S2 Excitation Energies

Ion pair formation competes with internal conversion from the S2 state in the chloranil-diphenylamine complex 78 and photoionization of 2,2 -bipyridine in aqueous solution involves multiphoton excitation of Ti and Si states.Monophotonic ionization from the vibrationally unrelaxed excited singlet state of N,N,N, N -tetraraethyl-p-phenylenediamine in acetonitrile shows ionization increases with energy in the Si state and slightly decreases for excitation around the S2 excitation energy level. 

The convergence of the linear polyene Sq to S2 excitation energy to a finite limit as the length of the conjugated chain increases is strong evidence that there is bond length alternation in the long chains. This is supported by the limited number of direct structure determinations that have been published. 

Dependence on Local Polarizability. Tlie solvent shift behavior of the S2 excitation energy for diphenylpolyenes with 1-6 and 8 double bonds in the polyene chain [65,75] are... 

Tables 11-6, 11-7, and 11-8 show calculated solvatochromic shifts for the nucle-obases. Solvation effects on uracil have been studied theoretically in the past using both explicit and implicit models [92, 94, 130, 149, 211-214] (see Table 11-6). Initial studies used clusters of uracil with a few water molecules. Marian et al. [130] calculated excited states of uracil and uracil-water clusters with two, four and six water molecules. Shukla and Lesczynski [122] studied uracil with three water molecules using CIS to calculate excitation energies. Improta et al. [213] used a cluster of four water molecules embedded into a PCM and TDDFT calculations to study the solvatochromic shifts on the absorption and emission of uracil and thymine. Zazza et al. [211] used the perturbed matrix method (PMM) in combination with TDDFT and CCSD to calculate the solvatochromic shifts. The shift for the Si state ranges between (+0.21) - (+0.54) eV and the shift for the S2 is calculated to be between (-0.07) - (-0.19) eV. Thymine shows very similar solvatochromic shifts as seen in Table 11-6 [92],...

Figure 1 Jablonski diagram showing energy levels and transitions F, fluorescence C, chemiluminescence P, phosphorescence CD, collisional deactivation IC, internal conversion ISC, intersystem crossing S0, ground singlet state S1( S2, excited singlet states Tl5 excited triplet state.

J. Li, C. J. Cramer, and D. G. Truhlar, A two-response-time model based on CM2/INDO/S2 electrostatic potentials for the dielectric polarization component of solvatochromic shifts on vertical excitation energies, Int. J. Quan. Chem. 77 264 (2000). 

Structural formulas of the ZP and ZP-I dyads are shown in Fig. 1. The absorption spectra of ZP-I dyads indicate that the excitation energies of the ZP S2 and S, states are practically the same throughout the ZP-I series. The peak position of the Soret absorption band of the ZP-I dyads are only slightly red shifted from that of free ZP, and are practically the same as that of ZnAr3P. These results indicate a rather weak D-A electronic interaction in the ground state of ZP-I systems despite of their directly linked structure, which can be ascribed to the nearly perpendicular conformation between ZP and I molecular planes [2],... 

Figure 4.25 Jablonski diagram of the heavy atom effect on photochemical reactivity. If excitation to S2 (hv2) is followed by intersystem crossing (isc) to T2, the quantum yield of reaction R decreases at higher excitation energies, ic = internal conversion, a = absorption, f = fluorescence, p = phosphorescence...

Fig. 1.11. (cont.) The Si and S2 potential energy surfaces have been calculated by Nonella and Huber (1986) and Suter, Briihlmann, and Huber (1990), respectively, whereas the PES for the So state is approximated by the sum of two uncoupled Morse oscillators. The shaded circles indicate the equilibrium region of the ground electronic state where the dissociative motion in the excited electronic states starts and the heavy arrows illustrate the subsequent dissociation paths. Detailed discussions of the absorption spectra and the vibrational state distributions of NO follow in Chapters 7 and 9. 

O - N vibrational levels of cluster S2 excitation using eq. (5-11). Note that the vibrational energy in the cluster is never sufficient to excite the OH stretch, either directly or indirectly. 

Coulson has recently discussed the bond angles in these molecules in a review paper,550 using two simple models. An ionic model would predict these molecules to be linear, but in a second model, in which valence states arising from s2- sp, s2- sd excitations were permitted, the first would lead to linear, the second to bent structures. From a consideration of the excitation energies, it was shown that when the central atom is heavy and the attached halogens are as electronegative as possible, the bent situation is favoured, in agreement with the experimental results. 

Figure 8-3. LIIC between the minimum of the S, state and the S, /S0 MXS in (a) aminopyrimidine and (b) pyridone. Calculations performed at (a) SA-3-CASSCF(8,7)/6-31G and (b) SA-3-CASSCF(10,8)/ 6-31G levels. Open symbols in (a) show the equivalent curves in 9H-adenine according to [7]. The S j and S2 vertical excitation energies are indicated at the left side of each graph...

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