Conjugated Molecules: Effects of Branching and Substitution

Photoexcited dynamics and nonlinear optical response of organic chromophores are investigated using time-dependent density functional theory (TDDFT). Closed expressions for frequency dependent polarizabilities up to the third order are derived and computationally implemented. We find that TDDFT reproduces well the energetics of both one-photon (linear absorption) and two-photon (TPA) states in a variety of donor-acceptor substituted molecules. The absolute magnitudes of TPA cross-sections have a good agreement with experiment as well. Several structurally related large chromophores of different symmetry are further investigated and compared for elucidation of the combined role of branching and charge symmetry on photoluminescence and nonlinear optical response. Branching is observed to lead to both cooperative enhancement of TPA while ensuring high fluorescence quantum yield. New strategies for molecular engineering of nonlinear optical materials are inferred