Singlet excitation energy

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

Figure 4. The INDO/S first singlet excitation energies of polysilanes, (SiH2)n full line, all trans broken line, all gauche.

Table I) is due to the so1ute-s01vent interactions that result in solvent induced shifts of the singlet-singlet excitation energies readily observable in solution absorption s p ec t r a (J 2). As expressed in... 

Tab. 4.5 Singlet excitation energies of a small subset of psoralen compounds. Experimental data in aqueous solution given in parentheses [99]. Experimental absorption coefficients are denoted w=weak (<8000), i = intermediate (8000-15000), and s = strong (>15000).

Interestingly, the flavin molecule is significantly easier to excite than the other heterocycles investigated. The first singlet excitation energy is only 3.0 eV (413 nm), and the probability of transition is intermediate. The second band with possible singlet excitation lies at 3.8 eV. Hence, if one intends to construct systems that are more readily excited, substituted flavins seem to be a more appropriate route than the furocoumarins, whereas the latter are easier to ionize. 

Figure 2. Calculated (INDO/S) first singlet excitation energy all-trans [(CHj)2Si]n (dot-dash) all-Irons [H2Si]n (full) all-gauche [H2Si]n (dashed).

M. Kupryszewska, I. Gryczynski, and A. Kawski, Intramolecular donor-acceptor separations in methionine- and leucine-enkephalin estimated by long-range radiationless transfer of singlet excitation energy, Photochem. Photobiol. 36, 499-502 (1982). 

In the second case, the HEI is formed in one or several reaction steps which are rate-limiting and the interaction of the ACT with this intermediate occurs in a rapid, kineticaUy unobservable, reaction step. Therefore, kinetic data cannot provide evidence of the occurrence of this interaction (Scheme 5). The classical example of this type of activated CL is the peroxyoxalate reaction , for which an electron transfer between a HEI and the ACT has already been postulated by Rauhut (see Section VI.C). In an elegant experiment, Lechtken and Turro confirmed the occurrence of an activated CL process in this reaction, using energy acceptors with different singlet excitation energies (Eg)... 

The condition for transfer of singlet excitation energy from a fluorophore (donor) to another chromophore (acceptor) to occur is spectral overlap of the donor s fluorescence emission spectrum with the acceptor s absorption spectrum. The efficiency of energy transfer E is defined in eq 15... 

The heart of photosynthesis is the absorption of sunlight and the subsequent conversion of singlet excitation energy to chemical potential energy in the form... 

On the other hand, development of the xc kernel beyond ALDA does not necessarily bring better results. To illustrate this point, the singlet-singlet excitation energies 0)ks calculated in [24] for the He atom with the combination (accurate vxc)/ALDA are compared in Table 2 with (Ok calculated in [46] with the accurate vxc and with the spatially non-local kernel /ffI(TD0F P)(ri, r2), which is a part of the exchange-only kernel of the time-de-pendent optimized effective potential method (OEP) [47]. They are also compared with the TDDFRT zero-order estimate 0)kso, the difference (10) of the orbital energies obtained with the accurate vxc. 

Table 2 Helium singlet excitation energies (in Hartrees) ...

This molecule consisted of two photochromes a fulgimide and a dithienylethene. Both of them could isomerize with UV light from the open form to the closed form. And each photo-unit was thermally stable in either isomeric state. Moreover, the photoisomerization of fulgimide could efficiently transfer singlet excitation energy... 

Table 7 contains the experimental and calculated excitation energies and assignments for Ni(CO)4 [10, 42, 64, 73, 74]. Difficulties with the choice of the active space reduces the accuracy of the CAS calculations but both TDDFT and SAC methods predict four dipole allowed excited states in the 4.0-6.5 eV range which is in agreement with the experimental results. All low-lying singlet excitation energies for TDDFT and SAC-CI calculations are presented in Table 8. 

Temperature Dependence of Singlet Excitation Energy Migration in Liquid Benzene as Revealed by Picosecond Laser Photolysis... 

Thus, due to the existence of the electron repulsion term — 2K,i the singlet excitation energy is only about half the orbital energy difference, and the exchange interaction is about one third of the Coulomb interaction J. ... 

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