Solvents play an important role in shaping the
intramolecular charge transfer (ICT) properties of π-conjugated molecules,
which in turn can affect their one-photon absorption (OPA) and two-photon
absorption (TPA) as well as the static (hyper)polarizabilities. Here, we study
the effect of solvent and donor-acceptor arrangement on linear and nonlinear
optical (NLO) response properties of two novel ICT-based fluorescent sensors,
one consisting of hemicyanine and dimethylaniline as electron withdrawing and
donating groups (molecule 1), respectively and its boron-dipyrromethene
(BODIPY, molecule 2)-fused counterpart (molecule 3). Density functional
theoretical (DFT) calculations using long-range corrected CAM-B3LYP and M06-2X
functionals, suitable for studying properties of ICT molecules, are employed to
calculate the desired properties. The dipole moment (µ) as well as the total
first hyperpolarizability (β<sub>total</sub>) of the studied molecules in the
gas phase is dominantly dictated by the component in the direction of charge
transfer. The ratios of vector component of first hyperpolarizability (β<sub>vec</sub>)
to β<sub>total</sub> also reveal unidirectional charge transfer process. The
properties of the medium significantly affect the OPA, hyperpolarizability and
TPA properties of the studied molecules. Time dependent DFT (TDDFT)
calculations suggest interchanging between two lowest excited states of molecule 3 from the gas phase to
salvation. The
direction of charge polarization and dominant transitions among molecular
orbitals involved in the OPA and TPA processes are studied. The results
presented are expected to be useful in tuning the NLO response of many
ICT-based chromophores, especially those with BODIPY acceptors.<br>
Tuning of Hyperpolarizability, One- and Two-Photon Absorption of D-A and D-A-A Type Intramolecular Charge Transfer Based Sensors