Counterintuitive absence of an excited-state intramolecular charge transfer reaction with 2,4,6-tricyanoanilines. Experimental and computational results.
Résumé
The fluorescence spectra of 2,4,6-tricyano-N,N-dimethylaniline (TCDMA), 2,4,6-tricyano-N-methylaniline (TCMA), and 2,4,6-tricyanoaniline (TCA) consist of a single emission band, even in the polar solvent acetonitrile (MeCN). This indicates that an intramolecular charge transfer (ICT) reaction from the initially prepared locally excited (LE) state does not take place with these molecules, in contrast to 4-(dimethylamino)benzonitrile (DMABN), although the electron accepting capability of the tricyanobenzene moiety in TCDMA, TCMA, and TCA is substantially larger than that of the benzonitrile group in DMABN. In support of this conclusion, the picosecond fluorescence decays of the tricyanoanilines are single-exponential. Only with TCDMA in MeCN at the highest time resolution, double-exponential decays are observed. On the basis of a similar temporal evolution of around 2 ps in the femtosecond excited-state absorption (ESA) spectra of TCDMA in this solvent, the time development is attributed to the presence of two rapidly interconverting S(1) conformers. The same conclusion is reached from CASPT2/CASSCF computations on TCDMA, in which two S(1) minima are identified. The ESA spectra of TCDMA, TCMA, and TCA resemble that of the LE state of DMABN, but are different from its ICT ESA spectrum, likewise showing that an ICT reaction does not occur with the tricyanoanilines. From the luminescence spectrum of TCDMA in n-propyl cyanide at -160 degrees C, it follows that intersystem crossing and not internal conversion is the main S(1) deactivation channel. The radiative rate constant of TCDMA in MeCN is smaller than that of TCMA and TCA, which indicates that the S(1) state of TCDMA has a larger ICT contribution than in the case of TCMA and TCA, in accordance with the results of the calculations, which show that the S(1) state displays ICT valence bond character. Extrapolated gas-phase data for TCDMA and TCA are compared with the results of the computations, revealing a good agreement. The calculations on TCDMA and TCA also lead to the conclusion that the lowest excited singlet state S(1) determines its photophysical behavior, without the occurrence of an LE --> ICT reaction, in the sense that the initially excited LE state has already a strong ICT character and there is no equilibrium between two electronic states with strongly different electronic structures (i.e., LE and ICT with very different dipole moments) leading to dual (LE + ICT) fluorescence.