Solvent effects on emission

Jones, G., Jackson, W.R., Choi, C.Y., and Bergmark, W.R. (1985) Solvent effects on emission yields and lifetime for coumarin laser dyes. Requirements for the rotatory decay mechanism.. Phys. Cbem. 89, 294. 

Because the physical description is correct and consistent, the method allows for arbitrary division of a system into different subsystems, which may be described either on the quantum-mechanical (QM) or the molecular mechanics (MM) level, without significant loss of accuracy. This allows for performing fully MM molecular simulations (Monte Carlo, molecular dynamics), which can subsequently be followed by performing QM/MM calculations on a selected number of representative snapshots from these simulations. These QM/MM calculations then give directly the solvent effects on emission or absorption spectra, molecular properties, organic reactions, etc... 

A) solvent effects on the fluorescence emission, and (5) the effects of additional reagents and catalysts normally encountered in HPLC assays. 

Shifts in absorption spectra due to the effect of substitution or a change in environment (e.g. solvent) will be discussed in Chapter 3, together with the effects on emission spectra. Note that a shift to longer wavelengths is called a bathochromic shift (informally referred to as a red-shift). A shift to shorter wavelengths is called a hypsochromic shift (informally referred to as a blue-shift). An increase in the molar absorption coefficient is called the hyperchromic effect, whereas the opposite is the hypochromic effect. 

The dipole moment of an excited state can be estimated from an analysis of solvent effects on absorption and emission spectra the... 

PCM originated as a method to describe solvent effects on ground state molecules [2], but the extension to excited states was realized only after the original presentation, with a model [3], which introduced nonequilibrium effects in the solvent response for the optical processes of photon absorption and emission. The nonequilibrium solvation regime has later been applied to vibrational spectroscopies... 

Trace B shows the emission spectrum obtained from a sample of crystals of AuVMeN = COMe)3 after pulsed irradiation, while Trace C shows the spectrum of the light emitted immediately after the addition of chloroform to a previously photo-irradiated sample of Au lMeN = COMe)3 crystals. The spectrum shown in Trace C is that from solvoluminescence. Notice that the emitted light corresponds to the lower energy emission seen for the solid and does not correspond to that seen in solutions of the trimer. Similar emission spectra have been obtained with a number of different solvents including dichloromethane, toluene, methanol, hexane, and even water. In all cases the spectra of the emissions show a maximum at 550 nm. Thus, there is no solvent effect on the emission. However, the intensity of the emission is greatest with those liquids (chloroform, dichloromethane) that are good solvents for the complex and rather feeble in those that are not (hexane, water). 

An illustrative example of solvent effects on absorption and emission spectra is given by the following positive solvatochromic laser dye 7-dialkylamino-4-(trifiuoromethyl)coumarin ( coumarin-153 ), in which the rotation of an amino group is restricted by incorporating it into a hexahydroquinolizine ring [341],... 

A brief review and reassessment of data on the photophysics of benzene has been presented by Pereira. Evidence for the l E2g valence state has been obtained by u.v. two-photon spectroscopy.Slow electron impact excites fluorescence in thin films of benzene at 77 K as well as emission from isomers." The fluorescence yields and quenching by chloroform of alkyl-benzenes and 1-methylnaphthalene after excitation into Si, Sz, and S3 states and after photoionization have been measured. The channel-three process has been reconsidered in terms of the effects of local modes and Morse oscillator potentials. Excited-state dipole moments of some monosubstituted benzenes have been estimated from solvent effects on electronic absorption spectra, Structural imperfections influence the photochemistry of durene in crystals at low temperatures. Relaxation time studies on excited oxido-substituted p-oligophenylenes have been made by fluorescence depolarization... 

Electron Transfer Reactions and Exciolexes - Photoinduced electron transfer is one of the most important areas of research. A review of photoinduced electron transfer and electron acceptor complexes usefully surveys the subject . Details of the mechanisms can be obtained by very short time resolution spectroscopy. Dynamic solvent effects on intramolecular electron-transfer involve solvent fluctuations. Time resolved ps emission spectroscopy has been used to examine the kinetics of intramolecular charge transfer in bis(4-aminophenyl)sulphone in ethanol as a function of temperature in this respect 2. it has... 

In calculating solvent effects on excitation energies one has to take into account that normally the excitation process is sufficiently rapid for the Franck-Condon principle to hold, which means that while the induced dipole of the solvent molecule can change in response to the change of solute dipole on excitation, the permanent dipoles cannot—that is, there is negligible dipole reorientation. Such reorientation can, however, occur in the time period between absorption and emission and lead to a large Stokes shift in the emission. (Cf. Section 5.3.1.)... 

Some emission lifetime data are presented in Table 14. Particularly of note are the lifetimes in frozen inert gas solutions, reflecting the enhancement of T -> Sq transition with increasing atomic number of the solvent an effect also noted by other authors. The important results of Haaland and Nieman (222) show a marked solvent effect on phosphorescence lifetimes. Per-fluorocarbons are the least perturbing solvents on phosphorescence, an effect consistent with results obtained in liquid solution (72,91). 

Figure 2. Comparison of solvent effects on absorption and emission processes where n Is the solvent linear refractive Index (a) dlphenyloctatetraene (DPO) and (b) bls(methylphenyl)dlvlnyl-dlacetylene (DVDAl). (Reproduced with permission from reference 28. Copyright 1973.)...

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