Ultrafast Intramolecular Charge Transfer with Strongly Twisted Aminobenzonitriles: 4-(Di-<i>tert</i>-butylamino)benzonitrile and 3-(Di-<i>tert</i>-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.本研究中新合成的带有两个大位阻氨基取代基的氨基苯甲腈类（aminobenzonitriles）化合物：4-(二叔丁基氨基)苯甲腈（4-(di-tert-butylamino)benzonitrile, DTABN）与3-(二叔丁基氨基)苯甲腈（3-(di-tert-butylamino)benzonitrile, mDTABN），其基态下氨基基团呈现强烈扭转结构。经X射线晶体分析（X-ray crystal analysis）测得，mDTABN的氨基扭转角θ为86.5°；而通过其在正己烷（n-hexane）中最低能量吸收带的消光系数（extinction coefficient）可推得，DTABN的氨基扭转角约为75°。由于上述大扭转角导致氨基与苯甲腈基团间发生电子解耦作用，DTABN与mDTABN在40000 cm⁻¹以下的吸收强度相对较弱，其消光系数约为平面型4-(二甲基氨基)苯甲腈（4-(dimethylamino)benzonitrile, DMABN）的1/25。无论是DTABN还是mDTABN，在非极性溶剂正己烷与极性溶剂乙腈（acetonitrile, MeCN）的单重激发态（singlet excited state）下均会发生高效的分子内电荷转移（intramolecular charge transfer, ICT）。其荧光光谱（fluorescence spectra）仅呈现ICT发射带，未观测到局域激发（locally excited, LE）荧光的特征信号。mDTABN能够发生高效ICT这一现象，与文献中已报道的所有其他N,N-二烷基氨基苯甲腈类化合物的行为截然不同——这类化合物仅对位取代衍生物才会出现ICT过程。通过溶剂化显色测量（solvatochromic measurements）可知，DTABN与mDTABN的ICT偶极矩（dipole moment）均为17 D，与DMABN的偶极矩相近。在25 ℃条件下，DTABN的皮秒级荧光衰减（picosecond fluorescence decays）测试（时间分辨率为3 ps）结果基本符合单指数衰减（single exponential）规律，其衰减时间等于ICT寿命τ‘0(ICT)；该寿命随溶剂极性增大而升高，在正己烷中为0.86 ns，在乙腈中则升至3.48 ns。在22 ℃条件下，DTABN在正己烷与乙腈中的飞秒级激发态吸收（femtosecond excited-state absorption, ESA）光谱显示：局域激发态吸收逐渐衰减，同时分子内电荷转移态吸收随之同步增长。该ICT过程的反应时间在正己烷中为70 fs，在乙腈中为60 fs。DTABN与mDTABN可能拥有与6-氰基苯并奎宁环（6-cyanobenzoquinuclidine）类似的强扭转ICT态，这一结构与DMABN的ICT态存在显著差异。