Intramolecular Charge Transfer with the Planarized 4-Aminobenzonitrile 1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6)

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Abstract
Fast and efficient intramolecular charge transfer (ICT) and dual fluorescence is observed with the planarized aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in a series of solvents from n-hexane to acetonitrile and methanol. Such a reaction does not take place for the related molecules with 1-isopropyl (NIC6) and 1-methyl (NMC6) groups, nor with the 1-alkyl-5-cyanoindolines with methyl (NMC5), isopropyl (NIC5), or tert-butyl (NTC5) substituents. For these molecules, a single fluorescence band from a locally excited (LE) state is found. The charge transfer reaction of NTC6 is favored by its relatively small energy gap ΔE(S1,S2), in accordance with the PICT model for ICT in aminobenzonitriles. For the ICT state of NTC6, a dipole moment of around 19 D is obtained from solvatochromic measurements, similar to μe(ICT) = 17 D of 4-(dimethylamino)benzonitrile (DMABN). For NMC5, NIC5, NTC5, NMC6, and NIC6, a dipole moment of around 10 D is determined by solvatochromic analysis, the same as that of the LE state of DMABN. For NTC6 in diethyl ether at −70 °C, the forward ICT rate constant (1.3 × 1011 s-1) is much smaller than that of the back reaction (5.9 × 109 s-1), showing that the equilibrium is on the ICT side. The results presented here make clear that ICT can very well take place with a planarized molecule such as NTC6, when ΔE(S1,S2) is sufficiently small, indicating that a perpendicular twist of the amino group relative to the rest of the molecule is not necessary for reaching an ICT state with a large dipole moment. The six-membered alicyclic ring in NMC6, for example, prevents ICT by increasing ΔE(S1,S2) relative to that of DMABN.

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