High-pressure studies of the viscosity effects on the formation of the twisted intramolecular charge-transfer (TICT) state in 4,4'-diaminodiphenyl sulfone (DAPS)

Abstract

To investigate the dynamic solvent effects on the formation of the "twisted intramolecular charge-transfer (TICT)" state, the pressure as well as temperature effects on the fluorescence spectra of 4,4'-diaminodiphenyl sulfone (DAPS) in alcohol solvents were examined. It is concluded that solvent viscous friction makes an important contribution to the rate of TICT-state formation of DAPS in alcohols. The contribution of the dynamic solvent effect on the kinetics is well-separated from that of the intrinsic static contribution defined by transition-state theory (TST). The contribution of the solvation effect to the activation volume and activation energy of the TICT-state formation is also discussed.

Pressure tuning of solvent viscosity for the formation of twisted intramolecular charge-transfer state in 4,4′-diaminodiphenyl sulfone in alcohol solution

“…21 However, as has been quantitatively described in our previous paper, 13 the effect of the change in dielectric constant in a single solvent with pressure on ␣ is 6.2ϫ10 Ϫ2 -6.9ϫ10 Ϫ2 , 22 which is much less than the present experimental error. This fact is gauged by the intrinsic activation volume defined by the transition state theory ͑TST͒ (⌬V TST ), which involves the contribution of solvent polarity effect.…”

“…19,20 It is found that the TICT-state formation clearly exhibits a viscosity dependence, just as has been discussed based on the yield ratio measurements in the previous paper. 13 The important feature is that ln k CT decreases with ln , whose slope is in the order of 1-propanol Ͼ1-butanolϾ1-pentanol. Supposing a linear relationship, we attempted to apply the power law form for this viscosity dependence; …”

“…The result at the highest viscosity region for 1-pentanol ͑␣ ϭ0.24͒ is in good agreement with that ͑␣ϭ0.2͒ evaluated previously by the yield ratio analysis. 13 They are signifi-cantly smaller than 1. Such a small viscosity dependence would be difficult to observe by means of the solvent changing method, because of the larger change in barrier energy.…”

“…The absolute value of ⌬V TST is much less than 1 cm 3 /mol. 13 The difference in k CT among different alcohols at a given can be considered as originating from the shift of barrier energy due to solvent polarity effect. Thus, we normalized them with respect to the k CT value at ϭ11.7 mPa s ͑k CT *͒.…”

“…In the previous article 13 we described the pressure effect on the fluorescence yield ratio for the TICT-state formation of DAPS in alcohols. It was shown that the observed activation volume can be explained by the contribution due to dynamic solvent effect and that the power law parameter ͑␣͒, which is used as a measure of viscosity dependence, ranges 0.7-0.2, decreasing with increasing solvent viscosity.…”