Ultrafast Intramolecular Charge Transfer with Strongly Twisted Aminobenzonitriles:  4-(Di-tert-butylamino)benzonitrile and 3-(Di-tert-butylamino)benzonitrile

The newly synthesized aminobenzonitriles with two bulky amino substituents 4-(di-tert-butylamino)benzonitrile (DTABN) and 3-(di-tert-butylamino)benzonitrile (mDTABN) have strongly twisted amino groups in the ground state. From X-ray crystal analysis it is found that the amino twist angle θ of mDTABN equals 86.5°, whereas a twist angle of around 75° is deduced for DTABN from the extinction coefficient of its lowest-energy absorption band in n-hexane. Because of the electronic decoupling between the amino and benzonitrile groups caused by these large twist angles, the absorption of DTABN and mDTABN is relatively weak below 40000 cm^-1, with extinction coefficients around 25 times smaller than those of the planar 4-(dimethylamino)benzonitrile (DMABN). DTABN as well as mDTABN undergo efficient intramolecular charge transfer (ICT) in the singlet excited state, in nonpolar (n-hexane) as well as in polar (acetonitrile) solvents. Their fluorescence spectra consist of an ICT emission band, without evidence for locally excited (LE) fluorescence. The occurrence of efficient ICT with mDTABN is different from the findings with all other N,N-dialkylaminobenzonitriles in the literature, for which ICT only appears with the para-derivative. From solvatochromic measurements, an ICT dipole moment of 17 D is determined for DTABN as well as for mDTABN, similar to that of DMABN. The picosecond fluorescence decays of DTABN (time resolution 3 ps) are effectively single exponential. Their decay time is equal to the ICT lifetime τ‘₀(ICT), which increases with solvent polarity from 0.86 ns in n-hexane to 3.48 ns in MeCN at 25 °C. The femtosecond excited-state absorption (ESA) spectra of DTABN in n-hexane and MeCN at 22 °C show a decay of the LE and a corresponding rise of the ICT absorption. The ICT reaction time is 70 fs in n-hexane and 60 fs in MeCN. DTABN and mDTABN may have a strongly twisted ICT state, similar to that of 6-cyanobenzoquinuclidine but different from that of DMABN.