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

Abstract

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 theta of mDTABN equals 86.5 degrees , whereas a twist angle of around 75 degrees 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 tau'0(ICT), which increases with solvent polarity from 0.86 ns in n-hexane to 3.48 ns in MeCN at 25 degrees C. The femtosecond excited-state absorption (ESA) spectra of DTABN in n-hexane and MeCN at 22 degrees 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.

Ultrafast photoinduced intramolecular charge transfer in push–pull distyryl furan and benzofuran: solvent and molecular structure effect

“…Photoinduced intramolecular charge transfer was first observed in 4-(N,N-dimethylamino)benzonitrile. 3 After the first observation many push-pull systems such as aminobenzonitrile derivatives, [4][5][6][7][8][9][10][11] transition metal complexes, 12 N-phenyl pyrrole derivatives, 13,14 bianthryl derivatives, 15 peryleneimide substituted oligothiophene, 16 tetrahydroquinoline derivatives, 17 pyrenyl biphenyl ester systems, 18 1,3-indandione derivatives, 19 pyrene-phenothiazine dyads, 20 push-pull polyenes, 21,22 push-pull stilbene derivatives [23][24][25][26][27][28] have been studied. Solvent dependent dual fluorescence or fluoro-solvatochromism has been found to occur in these systems.…”

Tuning dual emission behavior of p-dialkylaminobenzonitriles by supramolecular interactions with cyclodextrin hosts

“…12 The structural identity of the CT state has been the subject of active discussion in recent years mainly in relation to the rotation of the amino group, denoting it either as the twisted intramolecular charge transfer (TICT) state, or the planar intramolecular charge transfer (PICT) state or simply the intramolecular charge transfer (ICT) state. [14][15][16][17][18][19] In the present work, to avoid any ambiguity, we denote this longer wavelength emission band as the ICT emission. Since the LE to ICT conversion is highly sensitive to solvent polarity, considering that the dye experiences a much lower polarity inside a host cavity, it is expected that the dual emission behavior of these dyes can be largely modulated by inclusion complex formation.…”

A Dimethylaminophenyl‐Substituted Naphtho[1,2‐b]quinolizinium as a Multicolor NIR Probe for the Fluorimetric Detection of Intracellular Nucleic Acids and Proteins

“…In this particular case, the CS‐TICT transition requires a rotation about the biaryl axis, which is slowed down in more viscous media, resulting in an increased fluorescence quantum yield from the CS state [37] . Additionally, the solvation of the CS state in viscous polar media is slowed down [39a,42] preventing an efficient non‐radiative deactivation of the ICT states with low energy. Lastly, the increased emission intensity of compound 3 in acidic solution is caused by the complete suppression of the photoinduced CS by protonation of the aniline substituent, so that the regular and fairly emissive excited state of the quinolizinium unit is reached, as has been already observed for the resembling derivative 4 [23,43] …”