The Role of Hydrogen-Bonding Interactions in the Ultrafast Relaxation Dynamics of the Excited States of 3- and 4-Aminofluoren-9-ones (original) (raw)
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The role of hydrogen bonding in excited state intramolecular charge transfer
Physical Chemistry Chemical Physics, 2012
Intramolecular charge transfer (ICT) that occurs upon photoexcitation of molecules is a vital process in nature and it has ample applications in chemistry and biology. The ICT process of the excited molecules is affected by several environmental factors including polarity, viscosity and hydrogen bonding. The effect of polarity and viscosity on the ICT processes is well understood. But, despite the fact that hydrogen bonding significantly influences the ICT process, the specific role of hydrogen bonding in the formation and stabilization of the ICT state is not unambiguously established. Some literature reports predicted that the hydrogen bonding of the solvent with a donor promotes the formation of a twisted intramolecular charge transfer (TICT) state. Some other reports stated that it inhibits the formation of the TICT state. Alternatively, it was proposed that the hydrogen bonding of the solvent with an acceptor favors the TICT state. It is also observed that a dynamic equilibrium is established between the free and the hydrogen bonded ICT states. This perspective focuses on the specific role played by hydrogen bonding of the solvent with the donor and the acceptor, and by proton transfer in the ICT process. The utility of such influence in molecular recognition and anion sensing is discussed with a few recent literature examples in the end.
The Journal of Physical Chemistry A, 2001
Energy-gap dependency for radiationless deactivation from excited states of various molecules having strong intramolecular charge transfer (ICT) character has been investigated by observing fluorescence quenching on addition of alcohols. Molecules having strong ICT excited states were classified into three groups: (a) molecules that underwent considerable fluorescence quenching by ethanol (quenching constant, K SV > 20 M -1 ) and for which radiationless deactivation in protic solvents was much faster than anticipated from the ordinary energygap law observed in aprotic solvents, (b) molecules whose fluorescence exhibited substantial red shifts, and (c) molecules whose fluorescence were barely affected by the addition of ethanol (K SV < 1 M -1 ) and for which the energy-gap dependences on radiationless deactivation in protic solvents were not so different from those in aprotic solvents. Typical fluorophores for each case, i.e., a, b, and c, were aminoanthraquinone, aminophthalimide, and aminocoumarin, respectively. Differences in the fluorescence quenching phenomena are discussed in terms of the molecular structure and the hard-soft anionic character of the excited states, governed by changes in charge density on the carbonyl oxygen. An excited molecule having a hard anionic character on a specific site within the molecule, classified as group a, was concluded to undergo considerable fluorescence quenching through an intermolecular hydrogen bonding interaction with an alcohol having a hard cationic character. On the other hand, fluorescence of an excited molecule having a soft anionic character, classified as group c, cannot be quenched well by an alcohol because of the weak interaction on the carbonyl oxygen. The anomalous behavior of the excited aminophthalimides (group b), which are classified as hard anions but do not undergo fluorescence quenching, suggested the possibility that molecular rigidity is another factor controlling the radiationless deactivation process induced by hydrogen bonding.
Solvent Effects on Hydrogen BondsA Theoretical Study
The Journal of Physical Chemistry A, 2002
Hydrogen-bonded interactions in the acetic acid dimer and in complexes formed by acetic acid with acetaldehyde, acetamide, ammonia, methanol, and phenol and in corresponding complexes between the acetate anion and the same ligands as before were studied in the gas phase and in solution by means of quantum chemical DFT/BLYP calculations. Three solvents (heptane, DMSO, and water) of largely varying polarity were chosen. The polarized continuum model was used for the description of the solvent. Optimized geometries, reaction energies, and Gibbs free energies of complex formation were computed. In the neutral complexes an opening of the weaker of the two hydrogen bonds formed in the complex is observed with increasing polarity of the solvent. This opening is interpreted by the creation of optimal conditions for separate solvation of the subsystems of the hydrogen bond in competition with the geometrical requirements for the formation of this bond. Even though almost all reaction energies are found to be negative, only the strongly bound complexes, acetic acid dimer, and acetic acid-acetamide are stable according to Gibbs free energy results. The main factors for this finding are the entropy loss on the formation of the bimolecular complex and the changes of the free energy of solvation. Solvation effects are interpreted in terms of dipole moments, solvent-accessible surfaces, and cavity volumes of the separate molecules and of the complexes.
Chemical Physics, 2012
In this paper, we report the contrasting pattern of hydrogen bonding between solvents and 3-(phenylamino)-cyclohexen-1-one (PACO), an intramolecular charge transfer (ICT) molecule in the ground and excited states. The uniqueness of this molecule has been revealed through linear free energy relationship based Kamlet-Taft analysis which indicates that the polarizability (p ⁄) and the hydrogen bond acceptor abilities (b) of the solvent are mainly responsible for the observed absorption spectra of the probe while polarizability (p ⁄) and the hydrogen bond donor abilities (a) of the solvents mainly determine its emitting profile. This investigation helps us to decipher the ground and excited state behavior of the hydrogen bonding sites present in PACO. These findings are also expected to be useful in understanding the nature of other molecules containing multiple H-bonding sites.
The Role Played by Orbital Energetics in Solvent Mediated Electronic Coupling †
The Journal of Physical Chemistry A, 2002
Electron-transfer rates are measured for three supramolecular species, which contain an electron donor, electron acceptor, and rigid connecting bridge. Two of the species are linear and the third species is C-shaped. The latter topology produces a 10 Å wide, solvent accessible gap between the donor and the acceptor units. This molecular design allows the dependence of the electron-transfer rate on the solvent's electronic character to be evaluated. The results display a strong correlation between the energy of the solvent's lowest unoccupied molecular orbital and the magnitude of solvent mediated electronic coupling in systems with electronically excited donors. The variation of the electronic coupling with solvent modulates transfer rate constants by more than an order of magnitude. † Part of the special issue "Noboru Mataga Festschrift".
The Journal of Chemical Physics, 2003
The electronic and solvation structures of N,N-dimethylaniline in acetonitrile are examined by means of the ab initio reference interaction site model self-consistent-field theory coupled with the method to evaluate nonequilibrium solvation free energy developed by Chong et al. ͓J. Phys. Chem. 99, 10 526 ͑1995͔͒. The key quantities characterizing the solvation process-the free-energy profile governing the solvent fluctuations and solvent reorganization-are evaluated from first principles. A new scheme, which enables us to partition solvent reorganization into atomic contributions in the solute molecule, is proposed and used to analyze the process at the atomic level. We found that the linear response approximation holds well and the overall observable s is not much affected by solute geometry, while the individual atomic contribution is significantly changed, especially by the wagging motion of the amino group.