Lowest energy excited state substitutions

Two types of aromatic carbonyl compound give rise to lowest energy excited states not having (n, ti ) character. Amino substituted benzo-phenonra, especially, para-sul tituted compounds, form excited states which are neither (n, n ) nor (n, n ) in character, but which are better described as charge transfer states. A typical case is provided by p-aminobenzophenone whidi yields an excited state structure as shown ... 

The use of substituents with the Ru(II) system made possible more dramatic modification of quantum yields. In this case, however, the modifications can be attributed to changes in the order of excited states, specifically creating a situation where a substitution unreactive MLCT state becomes the lowest energy excited state. For these systems, it would appear likely that some new reaction pattern might prove characteristic of the MLCT state. 

The photochemical ligand substitution reaction of la was investigated by ultrafast TR-IR spectroscopy (Fig. 16) 51). An acetonitrile solution of la was irradiated by a 266-nm laser pulse ( 150 fs pulse width). A broad IR absorption band which was attributed to the reaction products in higher vibrational excited states was produced within 1 ps after the laser flash. The broad band sharpened and a vqo peak at 1828 cm of the reaction product was observed in the 50- to 100-ps duration. This time scale is much shorter than the decay of the lowest MLCT excited state (right-hand side of Fig. 16). The TR-IR results indicate that this photochemical reaction proceeds from higher vibrational states or high-energy electronic excited states instead of the lower vibrational excited states of MLCT and thermal accessible states from MLCT such as the LF state. 

The two-photon excitation spectrum of 22h taken as a neat film shows a broad peak with an onset of 1.32 eV and a maximum at 1.63 eV [391]. The larger TP excitation energy (1.32 eV 2hco = 2.65 eV) compared to OP excitation (2.34 eV) implies that the TP transition has a different selection rule than the OP transition. This is similar to behavior obtained in solution [384], A similar excitation pattern as disclosed in Figure 3.2a was obtained for such polymeric materials that is, the TP excited state possesses a higher excitation energy in comparison with the lowest OP excited state. Thus, the results obtained for diphenylpolyenes with more than two ethylene moieties, in which the Sj state is assigned to the TP excited state, are considered as an exception for chromo-phores with no donor substitution pattern [29]. 

Section 9.3 applied perturbation theory to the ground state of the helium atom. We now treat the lowest excited states of helium. The unperturbed energies are given by (9.48). The lowest unperturbed excited states have ] = 1, 2 = 2 or n, = 2, 2 = 1. and substitution in (9.48) gives... 

Azobenzenes do not emit noticeably at room temperature, but all three types show fluorescence in strong acids at 77K." Modern experimental techniques, however, allow one to study the very weak fluorescence of azobenzene and azobenzene functionalized molecules even at room temperature. Corner et al. observed a transient after laser flash photolysis of donor/acceptor-substituted azobenzenes which they assigned to the lowest energy triplet state of the E-isomer. Recent studies were directed at examining the behavior of the transient absorption of the singlet excited states of both geometric isomers with respect to time (see Section 89.4). 

A direct observation of a triplet state of substituted trimethylene biradical has recently been claimed.84) This is in good agreement with the prediction 104,105) that the lowest excited state of cyclopropane will have an energy minimum at a geometry in which one C—C bond is broken. 

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