A Computational Tddft Study On Intramolecular Charge Transfer In Di-tert-butylaminobenzonitriles And 2,4,6-tricyanoanilines.

Abstract

We have carried out TDDFT computational studies on the low-lying excited states of di-{\it tert}-butylaminobenzonitrile and 2,4,6-tricyanoaniline compounds that exhibit unusual photophysical behaviors associated with the intramolecular charge transfer (ICT). For both 3- and 4-di-{\it tert}-butylamino)benzonitriles ({\it m}-DTBABN and {\it p}-DTBABN, respectively) show the ICT formation, and {\it p}-DTBABN appears to be the only {\it meta}-substituted aminobenzonitrile that exhibits the ICT formation. The TDDFT calculations indicate evidence that the ultrafast ICT formation in {\it p}-DTBABN and {\it m}-DTBABN is due to the sequential state switches: $\pi\pi^{*}(L_{\rm a})\rightarrow \pi\sigma^{*}\rightarrow$ ICT in the presence of conical intersections among the three closely-lying excited-states. On the other hand, 2,4,6-tricyanoaniline does not show clear evidence for the LE (locally excited) state $\rightarrow$ ICT state formation from steady-state fluorescence studies, despite the greater electron acceptor strength of tricycanobenzene as compared to monocyanobenzene, which is part of a 4-(dimethylamino)benzonitrile ({\it p}-DMABN) compound.

However, it is predicted that 2,4,6-tricyano-{\it N,N}-dimethylaniline (TCDMA), but not 2,4,6-tricyanoaniline (TCA), possesses two ICT states, which show the ICT-characterized quinoidal structures and lie below the initially photo-excited $S_{1}(\pi\pi^{*})$ state. The CC2 calculations further predict two conformers as labeled with quinoidal (ICT--Q) and anti-quinoidal (ICT--AQ) structures are rapidly interconnecting with each other. The lower energy ICT--Q structure tends to be populated from the unstable ICT--AQ structure, which is responsible for the observed time-resolved fluorescence as well as the excited-state absorption from the mixed $S_{1}(\pi\pi^{*})$/ICT state of TCDMA. In both cases for TCDMA and TCA, the $\pi\sigma^{*}$ state locates significantly higher in energy than the $S_{1}(\pi\pi^{*})$ state (and the ICT state for TCA), thus precluding the $\pi\sigma^{*}\rightarrow$ ICT formation, which is believed to occur in a {\it p}-DMABN in polar environments.