Donor-bridge-acceptor systems based on boron dipyrromethene (BODIPY) are attractive candidates for bio-imagining and sensing applications because of their sensitivity to temperature, micro-viscosity and solvent polarity. The optimization of the properties of such molecular sensors requires a detailed knowledge of the relation between the structure and the photophysical behavior in different environments. In this work we have investigated the excited-state dynamics of three acceptor-donor-acceptor molecules based on benzodithiophene and BODIPY in solvents of different polarities using a combination of ultrafast spectroscopy and DFT-based electronic structure calculations. Transient absorption spectra show that upon photoexcitation an initial excited species with an induced absorption band in the near-infrared regime is formed independent of the solvent polarity. The subsequent photophysical processes strongly depend on the solvent polarity. In non-polar toluene this initial excited state undergoes a structural relaxation leading to a delocalized state with partial charge transfer character, while in the more polar tetrahydrofuran a fully charge separated state is formed. The results clearly show how factors such as donor-acceptor distance and restricted rotational motion by steric hindrance can be used to tune the excited state photophysics to optimize such systems for specific applications.

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. This journal is