Ultrafast Intramolecular Charge Transfer and Internal Conversion with Tetrafluoro‐aminobenzonitriles

Abstract

The five 2,3,5,6-tetrafluoro-4-aminobenzonitriles XABN4F with a dimethyl-amino (DMABN4F), diethyl-amino (DEABN4F), azetidinyl (AZABN4F), methyl-amino (MABN4F) or amino (ABN4F) group undergo ultrafast intramolecular charge transfer (ICT) at room temperature, in the polar solvent acetonitrile (MeCN) as well as in the nonpolar n-hexane. ICT also takes place with the corresponding non-fluorinated aminobenzonitriles DMABN, DEABN and AZABN in MeCN, whereas for these molecules in n-hexane only minor (DMABN, DEABN) or no (AZABN) ICT fluorescence is detected. For the secondary (MABN) and primary (ABN) amines, an ICT reaction does not occur, which makes ABN4F the first electron donor/acceptor molecule with an NH(2) group for which ICT is observed. The ICT state of the XABN4Fs has a dipole moment of around 14 D, clearly smaller than that of DMABN (17 D). This difference is attributed to the electron withdrawing from the CN group to the phenyl ring, exerted by the four F-substituents. The reaction from the initially prepared locally excited (LE) to the ICT state in n-hexane proceeds in the sub-picosecond time range: 0.35 ps (DMABN4F), 0.29 ps (DEABN4F) and 0.13 ps (AZABN4F), as determined from femtosecond transient absorption measurements. In the highly polar solvent MeCN, an ICT reaction time of around 90 fs is observed for all five XABN4Fs, irrespective of the nature of their amino group. This shows that with these molecules in MeCN the ICT reaction rate is limited by the solvent dielectric relaxation time of MeCN, for which a value of around 90 fs has been reported. It is therefore concluded that, during this ultrashort ICT reaction, a large-amplitude motion such as a full 90 degrees twist of the amino group is unlikely to occur in the XABN4Fs. The ICT state of the XABN4Fs is strongly quenched via internal conversion (IC), with a lifetime tau'(0) (ICT) down to 3 ps, possibly by a reaction passing through a conical intersection made accessible due to a deformation of the phenyl group by out-of-plane motions induced by vibronic coupling between low-lying pisigma* and pipi* states in the XABN4Fs.

Excited-State Dynamics of Donor−Acceptor Bridged Systems Containing a Boron−Dipyrromethene Chromophore: Interplay between Charge Separation and Reorientational Motion

“…The dynamics of solvation, especially that occurring through diffusive motion, is also related to viscosity. Given that for the present systems the stabilization of the CS state by solvation is crucial, a correlation between τ CS and the solvation time as predicted by theory, 73 and as recently reported for intramolecular charge-transfer processes, 74 would not be surprising. A plot of τ CS as a function of the diffusive solvation times reported for the nitriles 75,76 shows a reasonable correlation.…”

Evidence of coupled photoinduced proton transfer and intramolecular charge transfer reaction in para-N,N-dimethylamino orthohydroxy benzaldehyde: Spectroscopic and theoretical studies

“…Many experimental and theoretical studies have been performed on DMABN and related donor-acceptor charge transfer systems to establish the mechanism of excited state ICT process [21][22][23][24][25][26][27][28][29][30]. So far, models such as twisted intramolecular charge transfer (TICT) [22][23][24][26][27][28][29], planarized intramolecular charge transfer (PICT) [25], wagging intramolecular charge transfer (WICT) [30] have been proposed to establish ICT reaction in different donor-acceptor charge transfer molecular systems. According to the most accepted TICT model donor and acceptor, perpendicular geometry 0301-0104/$ -see front matter Ó 2008 Elsevier B.V. All rights reserved.…”

Quenching of fluorescence for fluoro derivatives of the laser dye DCM in polar solutions

“…The photophysical properties of organic compounds with electron-donor and electron-acceptor substituents have been widely studied recently [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. They are of interest in connection with their possible application as organic light-emitting diodes in color graphical displays [6], fluorescent labels and visualizing agents in biological systems [7,8], and also owing to their nonlinear optical properties [9, 10] (high quadratic polarizability β).…”