Periodic density functional theory (DFT) and hybrid ONIOM time-dependent DFT/MM cluster calculations have been carried out to investigate the ground- and excited-state properties of the crystalline structures of the enolic and ketonic tautomeric forms of a propoxy-substituted dibenzothiazolylphenol molecule (OPr), a prototype for systems undergoing the excited-state intramolecular proton transfer process. The crystalline structures of the tautomeric forms are well reproduced and, as expected, at the ground state, the enol polymorph is predicted to be more stable than the keto one. At the excited state, the effect of the environment on time-dependent DFT calculations has been accounted for by including a charge embedding scheme, and the influence of different kinds of point charges (Mulliken, CM5, RESP and QEq) in determining the optical properties of the central molecule has been investigated. The results reveal that, in fair agreement with experimental data, the absorption (emission) energies of the enol (keto) OPr molecule is red-shifted of about 3 (3) nm going from the gas phase to chloroform and blue-shifted of 10 (23) nm going from the gas to the crystal phase when the electronic embedding with Mulliken charges is employed. The electrostatic embedding influences the excited-state properties more severely than the ground-state properties, and apart the QEq charges, all other models provide Stokes shifts in reasonable agreement with experimental data.
Bibliographical noteFunding Information:
H. P. Hratchian and M. J. Frisch are kindly acknowledged. Computational resources for this work were granted by "Project 100339" (2013-2014) at GENCI-IDRIS (Orsay, France). This work was supported by the Italian Ministero dell?Istruzione, dell? Universit? e della Ricerca (MIUR) through the "Programma di Ricerca di rilevante Interesse Nazionale" (PRIN) Grant 2010C4R8M8_002 entitled "Nanoscale Functional Organization of (bio)Molecules and Hybrids for Targeted Application in Sensing, Medicine and Biotechnology" and the "Futuro in Ricerca" (FIRB) Grant RBFR1248UI 002 entitled "Novel Multiscale Theorethical/Computational Strategies for the Design of Photo and Thermo Responsive Hybrid Organic?Inorganic Components for Nanoelectronic Circuits." Published as part of the special collection of articles "Charge Transfer Modeling in Chemistry".
- ESIPT fluorophores
- Electrostatic embedding
- Molecular crystals