Impact of Cyclometalation and π-Conjugation on Photoisomerization of an N,C-Chelate Organoboron Compound

N,C-Chelate four-coordinate boron compounds that contain a B(ppy)Mes(2) unit (ppy = 2-phenylpyridyl, Mes = mesityl) are a new class of photochromic molecules discovered recently by our group that undergo photoisomerization upon exposure to light. To examine the influence of a covalently bound transition metal ion on the photochromic properties of this class of boron compounds, a new molecule (L1) that contains two linearly conjugated ppy units has been synthesized. A BMes(2) group was attached to L1 via chelation with one of the ppy units, producing a new four-coordinate boron compound, B1. The reactions of B1 with PtPh2(DMSO)(2) produced a Pt(II) cyclometalated compound, Pt!, where a PtPh(DMSO) unit is bound to the second ppy unit of B1. Replacement of DMSO in Pt1 by p-t-Bu-pyridine provided a new compound, Pt2. A third Pt(II) compound, Pt3, where a Pt(dpm) group (dpm = dipivaloylmethane) is chelated to the second ppy site of B1, was also synthesized successfully. The crystal structures of B1 and Pt3 have been determined by single-crystal X-ray diffraction analyses. The photophysical and photochromic properties of B1 and Pt1-Pt3 have been examined. Experimental and computational studies established that Pt(II) cyclometalation to B1 stabilizes a (LC)-L-3 state that involves pi ->pi* transitions localized on the ppy ppy conjugated backbone. This (LC)-L-3 state of the Pt(II) compounds is highly phosphorescent, with quantum efficiencies being 0.16, 0.13, and 0.45 for Pt!, Pt2, and Pt3, respectively, in toluene and at ambient temperature. The B(ppy)Mes(2) chromophore in all three Pt (II) compounds has been found to undergo photoisomerization in a similar manner to that of B1, but with a much lower quantum efficiency than B1. Deactivation of the photoisomerization process by the (LC)-L-3 state has been found to be most likely responsible for the low photoisomerization quantum efficiency of the Pt(II) compounds.