Charge transfer emission in oligotriarylamine-triarylborane compounds

Donor-acceptor compounds exhibiting charge transfer emission are of interest in a variety of different contexts, for example, for nonlinear optical processes and for sensor applications. Recently investigated triarylamine-triarylborane compounds represent an important class of donor-acceptor systems, and we explored to what extent their charge-transfer properties can be further improved by using stronger amine donors and borane acceptors than prior studies. The oligotriarylamine employed here is a much stronger donor than previously used triarylamines containing single nitrogen centers. In order to increase the acceptor strength, the electron-accepting unit was equipped with two (instead of one) dimesitylboron substituents. In our comparative study, six donor-acceptor compounds were synthesized and investigated by cyclic voltammetry and optical spectroscopy. An increase of the donor strength through replacement of an ordinary triarylamine by an oligotriarylamine unit leads to the expected energetic stabilization of charge transfer (CT) excited states, but the emission solvatochromism is not more pronounced. The attempted increase of the acceptor strength by substitution of the acceptor moiety by two (instead of one) dimesitylboron groups leads to a drastic decrease of emission quantum yields. On the basis of these results, our purely experimental study provides fundamental guidelines for the design of new triarylamine-triarylborane donor-acceptor compounds with favorable charge-transfer emission properties.