The role of π-linkers and electron acceptors in tuning the nonlinear optical properties of BODIPY-based zwitterionic molecules

Abstract

Intramolecular charge transfer process can play a key role in developing strong nonlinear optical (NLO) response in a molecule for technological application. Herein, two series of boron dipyrromethene (BODIPY)-based push-pull systems have been designed with zwitterionic donor-acceptor groups, and their NLO properties have been evaluated using a density functional theory-based approach. Different π-conjugated linkers and electron acceptor groups were used to understand their roles in tuning the NLO properties. The molecules were analyzed through HOMO-LUMO gaps, frontier molecular orbitals, polarizabilities, hyperpolarizabilities, Δr indices, transition dipole moment densities, ionization potentials, electron affinities and reorganization energies for holes and electrons. These observations correlated well with the computed absorption spectra of the molecules. It is found that with the introduction of different π-linkers in the molecule, planarity is maintained and the HOMO-LUMO gap is systematically decreased, which leads to a large NLO response. It was noted that the electronic absorption wavelength maxima were found in the near-infrared region (934-1650 nm). The results show that compared to the pyridinium acceptor group, the imidazolium acceptor group in the BODIPY systems amplifies the NLO response to a larger extent. It is also observed that the BODIPY-based dye with an imidazolium acceptor and thienothiophene π-linker shows the highest first hyperpolarizability value of 3194 × 10^-30 esu. Furthermore, the charge transfer occurs in the z-direction, as the z-component of the first hyperpolarizability is the dominant factor in this system. Here, the designed molecules show a characteristic reorganisation energy value, which is a deciding factor in the rate of hole/electron transport for favourable intermolecular coupling. As a whole, this theoretical work highlights that π-conjugated linkers and electron acceptor groups can be used judiciously to design new molecular systems for optoelectronic applications.

Fig. 1. Chemical structures of the BODIPY-based dyes under investigation.

Fig. 2. Energy level diagrams of the dyes in the gas phase at the B3LYP/6-311++G(d,p) level of theory.

Fig. 3. Frontier molecular orbitals of the dyes in the gas phase at the B3LYP/6-311++G(d,p) level of theory.

Fig. 4. First hyperpolarizabilities of BODIPY-based dyes studied at the B3LYP/6-311++G(d,p) level of theory.

Fig. 5. The transition dipole moment density of the studied molecules (1 to 2d). The isovalue is 2 × 10⁻⁴. The green colour implies charge increase upon excitation and the blue colour implies charge depletion.