Platinum Complexes of N,N',N″,N‴-Diboronazophenines.

Azophenine, (α-C6H5NH)2(C6H5-N═C6H2═N-C6H5), well known to be non-emissive, was rigidified by replacing two amine protons by two difluoroboranes (BF2+) and further functionalized at the para-positions of the phenyl groups by luminescent trans-ArC≡C-Pt(PR3)2-C≡C ([Pt]) arms [Ar = C6H4 (R = Et), hexa(n-hexyl)truxene) (Tru; R = Bu)]. Two effects are reported. First, the linking of these [Pt] arms with the central azophenine (C6H4-N═C6H2(NH)2═N-C6H4; Q) generates very low energy charge-transfer (CT) singlet and triplet excited states (3,1([Pt]-to-Q)*) with absorption bands extending all the way to 800 nm. Second, the rigidification of azophenine by the incorporation of BF2+ units renders the low-lying CT singlet state clearly emissive at 298 and 77 K in the near-IR region. DFT computations place the triplet emission in the 1200-1400 nm range, but no phosphorescence was detected. The photophysical properties are investigated, and circumstantial evidence for slow triplet energy transfers, 3Tru* → Q, is provided.