title: 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.
authors.affiliation: Max-Planck-Institut für biophysikalische Chemie, Spektroskopie und Photochemische Kinetik, 37070 Göttingen, Germany.
authors.forename: Victor A
authors.initials: VA
authors.lastname: Galievsky
authors.forename: Sergey I
authors.initials: SI
authors.lastname: Druzhinin
authors.forename: Attila
authors.initials: A
authors.lastname: Demeter
authors.forename: Yun-Bao
authors.initials: YB
authors.lastname: Jiang
authors.forename: Sergey A
authors.initials: SA
authors.lastname: Kovalenko
authors.forename: Luis
authors.initials: L
authors.lastname: Pérez Lustres
authors.forename: Karunakaran
authors.initials: K
authors.lastname: Venugopal
authors.forename: Nikolaus P
authors.initials: NP
authors.lastname: Ernsting
authors.forename: Xavier
authors.initials: X
authors.lastname: Allonas
authors.forename: Mathias
authors.initials: M
authors.lastname: Noltemeyer
authors.forename: Reinhard
authors.initials: R
authors.lastname: Machinek
authors.forename: Klaas A
authors.initials: KA
authors.lastname: Zachariasse