Ultrafast Dynamics of the Excited States of Hydrogen‐Bonded Complexes and Solvation

Abstract

In solutions, solute and solvent molecules interact to minimize the free-energy of the solute-solvent system [1][2][3][4][5]. Each solute molecule is surrounded by a shell of more or less tightly bound solvent molecules because of intermolecular forces between them; the phenomenon is well-known as "solvation" and "solvation energy" can be defined as the change of Gibb's free energy when a molecule is transferred from the gas phase into the solvent. Solvation interaction mainly arises due to the columbic forces between the dipoles of the solute and solvent molecules [6]. This is a long-range interaction, also known as a "non-specific" interaction, and for a particular solute molecule, the solvation energy is solely determined by the dielectric properties of the solvent. However, in addition to the dipolar solvation energy, the free energy of the solute-solvent system may be further lowered if a hydrogen bond is formed between the solute and a solvent molecule [7,8]. Since two specific solute and solvent molecules are involved in formation of the hydrogen bond, this kind of solvation interaction is known as "specific" interaction [9], which will be described here as formation of hydrogenbonded complex.Following photoexcitation of a solute molecule, the electronic charge distribution is changed. In such a case, in which the molecule is excited directly to an excited state having intramolecular charge transfer (ICT) character, the instantaneous and significant change in the polarity of the molecule following photoexcitation drives the dipoles of the surrounding solvent molecules to reorganize themselves to minimize the free energy of the solute-solvent system. This phenomenon is popularly known as "dipolar solvation" [10][11][12][13][14][15]. Following photoexcitation of the hydrogen-bonded complex formed in the ground electronic state of the solute, the

Fluorescence Quenching of Hydrogen-Bonded Coumarin 102-Phenol Complex: Effect of Excited-State Hydrogen Bonding Strength

“…4 Palit and coworkers considered dynamic equilibrium between H-bond breaking and reformation in the excited state of fluorenone and amino-fluorenones in many Hbond donating solvents. 5 Blank and coworkers studied the effect of H-bond on the excited C102 in acetonitrile−water binary mixture by both experiment and simulation. 6 Their model suggests a concurrent strengthening and weakening of the hydrogen bond between C102 and water.…”