Excited-state structures

Myers A B and Mathies R A 1987 Resonance Raman intensities A probe of excited-state structure and dynamics Biological Applications of Raman Spectroscopy yo 2, ed T G Spiro (New York Wiley-Interscience) pp 1-58... 

Several explanations have been given for this result. One possibility is the failure of the adiabatic assumption in this ionization process a transition state is generated that may not be a combination of the initial and final states, but may include mixing in of an excited state structure. This idea will reappear in the following pages. 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

The effect of substituents on colour in substituted anthraquinones may be rationalised using the valence-bond (resonance) approach, in the same way as has been presented previously for a series of azo dyes (see Chapter 2 for details). For the purpose of explaining the colour of the dyes, it is assumed that the ground electronic state of the dye most closely resembles the most stable resonance forms, the normal Kekule-type structures, and that the first excited state of the dye more closely resembles the less stable, charge-separated forms. Some relevant resonance forms for anthraquinones 52, 52c, 52d and 52f are illustrated in Figure 4.3. The ground state of the parent compound 52 is assumed to resemble closely structures such as I, while charge-separated forms, such as structure II, are assumed to make a major contribution to the first excited state. Structure II is clearly unstable due to the carbocationic centre. In the case of aminoanthraquinones 52c and 52d, donation of the lone pair from the... 

Usman A, Mohammed OP, Nibbering ET, Dong J, Solntsev KM, Tolbert LM (2005) Excited-state structure determination of the green fluorescent protein chromophore. J Am Chem Soc 127 11214-11215... 

OL behavior is assessed simply by monitoring the transmission of a (usually solution) sample as a function of the incoming laser fluence measured in joules per square centimeter (rather than intensity in watts per square centimeter).22,23 Limiting thresholds Fth, defined as the incident fluence at which the actual transmittance falls to 50% of the corresponding linear transmittance, are then commonly quoted. Since excited-state absorption processes generally determine the OL properties of molecules, the excited-state structure and dynamics are often studied in detail. The laser pulse width is an important consideration in the study of OL effects. Compounds (1-5)58-62 are representative non-metal-containing compounds with especially large NLO and/or OL... 

The Bom-Oppenheimer principle says that the atomic nuclei do not move during the electronic excitation only later will the excited state structure relax to minimize its conformational energy. An arrow represents the photo-excitation from the ground state to the excited state structures. The requirement for the excited-state structure to... 

We saw in Figure 9.15 how photon absorption leads to the excitation of an electron from the ground state to the first excited state. It is usual for the excited-state structure to form in a non-equilibrium state, so it must subsequently rearrange to achieve a lower energy. 

These data may be explained in terms of the above mechanism of the long-wavelength shift of fluorescence spectra for red-edge excitation. The properties of the environment of the tryptophan residues in the proteins studied are such that during the lifetime of the excited state, structural relaxation of the surrounding dipoles fails to proceed. Studies of the dependence of the... 

Yasuda N, Kanazawa M, Uekusa H, Ohashi Y (2002) Excited-state structure of a platinum complex by X-ray analysis. Chem Lett 1132-1133... 

Yasuda N, Uekusa H, Ohashi Y (2004) X-Ray analysis of excited-state structures of the diplatinum complex anions in five crystals with different cations. Bull Chem Soc Jpn 77 933-944... 

Kim CD, Pillet S, Wu G, Fullagar WK, Coppens P (2002) Excited-state structure by time-resolved X-ray diffraction. Acta Cryst A 58 133-137... 

Chen LX, Jennings G, Liu T, Gosztola DJ, Hessler JP, Scaltrito DV, Meyer GJ (2002) Rapid excited-state structural reorganization captured by pulsed X-rays. J Am Chem Soc 124 10861-10867... 

To help us understand how a polar solvent can help to stabilize an excited state, we will consider the ti tt transition of an alkene. We can represent the ground state and excited state species in a simple way with resonance structures. It is important to realize, though, that the dipolar structures in Scheme 2.1 are not the excited state but they do make a (minor) contribution to the excited state structure. 

Aqueous [Ru"(bpy)3]2+ is a model system for Metal-to-Ligand Charge Transfer (MLCT) reactions. Its excited state properties have been readily studied with optical spectroscopies [15,16]. However, little is known about its excited state structure, which we investigated via time-resolved x-ray absorption spectroscopy. The reaction cycle is described by Fig. 3 (where the superscripts on the left hand side of the ground and excited state compounds denote the... 

The fluorescence quantum yield of 448 is 0.14, a sixfold increase relative to that of the parent. In comparison, the fluorescence quantum yield of 449 (0.01) is comparable to that of the parent compound. The phosphorescence emission quantum yield of 449 is 0.56 in a frozen matrix as expected as a result of the intramolecular heavy atom effect. In contrast, the phosphorescence is effectively shut off in the anti-isomer where the quantum yield is only 0.04. This observation suggests that the electronic excited state structures and nonradiative decay channels very considerably with constitution of the isomers. The optical gap energy was 3.1 (3.3) eV for 448 (449). 

The numerical values obtained from Eq. (12-14) and the d s obtained from the fit using the coupled potential are <5co = 0.023 A, <5Pto = 0.011 A and da = 2.0°. The values obtained by using the uncoupled potentials are (5co = 0.021 A, t)Pt0 = 0.011 A and da = 2.0°. The differences between the distortions calculated with the coupled and uncoupled models are very small, probably smaller than the uncertainties introduced by using the approximations in Eqs. (12-14) for the normal coordinates. The excited state structure of Pt(hfacac)2 determined from the excitation spectrum in the molecular beam is shown schematically in Fig. 14. 

The rotational analysis gives a comprehensive picture of the excited state structure. Ingold and King (1953) established that the dipole moment associated with the transition is oriented along the inertial c axis (species Au), that is, perpendicular to the plane of the excited trans-bent structure hence the excited state symmetry species must be 1AU, its singlet character being inferred from the intensity of the transition (/ 10-4). Innes (1954) showed that the intensity alternation in the rotational lines requires the axis of greatest inertia to coincide... 

M. O.Trulson and R. A Mathies,/. Phys. Chem., 94,5741 (1990).Excited-State Structure and Dynamics of Isoprene from Absolute Resonance Raman Intensities. 

Back electron-transfer rate, cyanometallates, 116 Band analysis, excited-state structure, 211,25 Base, effect on doublet excited... 

---