The Lowest Energy Excited Singlet State

TABLE 2. Ground state frequencies for linear polyene C-C and C=C stretch frequencies. 

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Hudson BS and Kohler BE (1973) Polyene spectroscopy the lowest energy excited singlet state ofdiphenyloctatetraene and other linear polyenes. J Chem Phys 59 4984-5002 Hudson BS and Kohler BE (1974) Linear polyene electronic structure and spectroscopy. Ann Rev Phys Chem 25 437-460 Hudson BS and Kohler BE (1984) Electronic stmcture and spectra of finite linear polyenes. Synthetic Metals 9 241-253 Hudson BS, Ridyard JN and Diamond J (1976) Polyene spectroscopy. Photoelectron spectra of the diphenylpolyenes. J Am Chem Soc 98 1126-1129 Hudson BS, Kohler BE and Schulten K (1982) Linear polyene electronic structure and potential surfaces. In Lim EC (ed) Excited States, Vol 6, pp 1-95. Academic Press, New York Jones PF, Jones WJ and Davies B (1992) Direct observation of the 2 Ag electronic state of carotenoid molecules by consecutive two-photon absorption spectroscopy. Photochem Photohiol A Chem, 68 59-75... 

Fluorescence from molecules in liquid or solid solution almost always originates from the lowest energy excited singlet state. If this state is dipole allowed from the ground state, the absorption and fluorescence spectra will overlap with a mirror image relationship as indicated in Figure 3. In the case of the linear polyenes the presence of a significant gap between the onsets of absorption and fluorescence contradicted the assumption that the... 

When the lowest energy excited singlet state residing primarily on the triple bonds (l B,) is examined, it is Woodward-Hoffman spin forbidden due to the Cj, symmetry of 80. However, this excited state of 80 was found to relax through a second order Jahn-Teller distortion to a 2 A state of C, symmetry. This state has two major reaction paths, deactivation to the Sq state or Bergman cyclization, either of which result in Bergman cycHzation products. 

Molecular orbital calculations on ethylene indicate that the lowest energy excited singlet and triplet states have a twisted geometry.(2) This geometry helps minimize electron-electron repulsion. Figure 9.1 gives the calculated... 

Whilst chlorophyll absorbs light over the entire visible spectrum, relaxation to the lowest lying excited singlet state is extremely rapid. Thus singlet energy transfer occurs via the SqSi (Qy) transition. Unless otherwise stated all comments made here on chi spectroscopy concern this transition. 

The name of this diagram (or plot) honors the pioneering contributions of Jablonski, who recognized that phosphorescence involves emission from a metastable triplet state lying at lower energy relative to the lowest lying excited singlet state. 

The lowest-energy excited singlet and triplet states are called S, and Th respectively. In general, Tj has lower energy than S,. In formaldehyde, the transition n —>, it (T1) requires... 

Table 15-2. Comparison of TDDFT BP86/cc-pVDZ vertical excitation energies (in wavelengths) and oscillator strengths, /, of the lowest-energy tt-jt (Lb) and the lowest-energy Tru singlet states of DMABN with experimental transient absorptions in the visible region (400-700 nm). (Reprinted with permission from Ref. [62].)...

A test of the basis-set effects on the excitation energy and transition moments of vertically excited singlet states in pyrazine has been published [43]. Table IV shows some recent data on the basis-set dependency of the excitation energy for the lowest vertically excited singlet states of neutral butadiene and its positive and negative ions. In accord with the previous results, we conclude that basis sets of the contracted size (4s3pld/2s)... 

Figure 1 Relative positions of the potential energy (E) surfaces of the electronic states involved in a hypothetical chemiluminescent reaction as a function of intemuclear separation (r). P and P represent the ground and lowest electronically excited singlet states of the product of the reaction, respectively. R represents the ground electronic state of the reactant. AH is the enthalpy of the dark reaction while AHa is its enthalpy of activation. AH is the enthalpy of activation of the photoreaction, hv denotes the emission of chemiluminescence.

M. A. Buntine, C. Lee, G. E. Metha. The lowest-lying excited singlet and triplet electronic states of propanal an ab initio molecular orbital investigation of the potential energy surfaces, Phys. Chem. Chem. Phys., 6 688-696 (2004). 

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