Molecular Mechanism of the Intermolecular Hydrogen Bond between 2-Piperidinoanthraquinone and Alcohol in the Excited State:  Direct Observation of the Out-of-Plane Mode Interaction with Alcohol by Transient Absorption Studies

Abstract

Radiationless deactivation of 2-piperidinoanthraquinone (2PAQ) in its excited singlet state induced by an intermolecular hydrogen bond interaction was studied by picosecond and femtosecond transient absorption spectroscopy in benzene. On addition of alcohol, the transient absorption of excited 2PAQ exhibited a blue shift and was effectively quenched. Kinetic analyses of the transient decay obtained by experiments using a picosecond pulse compared well with those from dynamic fluorescence studies; the observed transient absorption was confirmed as being attributed to the excited singlet state of 2PAQ. Subpicosecond transient absorption experiments indicated that even at high concentrations of ethanol (1-3 M) in benzene the primary relaxation process was almost the same as that in neat benzene. A hydrogen-bonded excited state of 2PAQ, due to an out-of-plane mode interaction with ethanol, was successfully detected for the first time. Using detailed multicomponent analyses of the transient absorption, the spectra of both the out-of-plane mode species with λ max at 570 nm and the in-plane mode species with λ max at 550 nm were obtained.

Direct Observation of a Hydrogen‐Bonded Charge‐Transfer State of 4‐Dimethylaminobenzonitrile in Methanol by Time‐Resolved IR Spectroscopy

“…The strongly enhanced rate for IC to the ground state is consistent with the large effect of deuteration on DMABN fluorescence in alcohol solvents, [11] and we note that deactivation via IC resulting from hydrogen bonding has been proposed to account for the fluorescence quenching of many molecules having ICT states. [19] Our results show that the rates for radiative decay and ISC are unaffected by hydrogen bonding, illustrating the weakness of this interaction. It therefore seems reasonable to assume the "free" ICT in MeOH is the same as that in MeCN, in which case it follows that the radiative rates will be the same for ICT and HICT.…”

The Role of Hydrogen‐Bonding Interactions in the Ultrafast Relaxation Dynamics of the Excited States of 3‐ and 4‐Aminofluoren‐9‐ones

“…[84] The correlation time constant of the hydrogenbond length was observed to be approximately 500 fs, whereas the values of the dielectric relaxation time, t D , and t L for water are 8 and 2.6 ps, respectively. [85] From this comparison of the timescales, it is clear that the modulation of hydrogenbond length is a faster process than the orientation of the molecules. Hence, it is not surprising that the involvement of the thermalization process shortens the equilibration time of the hydrogen bond.…”

“…[42,[83][84][85] One should realize that hydrogen-bond reorientation, which is measured in the longitudinal relaxation or the rotational correlation experiments, and thermalization are not the same processes. The repositioning of the molecules in a thermalization process mainly involves a change of distance between the molecules, for example, hydrogen-bond lengths.…”

“…Since the solvent reorganization times, t R , determined here represent an average of multiexponential components, the motions of the nonpolar part of the solvent molecules due to translational diffusion, rather than the polar part, may also be responsible for the faster solvent reorganization process in alcohols with longer alkane chains. [42,85] Recently, Zhao and Han performed TDDFT calculations to investigate the excited-state hydrogen-bond dynamics of fluorenone in methanol solvent, and demonstrated that the intermolecular hydrogen bond between fluorenone and alcohol molecules is strengthened in the electronically excited state, since the calculated hydrogen-bond binding energy increases significantly from 27.85 kJ mol À1 in the ground state to 42.62 kJ mol…”

“…Hence, they explained their observations on molecules such as aminoanthraquinones and aminofluorenones by assuming the formation of two types of hydrogen-bonding modes, namely, an emissive in-plane mode of hydrogenbonded species and a nonemissive out-of-plane bending mode existing in the S 1 state. [21,23,85] The out-of-plane bending motion was proposed to promote effective radiationless deactivation. However, no specific reason was presented to explain why the out-of-plane hydrogen-bonding mode was so efficient in inducing the nonradiative deactivation process.…”