Energy transfer Stokes shift

Hypochromism in the absorption and redshifted emission were properly evaluated when the appropriate model (MUM) compounds were promptly available (see Table 9) and were found to be common features shared by the polybenzofulvene derivatives with well-known it-stacked macromolecules. When the study of the appropriate model compound was unfeasible, Stokes shifts (energy difference between the absorption and emission maxima) were used to evaluate the presence of stacking interactions. Indeed, it is generally accepted that excimer interactions and efficient energy transfer contribute to larger Stokes shifts. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

Stokes-shifted tautomer emission and high quantum yield in solid state (10%), DHBO was successfully used as an energy transfer donor in the photochromic switching system [88]. 

Because of the Stokes shift for vibrationally relaxed systems (the rate of transfer < the rate of vibrational relaxation), transfer between like molecules is less efficient than that between unlike molecules when acceptor is at a lower energy level (exothermic transfer). No transfer is expected if the acceptor level is higher than the donor level. If (i) the acceptor transition is strong (Emaz —- 10,000), (ii) there is significant spectral overlap, and (iii) the donor emission yields lie within 0.1 — 1.0, then R0 values of 50-100 A are predicted. 

Once a species, M, has absorbed a photon of energy hv, an excited state is created, M. Deactivation back to the ground state occurs through multiple steps, including very fast non-radiative processes that schematically correspond to energy transfers to the solvent. Radiative deactivation may also occur, leading to the emission ofa photon of energy hv. Due to the non-radiative processes, hv < hv (Stokes shift). The emission spectrum is composed of bands, that are characteristics of the species. 

S. C. Tucker, Solvent density inhomogeneities in supercritical fluids, Chem. Rev., 99 (1999) 391—418 O. Kajimoto, Solvation in supercritical fluids Its effects on energy transfer and chemical reactions, Chem. Rev., 99 (1999) 355-89 S. Nugent and B. M. Ladanyi, The effects of solute-solvent electrostatic interactions on solvatochromic shifts in supercritical C02, J. Chem. Phys., 120 (2004) 874-84 F. Ingrosso and B. M. Ladanyi, Solvation dynamics of C153 in supercritical fluoroform a simulation study based on two-site and five-site models of the solvent, J. Phys. Chem. B, 110 (2006) 10120-29 F. Ingrosso, B. M. Ladanyi, B. Mennucci and G. Scalmani, Solvation of coumarin 153 in supercritical fluoroform, J. Phys. Chem. B, 110 (2006) 4953-62 Y. Kimura and N. Hirota, Effect of solvent density and species on static and dynamic fluorescence Stokes shifts of coumarin 153, J. Chem. Phys., Ill (1999) 5474 ... 

---