Exploring excited states of Pt(ii) diimine catecholates for photoinduced charge separation

Abstract

The intense absorption in the red part of the visible range, and the presence of a lowest charge-transfer excited state, render Platinum(II) diimine catecholates potentially promising candidates for light-driven applications. Here, we test their potential as sensitisers in dye-sensitised solar cells and apply, for the first time, the sensitive method of photoacoustic calorimetry (PAC) to determine the efficiency of electron injection in the semiconductor from a photoexcited Pt(II) complex. Pt(II) catecholates containing 2,2′-bipyridine-4,4′-di-carboxylic acid (dcbpy) have been prepared from their parent iso-propyl ester derivatives, complexes of 2,2′-bipyridine-4,4′-di-C(O)OiPr, (COOiPr)2bpy, and their photophysical and electrochemical properties studied. Modifying diimine Pt(II) catecholates with carboxylic acid functionality has allowed for the anchoring of these complexes to thin film TiO2, where steric bulk of the complexes (3,5-di(t)Bu-catechol vs. catechol) has been found to significantly influence the extent of monolayer surface coverage. Dye-sensitised solar cells using Pt(dcbpy)((t)Bu2Cat), 1a, and Pt(dcbpy)(pCat), 2a, as sensitisers, have been assembled, and photovoltaic measurements performed. The observed low, 0.02–0.07%, device efficiency of such DSSCs is attributed at least in part to the short excited state lifetime of the sensitisers, inherent to this class of complexes. The lifetime of the charge-transfer ML/LLCT excited state in Pt((COO(I)Pr)2bpy)(3,5-di-(t)Bu-catechol) was determined as 250 ps by picosecond time-resolved infrared spectroscopy, TRIR. The measured increase in device efficiency for 2a over 1a is consistent with a similar increase in the quantum yield of charge separation (where the complex acts as a donor and the semiconductor as an acceptor) determined by PAC, and is also proportional to the increased surface loading achieved with 2a. It is concluded that the relative efficiency of devices sensitised with these particular Pt(II) species is governed by the degree of surface coverage. Overall, this work demonstrates the use of Pt(diimine)(catecholate) complexes as potential photosensitizers in solar cells, and the first application of photoacoustic calorimetry to Pt(II) complexes in general.

Square-Planar Heteroleptic Complexes of α-Diimine-NiII-Catecholate Type: Intramolecular Ligand-to-Ligand Charge Transfer

“…The synergistic existence of donor and acceptor organic parts in the coordination sphere of the metal center is a distinctive feature for a wide range of LL'CT chromophores. The acceptor moiety is usually represented by a neutral redox form of α-diimines (Scheme 1) with significant steric hindrances [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] and/or a rigid carbon skeleton [13,15,[25][26][27]. Thus, it provides the required planarity of the complex's structure.…”

Tuning PtII‐Based Donor–Acceptor Systems through Ligand Design: Effects on Frontier Orbitals, Redox Potentials, UV/Vis/NIR Absorptions, Electrochromism, and Photocatalysis

“…Group 10 metalsi nt heir d 8 electronic configuration have served to synthesize ar ange of donor-acceptor metalc omplexes. [1,[2][3][4][5] Thesec ompounds are usually characterized by intense ligand-to-ligand charge transfers (LL'CTs), which impart unique photochemical and photophysical properties on the resulting metal complexes. [2,6] Applications range from dye-sensitized solar cells to small molecule activation and catalysis.…”

“…[1,[2][3][4][5] Thesec ompounds are usually characterized by intense ligand-to-ligand charge transfers (LL'CTs), which impart unique photochemical and photophysical properties on the resulting metal complexes. [2,6] Applications range from dye-sensitized solar cells to small molecule activation and catalysis. [4,7,8] To obtain ac ompound with as trongL L 'CT transition, it is desirable to use as trong p-acceptor and as trong s-a nd/or p-donorl igand, with favorable orbitale nergies.…”

Stabilising the lowest energy charge-separated state in a {metal chromophore – fullerene} assembly: a tuneable panchromatic absorbing donor–acceptor triad

“…It is worth noting the sub-picosecond ISC has been reported for many small transition metal complexes, 61 and the lowest detected excited state in Pt( ii ) diimine catecholates has been assigned as a 3 MLCT/LLCT in the previous works. 38 , 62 , 63 It has also been proposed that S 1 → T 1 conversion in (bpy)Pt(cat) may be comparatively slow with regard to the IC deactivation of S 1 to the ground state if C 2v symmetry is maintained. 64 Since the early time spectra (∼0.5 ps), electronic and vibrational, of the model (diimine)Pt(cat) compounds and the asymmetric 3 are very similar, and no stimulated emission was detected in the transient absorption spectra, we propose that the first detected excited state as 3 CT, (C 60 -bpy˙ – )-Pt-(cat˙ + ) in its nature, and that electron transfer from bpy-anion to C 60 -moiety occurs in the triplet manifold.…”