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2007 | 112 | S | S-39-S-56
Article title

Intramolecular Charge Transfer with N,N-Dialkyl-4-(Trifluoromethyl)anilines and 4-(Dimethylamino)benzonitrile in Polar Solvents. Investigation of the Excitation Wavelength Dependence of the Reaction Pathway

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In the series of N,N-di-n-alkyl-4-(trifluoromethyl)anilines (DXCF3), with X = M (methyl), E (ethyl), Pr (n-propyl) and Pe (n-pentyl), an intramolecular charge transfer (ICT) reaction takes place in the polar solvent acetonitrile (MeCN). Such a reaction does not occur in the less polar solvents $n$-hexane and diethyl ether, in which a single fluorescence band from the locally excited (LE) state is observed. For DMCF3 in MeCN at 25ºC, the intramolecular charge transfer reaction efficiency is much smaller than for 4-(dimethylamino)benzonitrile (DMABN) in this solvent, as seen from the ICT/LE fluorescence quantum yield ratio Φ'(ICT)/Φ(LE): 0.06 for DMCF3 and 39.5 for DMABN. As predicted by the planar ICT (PICT) model, this difference is caused by the considerably larger energy gap Δ E(S_1,S_2) of DMCF3 (5750 cm^{-1}) as compared with that of DMABN (3130 cm^{-1}). When the alkyl chain length of DXCF3 becomes larger, Φ'(ICT)/Φ(LE) in MeCN at 25ºC increases, from 0.06 for DMCF3 to 0.64 for DPeCF3. This increase in ICT reaction efficiency is accompanied by a small but significant decrease of 380 cm^{-1} for Δ E(S_1,S_2) when going from DMCF3 to DPeCF3. Because of their relatively large Δ E(S_1,S_2) gap, the molecules DXCF3 are suitable for an investigation of the excitation wavelength dependence of the intramolecular charge transfer reaction. For DPrCF3 in MeCN, the same Φ'(ICT)/Φ(LE) is found for excitation in the S_1 or in the S_2 manifold. A similar result is obtained with DMABN in tetrahydrofuran. From these experiments it is concluded that for DPrCF3 and DMABN the intramolecular charge transfer reaction follows an adiabatic pathway. After excitation to a Franck-Condon S_n(FC) state, the electron donor/acceptor molecules first undergo ultrafast internal conversion to the relaxed LE(S_1) state, from which precursor the reaction proceeds to the ICT state. Experimental evidence for a nonadiabatic ICT reaction pathway, going directly from S_2(FC) via an S_2/S_1 conical intersection to the LE and ICT state, is not found. This pathway would lead to an enhancement of the ICT population, i.e., to an increase in Φ'(ICT)/Φ(LE), as compared with the adiabatic LE → ICT reaction.
Physical description
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