Role of Intramolecular Torsion and Solvent Dynamics in the Charge-Transfer Kinetics in Triphenylphosphine Oxide Derivatives and DMABN

Abstract

The photoinduced processes in three dimethylamino derivatives of the triphenylphosphine oxide (OMAP, ODAP, and OTAP) are studied in solution at room temperature by time-resolved fluorescence spectroscopy with a streak camera and a 500 fs UV laser excitation source. These compounds exhibit a dual fluorescence in polar solvents explained by the fast formation of an emissive charge-transfer state as in the model compound (dimethylamino)benzonitrile (DMABN). Fluorescence decays are also measured for solutions of DMABN under the same conditions. For both compounds, the intramolecular charge-transfer time is shown to vary from a few picoseconds to a few tens of picoseconds depending on the polarity of the solvent and, for the triphenylphosphine derivatives, on the number of dimethylamino substituents. The charge-transfer process is described as a barrier-activated process with a solvent polarity dependent height. The solvent dynamics and solvent viscosity effects on the charge-transfer rate are examined for both the (dimethylaminophenyl)diphenylphosphine oxide (OMAP) and DMABN. In protic solvents, the charge-transfer time is found to be shorter than the average solvation time for both compounds, suggesting that the charge-transfer mechanism involves an intramolecular coordinate in addition to the solvent coordinate. The charge-transfer times found for DMABN are in good agreement with those recently calculated by Kim and Hynes (J. Photochem. Photobiol. A 1997, 105, 337-343), who derived a two-dimensional model using the initially proposed twisting motion of the dimethylamino group as the intramolecular coordinate. The twisting motion of the whole aniline moiety is discussed as the possible intramolecular motion for OMAP on the basis of the solvent viscosity effects, which are found to differentiate this compound from DMABN.

Do fluorescence and transient absorption probe the same intramolecular charge transfer state of 4-(dimethylamino)benzonitrile?

“…[11][12][13] There are also observations that the decay time of the long-lived component of the LE fluorescence is not identical to the decay time of the ICT fluorescence, with the former being shorter for dialkylaminobenzonitriles. [14][15][16][17] These results strongly indicate that the actual mechanism of the ICT reaction in DMABN may be far more complex than previously assumed. In an effort to develop a more detailed picture of the ICT reaction in dialkylaminobenzonitriles, we have been carrying out a concerted experimental and ab initio computational study of the reaction pathways that lead photoexcited DMABN to the TICT state.…”

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

“…Neglect of this major factor is considered to account for much of the difficulty in interpreting the fluorescence results. [2,8,[11][12][13] A mechanism of this kind has not to our knowledge been proposed before. We believe this interpretation is applicable to other molecules with solvent-dependent dual fluorescence.…”

“…[1] Results of experiments using aprotic solvents are well described by models in which polarity is the only solvent property that affects the charge transfer reaction activation energy and the relative stabilization of the ICT and LE states. [2] Whilst much work continues to concentrate on determining the structures of the LE and ICT states, [3][4][5][6][7] the precise nature of the difference between the properties of the excited state in protic and aprotic solvents is little understood. For example, the fluorescence quantum yield of DMABN in protic solvents is lower and the fluorescence spectrum is further red-shifted and broadened, relative to measurements in aprotic solvents of the same polarity, [8,9] and the fluorescence decay kinetics are difficult to interpret.…”

“…For example, the fluorescence quantum yield of DMABN in protic solvents is lower and the fluorescence spectrum is further red-shifted and broadened, relative to measurements in aprotic solvents of the same polarity, [8,9] and the fluorescence decay kinetics are difficult to interpret. [2] Hydrogen bonding in protic solvents can lead to complicated interactions [10] but although specific solute-solvent and solute-solute interactions have been discussed, [8,[11][12][13][14] there is no generally accepted explanation. There are similar problems in other cases of dual fluorescence.…”

“…The charge-transfer fluorescence will therefore be composed of overlapping bands having intensities in proportion to the ICT/HICT population ratio of about 1/2. The overall fluorescence will be intrinsically triple, and show complicated kinetics [21,22] not predicted by the dual fluorescence model [2] appropriate for aprotic solvents. This can account for much of the confusion in the literature, especially the complicated fluorescence decay dynamics observed at different temperatures in alcohols, [2,22] where variations in the relative populations of LE/ICT and ICT/ HICT can be expected.…”