Conformational Effects on the Lowest Excited States of Benzoyl-Pyrrolopyridazine: Insights from PCM Time-Dependent DFT

Abstract

Time-dependent density functional theory (TD-DFT) computations and steady-state electronic spectroscopy measurements are performed on two recently synthesized pyrrolopyridazines to account for the detrimental effect of benzoyl substitution on the blue fluorescence emission. In the case of the highly fluorescent ester derivative, planar in ground state, we show that TD-DFT using the PBE0 and B3LYP hybrid functionals in the state-specific solvation approach provides an accurate description of absorption and emission properties. In benzoyl-pyrrolopyridazine, the (pretwisted) orientations of the benzoyl group and the solvent polarity are both found to modulate the nature of the lowest excited states. The first excited state has nπ* character at ground-state geometry of the main conformer (carbonyl group facing the diazine ring) in nonpolar solvents and become nearly degenerate with a ππ* state in polar solvents. The latter, lower than the nπ* state at the ground state geometry of a minor conformer, relaxes into a twisted intramolecular charge transfer. Experimental absorption and excitation spectra are consistent with the conformational-dependent picture of the lowest excited state (as derived from TD-DFT). A rather qualitative agreement in predicting the fluorescence emission wavelength is achieved in computations employing the CAM-B3LYP and BH&HLYP functionals, whereas global hybrids with low or moderate amounts of exact exchange exhibit the expected TD-DFT failure with up to 1 eV underestimated transition energies.