Excited states polarity effects

A study has been made of the emission of some related Tb and Eu macrocylic complexes, immobilized in a sol-gel glass, which is made pH-dependent either by perturbing the energy of the aryl singlet or triplet state, or by modulating the degree of quenching of the lanthanide excited state. The effect of bicarbonate chelation on the polarized luminescence from chiral... 

The importance of helical structures for enhancing the first hyperpolarizability has been discussed by Panda and Chandrasekhar 193 and SOS theory has been used by Moreau et a/.194 and Monshi et al 95 to investigate excited state polarizations with solvent effects. Torrens et al.196 have developed a scheme to predict molecular polarizabilities from the effect of interacting dipoles. 

GROUND-STATE VERSUS EXCITED-STATE POLARITY OF TRIAFULVENES A STUDY OF SOLVENT EFFECTS ON MOLECULAR ELECTRONIC SPECTRA[1]... 

The two phenomena, the solvent effect on the absorption spectrum and the solvent effect on the fluorescence spectrum are relevant to our problem of investigating the ground state vs excited state polarity. An estimation of the dipole moment of a polar solute in the excited state may be attempted using these two phenomena. 

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

More systematic study of the dynamics of radicals in solution should now be possible using CIDNP. Investigations so far reported have indicated that the rates of very rapid chemical reactions and other dynamic processes undergone by radicals can be measured in a crade way greater refinement should be possible. Special effects have been predicted for reactions in thin films (Deutch, 1972). Moreover the time-scale of polarization is such that the technique may prove capable of throwing new light on the dynamics of excited states. 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

Yamazaki S, Kato S (2007) Solvent effect on conical intersections in excited-state 9H-adenine radiationless decay mechanism in polar solvent. J Am Chem Soc 129 2901—2909... 

Third, there are clear differences in the images for dissociation at the same wavelength while probing different spin-orbit states. Two effects contribute to these differences. One is the slight difference in energy in the atomic states, most easily seen in the data at 234 nm where the 0(3P2) data clearly has the strong contribution from vibrationally-excited O2 while the other spin-orbit states do not. In this case the threshold for the dynamical process that forms the vibrationally-excited products has been crossed by the 158.265 cm-1 of the spin-orbit excitation. The second effect is that due to the nature of the J-level. It is known that there is a v—J correlation from the angular fits as well as from the fact that when the polarization of the... 

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