Second-order nonlinear polarizability of "Push-Pull" chromophores. A decade of progress in donor-π-acceptor materials

Fine tuning and switching of nonlinear optical response of ferrocene chromophores has been an area of considerable significance as evidenced by a large number of reports in the current literature. In this personal account, we present linear/nonlinear behavior and structure-activity relationships of several classes of donor-π-acceptor designs using organometallic and organic materials, developed by our research group during the last decade. The results especially the electronic absorption spectral and the hyper-Rayleigh scattering have been supported by theoretical calculations. Exploiting the redox behavior of ferrocene donor, we have demonstrated switching of quadratic nonlinear optical responses with reversible redox chemistry, which is a useful attribute of nonlinear optical materials. Based on the ease in synthesis, structure diversification and structure-based large and switchable second-order optical nonlinearity, these materials are potential candidates for electro-optic applications.

Structure-property relationships in multi-stimuli responsive BODIPY-biphenyl-benzodithiophene TICT rigidochromic rotors exhibiting (pseudo-)Stokes shifts up to 221 nm

Structure-property relationships of donor-π-acceptor (D-π-A) type molecular dyad (pp-AD) and triads (pp-ADA and Me-pp-ADA) based on benzodithiophene and BODIPY with biphenyl spacers have been reported. Rotors pp-AD and pp-ADA showed efficient twisted intramolecular charge transfer (TICT) with near infrared (NIR) emissions at ∼712 nm and ∼725 nm with (pseudo-)Stokes shifts of ∼208 nm and ∼221 nm, respectively, and prominent solvatochromism. A structurally similar triad, Me-pp-ADA, with tetramethyl substituents on the BODIPY core instead was TICT inactive and exhibited excitation energy transfer with a transfer efficiency of ∼88% as revealed using steady state emission and transient absorption measurements. Rotors pp-AD and pp-ADA showed NIR emission with an enhancement in intensity with the addition of water in THF solution as well as a pronounced change in emission intensity with temperature and viscosity variations, which justify their utility as temperature and viscosity sensors. Furthermore, the linear correlation of lifetime with fluorescence intensity ratios of the donor and acceptor justifies the rigidochromic behaviour of these rotors.

Modulating absorption and charge transfer in bodipy-carbazole donor–acceptor dyads through molecular design

Bodipy-based donor–acceptor dyads were evaluated using transient absorption spectroscopy to reveal the influence of beta vs. meso substitution on excited-state dynamics.