Rsc_cc_c0cc04443a 1..3

Platinum diimine bis(alkynyl), bis(thiolate) or bis(alkoxolate) complexes (diimine)Pt(XR)2 (XR=C CR, OR, SR) constitute a prominent class of photosensitizers with numerous applications in the fields of photocatalysis and light to (chemical) energy conversion; e.g., photocatalysed hydrogen production. The photoreactive state is usually accessed by excitation into the (RX)2Pt-diimine charge-transfer (CT) band with the strongly mixed metal alkynyl/thiolate/alkoxolate HOMO as the donor and a diimine based p* orbital as the acceptor orbital. The charge-separated state thus generated may be reactive by itself or can be quenched by a sacrificial electron donor to yield the strongly reducing (diimine)Pt(XR)2 as the reactive species. Despite their tremendous success (diimine)Pt(XR)2-type photosensitizers usually suffer from the rather moderate absorption coefficient of the (RX)2Pt-diimine CT band and from the limited spectroscopic window as defined by the envelope of the CT band that can be utilized to generate the photoreactive state. When considering the significant contributions of S or O atoms to the HOMO of diimine bis(thiolate) or -(alkoxolate) platinum complexes, it occurred to us, that their incorporation into the extended p-system of an organic dye might serve to funnel the energy of a higher lying dye-based excited state into the lower lying (RX)2Pt-diimine CT state as it has already been demonstrated for coumarine-[Ru(terpy)2] -type dyads. As a test of our hypothesis we prepared and investigated complexes 1–3 (see Chart 1) where the chalcogen donor atoms X are part of coumarine dyes. Coumarine dyes are particularly well suited for potential applications in photochemical hydrogen production owing to their stability in protic environments. Complexes 1–3 were synthesized from (Bu2bipy)PtCl2 by simple salt metathesis with the respective hydroxylato or thiolato substituted coumarine.w In the solid state, complexes 1 and 2 possess the expected square planar platinum coordination with the Pt–N (2.051(5), 2.052(5)) and Pt–S (2.291(1), 2.293(1) Å, 1) or Pt–N (range 1.992(7) to 2.012(6)) and Pt–O (range 2.009(6) to 2.026(8) Å for the three independent molecules of 2) bond lengths in the usual range.z Both coumarine substituents are rotated out of the PtN2S2 or PtN2O2 planes by 67.1 or 83.71 (1) or 46.9 to 77.31 (2) (Fig. 1, Fig. S1, ESIw). Absorption spectra of 1 and 2 in CH2Cl2 feature moderately intense (e E 3000 M 1 cm ) asymmetric bands with maxima at 470 and 410 nm with a moderate negative solvatochromism and much more intense (e E 38 000 M 1 cm ) absorptions at 390 and 360 nm, respectively, as well as bands in the 320–300 nm range. All higher-energy bands display much lesser positional dependence on solvent polarity (Fig. S2–S4, ESIw). Literature precedence suggests an assignment of the low-energy absorptions as (RX)2PtBu2bipy1* CT transition(s) with two or more underlying subbands. The intense band at higher