Di- and Tricyanovinyl-Substituted Triphenylamines: Structural and Computational Studies

Abstract

We report herein on the solid-state structures of three closely related triphenylamine derivatives endowed with tricyanovinyl (TCV) and dicyanovinyl (DCV) groups. The molecules described contain structural features commonly found in the design of functional organic materials, especially donor-acceptor molecular and polymeric architectures. The common feature noticeable in these structures is the impact of these exceptionally strong electron-accepting groups in forcing partial planarity of the portion of the molecule carrying these groups and directing the molecular packing in the solid state, resulting in the formation of π-stacks of dimers within the unit cell of each. Stacks are formed between phenyl groups bearing electron-accepting groups on two adjacent molecules. Short π-π stack distances ranging from 3.283 to 3.671 Å were observed. Such motif patterns are thought to be conducive for better charge transport in organic semiconductors and enhanced device performance. Intramolecular charge transfer is evident from the shortening of the observed experimental bond lengths in all three compounds. The nitrogen atoms (of the cyano groups) have been shown to be extensively involved in short contacts in all three structures, primarily through C-H···NC interactions with distances as short as 2.462 Å. The compounds reported here are (3,3-dicyano-2-(4-(diphenylamino)phenyl)-1λ³-allylidene)amide or tricyanovinyltriphenylamine, Ph₃NTCV (1); 2-(4-(diphenylamino)benzylidene)-malononitrile or dicyanovinyltriphenylamine, Ph₃NDCV (2); and (3,3-dicyano-2-(4-(di-p-tolylamino)phenyl)-1λ³-allylidene)amide or dimethyltricyanovinyltriphenylamine, Me₂Ph₃NTCV (3). Results of density functional theory calculations using DFT-B3LYP/6-31G(d,p) indicate the lowering of LUMO levels as a result of the introduction of these groups with band gaps of 3.13, 2.61, and 2.55 eV for compounds 1-3, respectively, compared with 4.65 eV calculated for triphenylamine. This is supported by the electronic and fluorescence spectra of these molecules with absorption λ_max of 483, 515, and 545 nm for compounds 1, 2, and 3, respectively.

Figure 1

Structures of molecules in this study.

Figure 2

UV–vis (a) and fluorescence (b) spectra of compounds 1–3. The emission spectra are excited at 355 nm. Both absorption and emission spectra were measured in dichloromethane.

Figure 3

π-Stacking and C–H···N interactions in compounds 1 (a), 2 (b), and 3 (c).

Figure 4

Bond lengths highlight the intramolecular charge transfer path in 1.

Figure 5

Calculated molecular orbitals for triphenylamine and compounds 1–3, DFT-B3LYP/6-31G(d,p).

Figure 6

Electrostatic potential map (EPM) as calculated at the DFT-B3LYP level of compound 1 shows a highly negative electrostatic potential (red) located on the TCV group.