Donor-and-acceptor substituted truxenes as multifunctional fluorescent probes

A series of dimesitylboryl acceptor (mesityl=2,4,6-trimethylphenyl) and/or diphenylamino donor (-N(Ph)2)-substituted truxene derivatives, classified as D-or-A substituted compounds and D-and-A substituted charge-transfer compounds, have been synthesized. Two D-and-A substituted truxene compounds, namely, 2-dimesitylboryl-7,12-di(N,N-diphenylamino)-5,5',10,10',15,15'-hexaethyltruxene (BN2) and 2,7-di(dimesitylboryl)-12-(N,N-diphenylamino)-5,5',10,10',15,15'-hexaethyltruxene (B2N), exhibit extraordinarily large solvatochromism ranging from 420 nm (in hexane) to 580 nm (in acetonitrile) in aprotic solvents, which can be used to probe the polarity of the solution environment. Due to proton-donor interactions, the solvatochromic red shift of BN2 and B2N in protic solvents has been significantly decreased, and this effect can be used to identify local protic and aprotic environment. Furthermore, because of the interaction between F- and acceptor, BN2 and B2N show sharp spectral response to fluoride ion concentration. The simultaneous "turn-off" at 500 nm and "turn-on" at 380 nm of the fluorescence signal have provided a good example of a fluorescent ratiometric method, which can greatly enhance the sensitivity of the fluoride ion probe. Underlying these interesting spectral phenomena and multifunctional probe properties is the charge-transfer strategy of grafting donor and acceptor moieties, as A-pi-D2 or A2-pi-D style, to the triangular truxene.