Luminescent Organometallic Complexes Built upon the Nonemissive Azophenine

Azophenine, C6H2(NPh)2(NHPh)2, is renowned to be nonemissive
in solution or
in the solid state at 298 and 77 K. It was rendered luminescent in
solution at room temperature without using any cyclization strategy
of the N∧N end by anchoring two or four trans-RCCPt­(PBu3)2(CC)
units (R = hexa-n-hexyltruxene (Tru))
on the azophenine. Complexes of the general formulas C6H2(NC6H4CCSiMe3)2(NHPtTru)2 (DiPtTruQ) and C6H2(NPtTru)2(NHPtTru)2(TertPtTruQ), where Pt = trans-C6H4CCPt­(PBu3)2CC, exhibit
fluorescence (420 nm) and phosphorescence (512 nm) bands arising from
upper localized ππ*/C6H4CC
to TruCC charge transfer singlet and triplet
excited states in 2MeTHF at 298 and 77 K. This latter assignment is
based on DFT computations (B3LYP). Moreover, DiPtTru and TertPtTru exhibit low-energy absorption bands with maxima
in the 470–485 nm range extending all the way to 600–650
nm. These spectral features are associated with charge transfer (CT)
excited states: namely, TruPt → Q (Q = C6H2N2(NH)2). No emission band (fluorescence or phosphorescence) associated
with these CT states has been detected at 298 K, but weak fluorescence
bands (λmax ∼750 nm) decaying on the picosecond
time scale have been observed in both cases. Biexponential decays
were also often noted and likely reflect the presence of the possible
conformers associated with the two possible dihedral angles made by
the C6H4 plane and the central C6H2N2(NH)2 core. No evidence for
electron transfer between the TruPt arms and Q was observed.