First excited singlet state

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

This dialog box also contains the option for specifying that the molecular system is in the first excited singlet state (Next lowest) or the Lowest electronic state. 

The first excited singlet state, 2 Sq, is also metastable in the sense that a transition to the ground state is forbidden by the Af selection rule but, because the transition is not spin forbidden, this state is not so long-lived as the 2 Si metastable state. 

Discuss, briefly, the valence molecular orbitals of AIH2 and the shape of the molecule in the ground and first excited singlet states. 

One characteristic property of dyes is their colour due to absorption from the ground electronic state Sq to the first excited singlet state Sj lying in the visible region. Also typical of a dye is a high absorbing power characterized by a value of the oscillator strength/ (see Equation 2.18) close to 1, and also a value of the fluorescence quantum yield (see Equation 7.135) close to 1. 

Fig. 1. Schematic energy-level diagram for a dye molecule. Electronic states Sq = ground singlet state = first excited singlet state S2 = second excited singlet state Tj = first excited triplet state T2 = second excited triplet state EVS = excited vibrational states. Transitions A = absorption excited states ...

I won t give you the results in full, but on matrix diagonalization we find a deseription of the first excited singlet state as... 

Calculations account for the experimental results (Fig. 20) (99MI233). The first excited singlet state (which accounts for the absorption at 269 nm calculated value 267 nm) was converted into the corresponding triplet state (69 kcal moF experimental 68 kcal moF ). The triplet state gives the cleavage of the S—N bond with the formation of the biradical intermediate. This intermediate leads to the product. 

Oxadiazole was obtained through the first excited singlet state. When the reaction was carried out in the presence of a triplet sensitizer, 99 was not detected but the quinazolinone 100 was obtained (Scheme 41) [91JCS(P2)187]. Compound 99 cannot be obtained via the Dewar isomer. The author supposed the formation... 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

Fig. 14 Schematic representation of the electronic transitions of photochemically excited substances Sq = ground state, Sj = first excited singlet state, T = forbidden triplet transition, N = ground state of a newly formed compound, A = absorption, F = fluorescence, P = phosphorescence.

Spectral Transparence Starting from 230 nm HN—< CH3 4-Toluidino p-Naphthoxy Substituent Photoactive from the First Excited Singlet State Polymers Photochemically Active from the First Excited Singlet State of the Phosphorus Substituents - " - "... 

Andreasson, J., Kodis, G., Lin, S., Moore, A.L., Moore, T.A., Gust, Martensson, J. and Albinsson, B. (2002) The gold porphyrin first excited singlet state. Photochemistry and Photobiology, 76,... 

It should be noted that no geometrical relaxation of the excited state geometry was considered in our calculations. Such relaxations may well occur to some degree and would then enhance the localization of the electronic excitation. However the relatively narrow spectral bandwidth suggests that the geometrical changes associated with excitation into the first excited singlet state are only minor. 

Figure 5.14. Geometry of (a) ground and (b) first excited singlet states of acetylene.

Figure 9.1. Potential energy diagram for the electronic states of ethylene N, ground state (w)2 r(3Biu) first excited triplet state (mr ) V, first excited singlet state (wir ) Z, two-electron excitation (w )2. For the ion C2H + R and R, Rydberg states, /, / ground and excited states. [From Ref. 2(c).]...

This is likely a [ 2, + 2J cycloaddition from the first excited singlet state, and suggests that this state either does not have the twisted geometry noted for... 

The positional specificity of these nucleophilic and electrophilic photosubstitutions can be readily understood by examination of the calculated charge densities of some of these molecules in their ground and first excited singlet states. For example, 4-nitrocatechol has the charge densities shown in (82) and (83).<139> Thus on this basis one would predict that 4-nitroveratrole... 

---