1B„ states

Koyama, Y., F. S. Rondonuwu, R. Fujii, and Y. Watanabe. 2004. Light-harvesting function of carotenoids in photosynthesis The roles of the newly found 1B state. Biopolymers 74 2-18. 

Cook (Ref 1), in describing thermal decomposition of some HE s conducted in the quartz spring apparatus (described in Ref 1, p 175 and shown there in Figs 8.1a 8.1b), stated that PETN, RDX, Tetryl and to a small extent TNT decomposed autocatalyti-cally. EDNA followed the first-order decomposition law only until about 5% of the explosive had decomposed and then the reaction stabilized. The term autostabilization was applied here on the supposition that one of the condensed decomposition products of EDNA which accumulated in the explosive apparently tended to stabilize the bulk of expl and thus slow down the decomposition. After about 10% of the expl had decompd, however, the "autocatalysis developed. 

Table 3. One and two-photon thresholds (in eV) of trans-polyenes, CnHn+2, in alkane matrices[58] and in the Pariser-Parr-Pople model[21]. The solid-state shift of 0.40eV of the ionic 1 1B state yields gas-phase values up to n = 12 covalent 2M+ shifts of 0.05eV are neglected.

At present the basic red shift and the accompanying solvent effects have been extensively investigated only as far as the lowest 1B+ state of PRSB is concerned. Theoretical calculations predict a decrease in the energy of the "cis" - -Aj state upon protonation which is smaller (121) or comparable (126) to that of the - -bJ state. The latter prediction seems to be consistent with the 340-nm location of band II for PRSB in solution (127,128). 

Rosenfeld et al. (144) have recorded the absorption spectrum of the fluorescent states of retinol, retinyl acetate, and retinyl-n-butylamine, using pulsed laser photolysis. Theoretical calculations (145) have closely reproudced the observed 435-nm band assuming that emission originates from the lowest - Ag state. Unfortunately, these results are not discriminative as far as identification of the lowest singlet state is concerned, since a strong absorption in the same region is also predicted for the 1B+ state (145). 

As discussed in Section 8.2, superexcited states, AB, can decay by both autoionization and dissociation (more specifically, by predissociation). Decay by spontaneous fluorescence can be neglected for superexcited states because, generally, the predissociation or autoionization rates (l/rnr 1012 to 1014s-1) are much faster than the fluorescence rate (l/rr < 108s-1). Only two examples of detected spontaneous fluorescence from superexcited states have been reported (for H2, Glass-Maujean, et ai, 1987, for Li2, Chu and Wu, 1988). The H2 D1 e-symmetry component is predissociated by an L-uncoupling interaction with the B 1B+ state (see Section 7.9 and Fig. 7.27). Since a 4E+ state has no /-symmetry levels, the /-components of the D1 A-doublets cannot interact with the B E+ state and are not predissociated. The v = 8 level of the D1 state, which lies just above the H/ X2E+ v+ = 0 ionization threshold, could in principle be autoionized (both e and / components) by the X2E+ v+ = 0 en continuum. However, the Av = 1 propensity rule for vibrational autoionization implies that the v = 8 level will be only weakly autoionized. Consequently, the nonradiative decay rate, 1 /rnr, is slow only for the /-symmetry component of the D1 v = 8 state. Thus, in the LIF spectrum of the D1] —... 

Intersystem crossing (ISC) and formation of TE from the lowest 1B state to the lowest state in the triplet manifold, the 1 B state. ISC rates fcjsc determined for conjugated polymers are found in the range of 10 —10 s [2]. 

Thus, a particle-hole excitation from the HOMO to the LUMO must have overall odd symmetry. This is the state. The first Ag excitation (the 2Ag state) will be HOMO—1 to LUMO (or, equivalently HOMO to LUMO+1). Such an excitation will lie higher in energy than the 1B state.These transitions are shown in Fig. 3.8. 

Pairs of solitons are the natural excitations from the ground state. This is shown in Fig. 4.6, which shows the bond dimerization of the 1B state, obtained by iterating eqn (4.21). The bond dimerization fits the functional form (Brazovskii and Kirova 1981 Campbell and Bishop 1981),... 

Fig. 4.11. Probability distribution functions of the soliton defects in the noninteracting limit on a 102-site chain for the 1B state. Left defect, or soUton (filled symbols), right defect, or antisoliton (open symbols) extrinsic dimerization, = 0 (circles), Je = 0.1 (squares) and A = 0.1.

These data indicate more clearly the problems theoretical methods (TDDFT in this case) have in accovmting for the change of electronic structure upon excitation. For the ionic La states of the aromatic compounds (using Platts nomenclature derived from the perimeter model, see. Ref. 9 e.g.) and the 1B state of the polyene, a systematic underestimation of the excitation energies is observed while the opposite is true for the other more covalent states that exhibit stronger multiconfigurational character (for a more detailed discussion of these problems see Refs. 35 and 36). 

A weak first absorption is found experimentally (AE° ° = 3.59 eV) corresponding to the electric dipole-forbidden lAg 2Ag transition. The second (intense) band results from the allowed lAg 1B transition (AE ° = 4.41 eV). This situation is challenging for any theoretical method because the character of both states is very different (single-configurational, ionic 1B state vs. multiconfigurational, covalent 2Ag state with significant contributions from double excitations). 

For short oligomers, the contribution from non-linear terms could be rather large, so that predictions of convergence or crossover of the Ag and Bu states based on 1/n-type extrapolations from short oligomers should be treated with caution. The theoretical work of Mazumdar et al. [65] and also experimental studies on carotenes [66, 67] suggest that the 2Ag state may only be weakly coupled to the 1B state and... 

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