Effect of solvent and temperature on proton transfer reactions of excited molecules

Abstract

The spectral difference in fluorescence emitted from the forms (I), ( 11) and (III) has been used to study proton transfer reactions of electronically excited molecules : * * *o 0 (I) (10 (111) A H + B + A H . . . B-tA . . . HE3 by fluorescence measurements.With intermolecular hydrogen bonds, pronounced effects of solvent and of temperature have been observed which indicate that in this case proton transfer is strongly facilitated by an increase in polarity of the surrounding medium. In polar solvents considerable changes in solvent orientation are required and can become rate-determining, particularly at low temperatures. With intramolecular proton transfer equilibria the solvent effect is weaker and in the opposite direction. This is attributed to compensation by intramolecular electron migration of the dipole moment of form (111). An upper limit of 0.12 kcal/mole has been obtained for the activation energy of the intramolecular proton transfer.

Some aspects of stabilization of polymers against light

“…The cnergy difference betwcen the 'enol' and the 'keto' -forms in the ground state of methyl salicylate can be estimated from measurements by Weller 6 to be around 15 kcal/molt. The difference in pK values between proton donor and acceptor in this molecule is estimated at 16 to 18 units 10 . This coincides very closely with the corresponding values for benzophenone and benzotriazole u.v.-absorbers (cf.…”

“…Hence the energy difference between thc excited 'enol'-and 'keto'-forms of these molecules also should not excecd this valuc of 5 kcaljmol. In fact Weller found 6 an enthalpy difference of 1 kcaljmol between the excited 'cnol'-and 'keto'-forms of methylsalicylate.…”

“…Examples are derivatives of salicylic acid 6 or salicylidene anilines and 2-(2-hydroxyphenyl)-benzothiazole 7 • The 'keto'-fluorescence can be recognized by a red shift of about 4000 to 5000 cm-1 with respect to the 'enol' -fluorescence and in any case by a Stokes shift of about 10000 cm-1 • Our own investigations show that 'keto'-fluorescences are also formed in 2-(2-hydroxyphenyl)-benzoxazole and -imidazole. In polar solvents the imidazole derivative shows even in the ground state a small amount of 'keto' -form.…”

Intramolecular Proton Transfer in Electronically Excited Molecules

“…is the stretching frequency, k > l / r o (>lo s and From 4' K experiments an even smaller limit of E 5 0.1 kcal/mol is derived, suggesting proton tunneling as transfer mechanism (24). of intersystem crossing or internal conversion in A* and B* is not known.…”

Synthesis, spectral characterization and in vitro biological studies of Co(II), Ni(II) and Cu(II) complexes with 1,2,4-triazole Schiff bases

“…The changes clearly indicate that, proton transferred (H-bonded ion pair) species exist in equilibrium [45] and also we observed the Xmax of the Schiff bases undergoes red shift in DMSO, DMF, and MeCN and blue shift in Dioxan solutions is due to hydrogen bond formation [46].…”