Solvent and temperature effects on the deactivation pathways of excited ion pairs produced via photoinduced proton transfer

Abstract

The kinetics of the energy dissipation pathways of the excited ion pairs formed upon light absorption of the hydrogen bonded complex between N-methyl-3-hydroxynaphthalimide (3HONI) and 1-methyl-imidazole (MIm) has been studied in a wide temperature range in solvents of various polarities. In polar media such as butyronitrile, three emitting species are found which are assigned as hydrogen bonded, solvent separated and free ion pairs. In contrast, less polar solvents do not favour free ion pair formation, and reversible transition occurs between the excited hydrogen bonded (HBIP) and solvent separated (SSIP) ion pairs around room temperature. Systematic steady-state and time-resolved fluorescence measurements combined with computer modelling enabled the deduction of the kinetic parameters for all excited state deactivation processes in solvents of moderate polarity. The Arrhenius parameters of the interconversion between the ion pairs, the A-factor for the radiationless deactivation of SSIP and the radiative rate constant of HBIP diminish with the increasing dielectric constant of the media. The enthalpy and the entropy change in the SSIP formation from HBIP become less negative in more polar solvents. Going from ethyl acetate to CH 2 Cl 2 the most profound decrease (more than three orders of magnitude) is observed for the A-factor of SSIP HBIP reaction.

Energy Dissipation Processes of singlet‐excited 1‐Hydroxyfluorenone and its Hydrogen‐bonded Complex with N‐methylimidazole¶

“…Consequently, proton back transfer followed by dissociation to 1HOF* andground‐state MIm can readily compete with the other deactivation processes of the HBIP*. This behavior is in sharp contrast with the previous results on N ‐methyl‐3‐hydroxynaphthalimide‐MIm system, where the photoinduced proton transfer proved to be an irreversible process and the fluorescence of the HBIP and solventseparated ion pair could be distinguished (42). In solutions containing lHOF and MIm, no evidence has been found for excited solvent‐separated ion pair formation because the tluorescence maximum and the spectral shape exhibit marginal change with increasing solvent polarity in toluene, ethyl acetate, butylchloride and CH 2 Cl 2 .…”

“…The Φ F (HBIP*) data above 0.4 M MIm concentration were excluded from the fit because a slow decrease appears, which seems to originate partly from the gradual lessening of k F . It has been shown previously that the radiative rate constant of a HBIP can diminish more than factor 3 going from ethyl acetate to CH 2 Cl 2 (42). When 1HOF is solved in toluene, the high amount of MIm additive increases the local polarity around HBIP* and this effect may lead to lower k F value.…”

Complexes of a naphthalimide photoacid with organic bases, and their excited-state dynamics in polar aprotic organic solvents

“…[16][17][18]22,23 In the first step, fast short-range proton rearrangement takes place producing the hydrogen-bonded ion pair, where the proton resides on the basic nitrogen of the proton acceptor. Full dissociation takes place in the excited state in polar solvents demonstrated by the increased red shift of the emitting anionic species.…”

“…[9][10][11][12][13][14][15] Secondly, most of the studies are aimed at looking at the decay of the neutral form (usually twoexponential) and appearance of the anionic form whereas a detailed analysis of the decay of the anionic species is required for observation of all the species. 22,24 Interestingly, excitation of the deprotonated DBU complex results in dissociation kinetics similar to those of the NMI complex. A larger extent of ICT results in stronger stabilization of the excited-state anion, decreasing the emission energy.…”

Energy Dissipation Processes of Singlet Excited 1-Hydroxyfluorenone and its Hydrogen-Bonded Complex with N-Methylimidazole

“…The ratio of the molar absorption coefficients for lHOF and MIm- The QF(HBIP*) data above 0.4 M MIm concentration were excluded from the fit because a slow decrease appears, which seems to originate partly from the gradual lessening of kb. It has been shown previously that the radiative rate constant of a HBIP can diminish more than factor 3 going from ethyl acetate to CH2C12 (42). When lHOF is solved in toluene, the high amount of MIm additive increases the local polarity around HBIP*, and this effect may lead to lower k; value.…”