Solvatochromism upon excitation

Mukherjee, S., Chattopadhyay, A., Samanta, A. and Soujanya, T. (1994). Dipole-moment change of Nbd group upon excitation Studied using solvatochromic and quantum-chemical approaches - Implications in membrane research. J. Phys. Chem. 98, 2809-2812. 

When only nonspecific effects are responsible for the solvatochromic behavior, the change of the molecule dipole moment upon excitation may be estimated from a continuum model. 

If, as is more common, the dipole moment decreases in the excited state (or changes direction) the resulting blue shift, which increases with increasing solvent polarity, will most probably cause an overall blue shift in the absorption, compared to the gas phase. In some complexes there will be a change in orientation of the dipole moment upon excitation, which can also give rise to negative solvatochromism. 

Solvatochromic shifts are rationalized with the aid of the Franck-Condon principle, which states that during the electronic transition the nuclei are essentially immobile because of their relatively great masses. The solvation shell about the solute molecule minimizes the total energy of the ground state by means of dipole-dipole, dipole-induced dipole, and dispersion forces. Upon transition to the excited state, the solute has a different electronic configuration, yet it is still surrounded by a solvation shell optimized for the ground state. There are two possibilities to consider ... 

Prabhumirashi and Kunte [181] have proposed a new procedure employing Bakhshiev s equation for solvatochromic frequency shifts for excited-state dipole moments and specific solute -solvent interaction energies based on absorption spectra only, without using emission spectra. Suppan [182] has expanded upon the solvatochromic shift method and discussed the effect of the medium on the energies of electronic states and Ghoneim and Suppan [183] discussed solvatochromic shifts of non-dipolar molecules in polar solvents. 

An example of solvent-induced solvatochromic shifts (calculated at a characteristic snapshot from the MD trajectory for each solvent) on different electronic excited states is shown in Fig. 5.5. Inspection of this plot reveals that the electronic states with the dipole moments that are larger than the dipoles in the ground state (shown as solid red curves in Fig. 5.5) become increasingly stabilized (red-shifted) in polar solvents. For example, l Ai, l Bi, 2 Bi states, which dipoles are larger than in the ground state dipole (7.7 Debye), demonstrate systematic red shifts upon solvation. The red shift increases in more polar solvents (in the order of c-hexane, dioxane, and water). The most dramatic red shift is experienced by the experimentally observed l Ai charge-transfer state with the (gas-phase) dipole moment of 12.9 D. It is quite intriguing that this state (the lowest red state in Fig. 5.5) is only the third lowest excited state in the gas phase but becomes the lowest excited state in water. On the... 

Both absorption and emission electronic transitions are acceptably described by our scales, as shown by a recent study on the solvation of a series of probes containing an intramolecular hydrogen bond, so no further comment is made here other than the following even if one is only interested in evaluating the change in dipole moment upon electronic excitation via solvatochromic analysis, a multi-parameter analysis must be conducted in order to isolate the shifts corresponding to the pure dipolar effect of the solvent. ... 

Solvatochromism has its origin in changes in both dipole moment and polarizability of the dye upon electronic excitation provoking differential solvation of the ground and excited states. The dipole moment, p.e> of the excited state can be either smaller or larger than the ground state value ig. In the former case one speaks about a negatively solvatochromic... 

Solvatochromism, which is generally considered as indicative of intramolecular charge transfer upon electronic excitation and as a basic requirement for high NLO responses, is not observed, however. 

Fluorophores capable of CHEF and other types of environment-sensitive (or solvatochromic) fluorophores have been utilized in sensors for kinase activity. Chelation-sensitive fluorophores manifest altered fluorescent properties upon chelation of various metal ions, while environment-sensitive fluorophores exhibit altered excitation and emission properties with changing environment, such as solvent polarity (Figure 1.1). 

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