Two-State Intramolecular Charge Transfer (ICT) with 3,5-Dimethyl-4-(dimethylamino)benzonitrile (MMD) and Its Meta-Isomer mMMD. Ground State Amino Twist Not Essential for ICT

From X-ray structure analysis, amino
twist angles of 90.0°
for 2,4-dimethyl-3-(dimethylamino)­benzonitrile (mMMD), 82.7°
for 4-(di-tert-butylamino)­benzonitrile (DTABN), and
88.7° for 6-cyanobenzoquinuclidine (CBQ) are determined, all
considerably larger than the 57.4° 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
°C consists of two emissions, from a locally excited (LE) and
an intramolecular charge transfer (ICT) state, with a fluorescence
quantum yield ratio Φ′(ICT)/Φ­(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
μe(ICT) of MMD (16 D) and mMMD (15 D) are obtained.
Femtosecond excited state absorption (ESA) spectra at 22 °C,
together with the dual (LE + ICT) fluorescence, reveal that MMD in n-hexane undergoes a reversible LE ⇄ ICT reaction,
with LE as the precursor, with a forward rate constant ka = 5.6 × 1012 s–1 and
a back-reaction kd ∼ 0.05 ×
1012 s–1. With MMD in the strongly polar
solvent MeCN, ICT is faster: ka = 10 ×
1012 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 ×
1012 s–1. The ICT reactions with MMD
and mMMD are much faster than that of the parent compound DMABN in
MeCN, with ka = 0.24 × 1012 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 (ka = 0.24 × 1012 s–1 in
MeCN) is caused by a smaller energy gap ΔE(S1,S2) between the lowest singlet excited states
and not by the large amino twist angle. Similarly, the larger ΔE(S1,S2) of mMMD as compared with
MMD is held responsible for its smaller ICT efficiency (no reaction
in n-hexane).