Second solvatochromism

When Ajx = /positive solvatochromism resulting in the red shift of transitions otherwise, when A/r is negative, the spectra move to the blue and that is the case of negative solvatochromism. As follows from this expression, the change of dipole moments A/r during electronic transition is a necessary condition for observed solvatochromic shift the more this difference -the stronger is the solvatochromic shift. The second important parameter is the local... 

It is also worth noting that, due to their dipolar and conjugated donor-acceptor nature, all these amino-terminated Group 6 metaUacumulenes exhibit a strong negative solvatochromic effect and they show significant second-order NLO properties [68]. 

As mentioned in Section 4, the analysis of rate data resulting from unimolecular reactions is considerably easier than the analysis of such data for bimolecular reactions, and the same is true for pseudounimolecular reactions. Kinetic probes currently used to study the micellar pseudophase showing first-order reaction kinetics are almost exclusively compounds undergoing hydrolysis reactions showing in fact pseudofirst-order kinetics. In these cases, water is the second reactant and it is therefore anticipated that these kinetic probes report at least the reduced water concentration (or better water activity in the micellar pseudophase. As for solvatochromic probes, the sensitivity to different aspects of the micellar pseudophase can be different for different hydrolytic probes and as a result, different probes may report different characteristics. Hence, as for solvatochromic probes, the use of a series of hydrolytic probes may provide additional insight. 

Transition moments, ioi> to the first excited state can be calculated from the integrated absorption of the linear electronic spectrum. This can be used to calculate -Af poi n the first term (defined here as yc) of the two-level model. The second term (Af p.oiAjioi m) from Equation (12) (defined here yn) involves Ajioi, which can be determined directly from solvatochromism( 19), or from a two-level analysis of a molecular EFISH measurement of p. 

R. Cammi and J. Tomasi, Nonequilibrium solvation theory for the polarizable continuum model - a new formulation at the SCF level with application to the case of the frequency-dependent linear electric-response function, Int. J. Quantum Chem., (1995) 465-74 B. Mennucci, R. Cammi and J. Tomasi, Excited states and solvatochromic shifts within a nonequilibrium solvation approach A new formulation of the integral equation formalism method at the self-consistent field, configuration interaction, and multiconfiguration self-consistent field level, J. Chem. Phys., 109 (1998) 2798-807 R. Cammi, L. Frediani, B. Mennucci, J. Tomasi, K. Ruud and K. V. Mikkelsen, A second-order, quadratically... 

This technique has been applied to organic compounds where charge transfer is dominated by one transition this is not often the case for organometallics. The applicability of this technique to organometallics thus far has not been tested there are no reports where solvatochromism has been used to examine the second-order nonlinearities of transition-metal organometallics, and only one report of its application to two organoboron compounds.30... 

Therefore, we have quoted second-order polarizabilities in terms of /3o of p-nitroaniline in dioxane (A a = 354 nm, /3o = 13.5 Cm T convention, relative to quartz dn = 0.5 pm/V at 1064 nm). p-Nitroaniline is a truly one-dimensional NLO-phore with one significant component as has been verified experimentally by depolarized EFISHG (Wortmann et al, 1993). If different standards and conventions are taken into account, the values measured by different groups are quite consistent. Note that the intrinsic /3o of p-nitroaniline depends on the solvent, even when normalized for the solvatochromic shift of the CT absorption. We have chosen the lowest intrinsic /3o it is higher by a factor of 1.6 in very polar solvents (see p. 183). Also note that /3 values from HRS measurements of molecules with several significant tensor elements will not allow a true comparison of... 

Several conclusions can be drawn from Table 3. First, in accordance with the two-state model, /So and jSj all increase with decreasing HOMO-LUMO gap. Second, the intrinsic second-order polarizability of p-nitroaniline is increased by two-thirds when the solvent is changed from p-dioxane to methanol or A-methylpyrrolidone, even when the values are corrected for the differences in (A ). As we have adopted the value for p-nitroaniline in dioxane as a standard, it should therefore be noted that molecules that truly surpass the best performance of p-nitroaniline should have a second-order polarizability of l. p-nitroaniline (dioxane). As a third conclusion, there is a poor correlation between and the static reaction field as predicted by (91). This is in part due to the fact that the bulk static dielectric constant, E° in (89), differs from the microscopic dielectric constant. For example, p-dioxane has long been known for its anomalous solvent shift properties (Ledger and Suppan, 1967). Empirical microscopic dielectric constants can be derived from solvatochromism experiments, e.g. e = 6.0 for p-dioxane, and have been suggested to improve the estimation of the reaction field (Baumann, 1987). However, continuum models can only provide a crude estimate of the solute-solvent interactions. As an illustration we try to correlate in Fig. 7 the transition energies of p-nitroaniline with those of a popular solvent polarity indicator with negative solvatochromism. 

This approach to separating the different types of interactions contributing to a net solvent effect has elicited much interest. Tests of the ir, a, and p scales on other solvatochromic or related processes have been made, an alternative ir scale based on chemically different solvatochromic dyes has been proposed, and the contribution of solvent polarizability to it has been studied. Opinion is not unanimous, however, that the Kamlet-Taft system constitutes the best or ultimate extrathermodynamic approach to the study of solvent effects. There are two objections One of these is to the averaging process by which many model phenomena are combined to yield a single best-fit value. We encountered this problem in Section 7.2 when we considered alternative definitions of the Hammett substituent constant, and similar comments apply here Reichardt has discussed this in the context of the Kamlet-Taft parameters. The second objection is to the claim of generality for the parameters and the correlation equation we will return to this controversy later. 

---