Solvatochromism in a pyridinium cyclopentadienylide: insights from a sequential Car-Parrinello QM/MM and TD-DFT/semicontinuum approach.

Abstract

Understanding the working mechanism and establishing structure-property relationships for optical probes is an essential step to develop design principles for novel molecular probes. Here we study optical properties of a small-sized dielectric probe, namely, 4-carbamido pyridinium cyclopentadienylide (CPYC) in benzene and in water solvents using a sequential approach. In particular, the structure modeling has been carried out using a Car-Parrinello hybrid QM/MM molecular dynamics approach, while the excitation energies were computed using time dependent density functional theory. To incorporate the solvent effect either a polarizable continuum model or a semicontinuum description was employed. The molecular dipole moment of CPYC in water is more than two times larger than in benzene solvent. The positive and negative charges tend to accumulate on pyridinium and cyclopentadienylide rings, respectively, with increasing solvent polarity. Significant solvent-induced geometrical changes have been reported in CPYC and this contributes to a significant red shift in spectra. Even though the absorption maxima for CPYC in benzene and water solvents were underestimated, the solvatochromic shift has been reproduced in good agreement with experiments. We also report that CPYC can be used as a two photon probe.