Trivalent Boron as an Acceptor in Donor–π–Acceptor‐Type Compounds for Single‐ and Two‐Photon Excited Fluorescence

The synthesis, structure, and fluorescence properties of a series of new donor-pi-acceptor (D-pi-A) type compounds, with a trivalent boron, protected by two mesityl groups, as acceptor, and with various typical donors and different pi-conjugated bridges, are reported. All these stable organoboron compounds show intense single-photon excited fluorescence (SPEF) and two-photon excited fluorescence (TPEF) in a wide spectral range from blue to green, with the spectral peak position of the SPEF being basically the same as that of the TPEF. The remarkably strong Cbond;B(mesityl)(2) bonding, and the well-conjugated pi-system, shown in X-ray crystal structures of two compounds, indicate some charge transfer features of the ground state. Meanwhile, spectral data indicate that the charge transfer from donor to acceptor is greatly enhanced in the excited states. Based on typical structural data and comprehensive spectral data, the following structure-property relationships can be drawn: 1) the moderate arylamino donor can more effectively enhance the SPEF and TPEF intensities than can the strong alkylamino donor; 2) stilbene is a better pi-bridge than styrylthiophene for its capability of enhancing and blue-shifting the SPEF and TPEF of the corresponding D-pi-A compounds; and 3) when compared to its boron-free precursors and other analogues, -B(mesityl)(2) invariably and consistently acts as an effective SPEF and TPEF fluorophore in all this series of organoboron compounds, which may result from its strong pi-electron-withdrawing and charge transfer-inducing nature in the ground-state and, more dominantly, in the excited-state. Combining all the above positive structure factors, trans-4'-N,N-diphenylamino-4-dimesitylborylstilbene (compound 3) stands out as the optimized green SPEF and TPEF emitter. This compound exhibits an SPEF quantum yield Phi of 0.91 at 522 nm in THF, a TPEF cross-section sigma' that is an order of magnitude larger than that of its boron-free precursor upon excitation by 800 nm femto-second laser pulses, and a two-photon absorption section sigma of 3.0 x 10(-48) cm(4) s. In the blue light region, trans-4'-N-carbazolyl-4-dimesitylboryl-stilbene (compound 4) shows significant SPEF and TPEF properties, with Phi=0.79 at 464 nm in THF and a large sigma' value, which is five times that of fluorescein upon excitation by 740 nm femto-second laser pulses.