Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, USA.
J Phys Chem B. 2010 Nov 18;114(45):14440-9. doi: 10.1021/jp101528z. Epub 2010 May 14.
The synthesis, photophysics, and excited state absorption properties of three platinum(II) terpyridyl acetylide charge transfer (CT) complexes possessing a lone ancillary ligand systematically varied in phenylacetylide (PA) π-conjugation length, [Pt((t)Bu(3)tpy)(C≡C-C(6)H(4)-H)]ClO(4) (n = 1, 2, 3), are described. Density functional theory (DFT) calculations performed on the ground states of complexes 1, 2, and 3 reveal that their HOMOs reside mainly on the ancillary π-conjugated PA moiety, ranging from 86 to 97%, with LUMOs predominantly centered on the terpyridyl acceptor ligand (91-92%). This electronic structure leads to the production of a triplet ligand-to-ligand CT ((3)LLCT) excited state upon visible light excitation with minor contributions from the corresponding triplet metal-to-ligand CT ((3)MLCT) excited state. Unusually strong red-to-near-IR transient absorptions are produced in the excited states of these molecules following selective long wavelength visible excitation of the low energy CT bands that do not emanate from the terpyridyl radical anion produced in the CT excited state or from an arylacetylide-based triplet intraligand ((3)IL) excited state. The extinction coefficients of these low energy absorption transients were determined using the energy transfer method with anthracene serving as the triplet acceptor. A detailed theoretical investigation using DFT and TDDFT methods reveals that these intense near-IR transient absorptions involve transitions resulting from transient oxidation of the PA subunit. In essence, the production of the (3)LLCT excited state transiently oxidizes the PA moiety by one electron, producing the corresponding highly absorbing radical cation-like species, analogous to that experienced in related intramolecular photoinduced electron transfer reactions. The computational work successfully predicts the oscillator strength and peak wavelength of the measured excited state absorption transients across this series of molecules. In the present effort, there is a convergence of theory and experiment given that the excited state absorption properties of these Pt(II) chromophores are determined by localized transitions that resemble open shell radical cation species.
三种铂（II）三吡啶基乙炔电荷转移（CT）配合物的合成、光物理和激发态吸收性质被描述，这些配合物具有系统变化的苯乙炔（PA）π共轭长度的孤立辅助配体，[Pt((t)Bu(3)tpy)(C≡C-C(6)H(4)-H)]ClO(4)（n = 1、2、3）。对配合物 1、2 和 3 的基态进行的密度泛函理论（DFT）计算表明，它们的 HOMO 主要位于辅助π共轭 PA 部分，范围为 86-97%，而 LUMO 主要集中在三吡啶受体配体上（91-92%）。这种电子结构导致在可见光照激发下产生三重态配体-配体 CT（（3）LLCT）激发态，而相应的三重态金属-配体 CT（（3）MLCT）激发态的贡献较小。在这些分子的激发态中，在选择性长波长可见光激发低能量 CT 带时，会产生异常强的红到近红外瞬态吸收，这些 CT 带不源于 CT 激发态中产生的三吡啶自由基阴离子或基于芳基乙炔的三重态内配体（（3）IL）激发态。使用蒽作为三重态受体，通过能量转移法确定了这些低能量吸收瞬变的消光系数。使用 DFT 和 TDDFT 方法进行的详细理论研究表明，这些强近红外瞬态吸收涉及由 PA 亚基瞬态氧化引起的跃迁。本质上，（3）LLCT 激发态的产生通过一个电子暂时氧化 PA 部分，产生相应的高吸收自由基阳离子样物质，类似于在相关的分子内光诱导电子转移反应中经历的那样。该计算工作成功预测了一系列分子中测量的激发态吸收瞬变的振子强度和峰值波长。在目前的研究中，理论和实验的结果趋于一致，因为这些 Pt(II) 生色团的激发态吸收性质由类似于开壳自由基阳离子物质的局域化跃迁决定。
