Pentacyano-N,N-dimethylaniline (PCDMA) does not undergo an intramolecular charge transfer (ICT) reaction, even in the strongly polar solvent acetonitrile (MeCN), in clear contrast to 4-(dimethylamino)benzonitrile (DMABN). Within the twisted ICT (TICT) model, this is unexpected, as the electron affinity of the pentacyanobenzene moiety of PCDMA is much larger than that of the benzonitrile subgroup in DMABN. According to the TICT model, the energy of the ICT state of PCDMA would be 2.05 eV (∼16550 cm-1) lower than that of DMABN, on the basis of the reduction potentials E(A-/A) of pentacyanobenzene (-0.29 V vs saturated calomel electrode (SCE)) and benzonitrile (-2.36 V vs SCE), more than enough to compensate for the decrease in energy of the locally excited (LE) state of PCDMA (E(S1) = 19990 cm-1) relative to that of DMABN (E(S1) = 29990 cm-1). This absence of a LE → ICT reaction shows that the TICT hypothesis does not hold for PCDMA in the singlet excited state, similar to what was found for DMABN, N-phenylpyrrole, and their derivatives. In this connection, the six dicyano-substituted dimethylanilines are also discussed. The energy gap ΔE(S1,S2) between the two lowest singlet excited states is, at 7170 cm-1 for PCDMA in MeCN, considerably larger than that for DMABN (2700 cm-1 in n-hexane, smaller in MeCN). The absence of ICT is therefore in accord with the planar ICT (PICT) model, which considers a sufficiently small ΔE(S1,S2) to be an important condition determining whether an ICT reaction will take place. The fluorescence quantum yield of PCDMA is very small: Φ(LE) = 0.0006 in MeCN at 25 °C, predominantly due to LE → S0 internal conversion (IC), as the intersystem crossing yield Φ(ISC) is practically zero (<0.01). From the LE fluorescence decay time of 27 ps for PCDMA in MeCN at 25 °C, a radiative rate constant kf(LE) = 2 × 107 s-1 results, comparable to the kf(LE) of DMABN (6.5 × 10 7 s-1) and 2,4,6-tricyano-N,N-dimethylaniline (TCDMA) (1.2 × 107 s-1) in this solvent, but clearly larger than the k′f(ICT) = 0.79 × 107 s-1 of DMABN in MeCN. The IC reaction with PCDMA in MeCN at room temperature, with a rate constant kIC of 3.6 × 1010 s-1, is much faster than with TCDMA (25 × 107 s-1) and DMABN (1.3 × 107 s-1, in n-hexane). This is connected with the nonzero (37°) amino twist angle of PCDMA, which leads to a decrease of the effective LE-S0 energy gap. The femtosecond excited state absorption (ESA) spectra of PCDMA in MeCN at 22 °C are similar to the LE ESA spectra of TCDMA and DMABN and are therefore attributed to the LE state, confirming that an ICT reaction does not occur. The decay of the LE ESA spectra of PCDMA is single exponential, with a decay time of 22 ps, in reasonable agreement with the LE fluorescence decay time of 27 ps at 25 °C. The spectra decay to zero, showing that there is no triplet or other intermediate.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry