Experimental Excited-State Dipole Moments

Experimentally, these effects are tested by fluorescence and absorption measurements. These directly probe solvent polarization dynamics on molecular time-scales [100, 101]. For instance, the time resolved fluorescence spectrum of a chromophore, whose excited state dipole moment is subject to interactions with the surrounding solvent molecules, will exhibit fluorescence spectra that are strongly solvent dependent. The solvent molecules attempt to compensate the changes of charge density in the chromophore and, in sum, the fluorescence... 

The experimental data reported in the Table for gas phase have been extracted from measurements in dioxane solution by applying the Onsager reaction field model to eliminate the solvent effect [37], By contrast, the cyclohexane experimental dipole moments have been obtained from those reported in Ref. [37] re-including the proper reaction field factors. Once recalled these facts, we note that the observed solvent-induced changes on both ground and excited state dipole moments are quantitatively reproduced by the calculations. 

In the calculation of the first excited singlet-state dipole moments, the a moment was assumed to be the same as in the ground state and only changes of the it moment were considered. The authors also found a relatively good agreement between the calculated excited-state dipole moments and the average values of the experimental excited-state dipole moments (Table 5) [17]. For comparison, another set of calculated excited-state dipole moments is presented in Table 5 [25,91,92]. 

The two-state model is also applied to determine the first-order hyperpolarizabilities based on the experimental measurements of the spectroscopic quantities [103]. The ground state and the CT excited state dipole moments, excitation energy, as well as transition dipole moment (or oscillator strength) can be determined through the solvatochromic effect measurements. In particular, the first-order hyperpolarizability can be obtained in such a way by employing Eq. (4) or Eq. (6a). 

Change in the dipole moment and polarizability upon electronic excitation was determined for various linear polyenes by adopting electric field-induced changes in the optical absorption spectrum [70]. Polyenes studied (Exhibit 2) were diphenylbutadiene (DPB 7), diphenylhexatriene (DPH 8), diphenyloctate-traene (DPO 9), diphenyldecapentaene (DPD 10), and all-tranj-retinal (3). Exhibit 2 describes the experimental values determined. It is proposed that (based on the results obtained) the excited state dipole moments determined on these polyene systems have a role to play in the mechanism of trans-cis photoisomerization. 

In general, experimental methods for the determination of excited-state dipole moments are based on experimental ground-state dipole moments and a change of the position of a spectral band (in an electronic spectnnn) caused by an electric field which can be external (electro-... 

The number of calculated excited-state dipole moments is still relatively limited (see the references in our publications mentioned above) and, because of the numerous approximations and problems related concerning experimental excited-state dipole moments, the agreement between the experimental and calculated values is often poor. 

In principle, experimental and calculated triplet excited-state dipole moments can be obtained in the same fashion as singlet excited-state dipole moments. However, the number of studies in this area is limited (references [17,162,190-193]) can serve as examples. 

Additional work in the area of excited-state dipole moments is needed, to make the agreement between experimental and theoretical values better and the values more reliable than they are today. However, the results obtained so far seem to be promising. 

An alternative means for the experimental evaluation of //da is by means of electroabsorption spectroscopy. " The electric field dependence of the absorption spectrum can be used to obtain the difference in the ground state and excited state dipole moments, A/UdaI- This may be combined with the transition dipole moment (in Debyes), l gel = 16[Cda/(108 x 10 da)]. to obtain (neglecting vibronic and nonresonance overlap contributions), ... 

We have applied the Ito technique to 4,4-dicyano-l,2-diphenyltriafulvene (VI) and have found a linear relationship between A v and D — ) D + 2). If the dipole moments in the ground and excited states are assumed parallel, and if a, the cavity radius is assumed to be 4 A, and taking the experimental dipole moment in the ground state as 7.9 D, the excited state dipole moment of the triafulvene is calculated to be 5.5 D. Once again, this quantitative result should be treated very cautiously, in view of the many approximations involved. But qualitatively, there is no doubt that tiedipole moment decreases appreciably. 

The basic problem of these models is, of course, related to the oversimplified cavity shape, as evidenced in Fig. 17.5, which compares spherical and van der Waals cavities for a typical dye. A quite successful empirical recipe was suggested by Kawski et al. who showed that if the condition a/a = 1/2 is verified (a being the isotropic polarisability of the solute), the specific choice of the solute cavity shape (sphere or ellipsoid of revolution) affects negligibly the value of the excited state dipole moment. It is worth noting that such a condition occurs quite commonly [193-197] and for instance for phtalimide and stilbene derivatives the experimental values found for the radius and the polarisability are in a range from 0.41 to 0.62 [193], and from 0.53 to 0.68 [198, 199], respectively. 

The second experimental measure of the orientation of the dipole moment in the ground state of the bases was made by Seibold and Labhart405 for uracil and thymine. The direction of the ground state dipole moment with respect to the transition moment in the lowest excited singlet state has been determined from the influence of an... 

Dipole moments have been measured in the ground and excited states for three derivatives of compound (1) <83CPL58>. Ground-state dipole moments are shown in Figure 4. The ground-state dipole moment of the parent structure (1) has been found experimentally to be 4.87 x 10-30 C m (1.46 D) <81JPC3181>. 

To determine the experimental first excited singlet-state dipole moments of the BPHTs, the authors established the Bakhshiev and Kawski-Chamma-Viallet solvatochromic equations, as described in previous works ([25] and references therein) (Table 5). For non-fluorescent compounds 7 and 8, no excited singlet-state dipole moment value could be obtained. 

As an example, the experimental and calculated first excited singlet-state dipole moments for selected fluorescent quinazolines are presented in Table 3, with the ground-state values included for comparison [120], For most compounds, the agreement is good or at least acceptable. 

Experimental and calculated ground and first excited singlet-state dipole moments of selected quinazolines ... 

Abs absoiption transitions of cytosine in water experimental data for keto-N3H and imino tautomers correspond to 3-methylcytosine and 3-methylcytidine, respectively, obtained in aqueous solution at pH 11 [279], °ground state dipole moments of keto-N3H, imino and enol tautomers at the HF/6-311G(d,p) level are 8.15, 5.12 and 3.34 Debye, respectively, scaled (scaling factor 0.72) excitation energies, Rydberg contamination. 

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