How intermolecular interactions influence electronic absorption spectra: insights from the molecular packing of uracil in condensed phases

The photoexcitation mechanism in photochemistry and photophysics is a key to understanding the photostability and photoreaction of nucleobases. Using a combination of the generalized energy-based fragmentation (GEBF) and quantum mechanical and molecular mechanical (GEBF-QM/MM) approach and the QM/MM approach, we have investigated the electronic absorption spectra for the π–π* transition of uracil in aqueous solution, amorphous solid, and crystal. Our results indicate that the intermolecular interactions in terms of molecular packing are crucial for the investigation of the absorption spectra of uracil in different environments. There is a large red-shift (relative to uracil in the gas-phase) for uracil in the amorphous phase, which arises from hydrogen-bonding (HB) and close π–π stacking interactions. In contrast, the relatively smaller red-shift of uracil in aqueous solution can be attributed to the cooperative HB and long-range electrostatic and polarization interactions. Due to the HB and weak π–π interactions, the red-shift of the crystal is smaller than that of amorphous uracil. Furthermore, the results suggest that a large system is required to obtain the accurate absorption spectra of solutions, whereas small electrostatically embedded cluster models could be used to obtain the corresponding results for amorphous solids and molecular crystals.