Synthesis, Structural, and Photophysical Studies of Phenylquinoline
and Phenylquinolinyl Alkynyl Based Pt(II) Complexes
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Abstract
We present the synthesis, characterization, and photophysical properties
of two series of alkynyl–platinum(II) complexes containing
the phenylquinoline (Hpq) or the cyclometalated phenylquinolinyl (pq)
ligand. Unusual monoanionic complexes (NBu<sub>4</sub>)[Pt(CCR)<sub>3</sub>(Hpq)] (R = Tol <b>1</b>, C<sub>6</sub>H<sub>4</sub>OMe-3 <b>2</b>, C<sub>6</sub>H<sub>4</sub>CF<sub>3</sub>-4 <b>3</b>) were generated in moderate yield by treatment of [Pt(pq)(μ-Cl)]<sub>2</sub> with an excess of LiCCR (1:8), followed by subsequent
treatment with NBu<sub>4</sub> in <i>i</i>PrOH. However, a similar reaction with LiCC<i><sup>t</sup></i>Bu evolves with formation of [Pt(pq)(μ–<i>κC</i><sup>α</sup>:η<sup>2</sup>-CC<sup><i>t</i></sup>Bu)]<sub>2</sub> <b>4</b>. Related
dimers [Pt(pq)(μ–<i>κC</i><sup>α</sup>:η<sup>2</sup>-CCR)]<sub>2</sub> (R = Tol <b>5</b>, C<sub>6</sub>H<sub>4</sub>OMe-3 <b>6</b>, C<sub>6</sub>H<sub>4</sub>CF<sub>3</sub>-4 <b>7</b>) were
successfully prepared by modification of the reaction conditions.
Single-crystal studies were performed on <b>2</b>, <b>4</b>, and <b>5</b>, revealing the presence of <i>intramolecular</i> (H···Pt and H···CC) and <i>intermolecular</i> O<sub>methoxy</sub>···H<sub>Hpq</sub> contacts in <b>2</b> and the formation of butterfly
structures in <b>4</b> and <b>5</b>. In the <i><sup>t</sup></i>Bu derivative <b>4</b>, a shorter intramolecular
Pt···Pt separation was found [3.0936(3) Å in <b>4</b> vs 3.2109(3) Å in <b>5</b>], which is related
to the nature of η<sup>2</sup>-alkynyl linkages (η<sup>2</sup>-<i>asymmetrical</i> in <b>4</b> vs η<sup>2</sup>-<i>symmetrical</i> in <b>5</b>). Anionic <b>1</b>–<b>3</b> display bright emissions in rigid
media (solid, glass) with resolved side vibronic bands at 77 K. The
energy of the emissions (supported by computational studies on <b>2</b>) allows us to ascribe these emissions primarily to a mixed
alkynyl–metal to the phenylquinoline <sup>3</sup>[(L′
+ M)LCT] excited state, being perturbed in <b>2</b> (solid)
by the <i>intermolecular</i> O<sub>methoxy</sub>···H<sub>Hpq</sub> contacts. Whereas diplatinum complexes <b>5</b>–<b>7</b> have no visible emission at 298 K in the solid state, <b>4</b> shows a structureless band (578 nm), which is significantly
red-shifted at 77 K (595 nm). Theoretical studies suggest a notable
distortion in the excited state in relation to S<sub>0</sub> and identify
the emission within the dimetallacycle Pt<sub>2</sub>(CC)<sub>2</sub> with alkynyl to pq charge-transfer <sup>3</sup>[(MM + L′)LCT]
[with minor <sup>3</sup>IL (pq) contribution], whereas, in <b>5</b>, the emission is tentatively ascribed to a mixture of <sup>3</sup>L′LCT with minor <sup>3</sup>MMLCT character. The emission
profiles observed in glassy solution for all <b>4</b>–<b>7</b> complexes (556–562 nm, CH<sub>2</sub>Cl<sub>2</sub>) and in the solid state for <b>6</b>–<b>7</b> are consistent with those exhibited by mononuclear related chromophores <sup>3</sup>[(L′ + M)LCT] (L′ = CCR, L = pq), suggesting
negligible contribution of the Pt···Pt interaction
