Abstract
From X-ray structure analysis, amino twist angles of 90.0 degrees for 2,4-dimethyl-3-(dimethylamino)benzonitrile (mMMD), 82.7 degrees for 4-(di-tert-butylamino)benzonitrile (DTABN), and 88.7 degrees for 6-cyanobenzoquinuclidine (CBQ) are determined, all considerably larger than the 57.4 degrees of 3,5-dimethyl-4-(dimethylamino)benzonitrile (MMD). This large twist leads to lengthening of the amino-phenyl bond, 143.5 pm (mMMD), 144.1 pm (DTABN), 144.6 pm (CBQ), and 141.4 pm (MMD), as compared with 136.5 pm for the planar 4-(dimethylamino)benzonitrile (DMABN). As a consequence, the electronic coupling between the amino and phenyl subgroups in mMMD, DTABN, CBQ, and MMD is much weaker than in DMABN, as seen from the strongly reduced molar absorption coefficients. The fluorescence spectrum of MMD in n-hexane at 25 degrees C consists of two emissions, from a locally excited (LE) and an intramolecular charge transfer (ICT) state, with a fluorescence quantum yield ratio Phi'(ICT)/Phi(LE) of 12.8. In MeCN, a single ICT emission is found. With mMMD in n-hexane, in contrast, only LE fluorescence is observed, whereas the spectrum in MeCN originates from the ICT state. These differences are also seen from the half-widths of the overall fluorescence bands, which in n-hexane are larger for MMD than for mMMD, decreasing with solvent polarity for MMD and increasing for mMMD, reflecting the disappearance of LE and the onset of ICT in the overall spectra, respectively. From solvatochromic measurements the dipole moments mu(e)(ICT) of MMD (16 D) and mMMD (15 D) are obtained. Femtosecond excited state absorption (ESA) spectra at 22 degrees C, together with the dual (LE + ICT) fluorescence, reveal that MMD in n-hexane undergoes a reversible LE reversible arrow ICT reaction, with LE as the precursor, with a forward rate constant k(a) = 5.6 x 10(12) s(-1) and a back-reaction kd similar to 0.05 x 10(12) s(-1). With MMD in the strongly polar solvent MeCN, ICT is faster: k(a) = 10 x 10(12) s(-1). In the case of mMMD in n-hexane, the ESA spectra show that ICT does not take place, contrary to MeCN, in which ka = 2.5 x 10(12) s(-1). The ICT reactions with MMD and mMMD are much faster than that of the parent compound DMABN in MeCN, with k(a) = 0.24 x 10(12) s(-1). Because of the very short ICT reaction times of 180 fs (MMD, n-hexane), 100 fs (MMD, MeCN), and 400 fs (mMMD, MeCN), it is clear that the picosecond fluorescence decays of these systems appear to be single exponential, due to the insufficient time resolution of 3 ps. It is concluded that the faster LE -> ICT reaction of MMD as compared with DMABN (k(a) = 0.24 x 10(12) s(-1) in MeCN) is caused by a smaller energy gap Delta E(S1,S2) between the lowest singlet excited states and not by the large amino twist angle. Similarly, the larger Delta E(S1,S2) of mMMD as compared with MMD is held responsible for its smaller ICT efficiency (no reaction in n-hexane).
