Electron delocalization in the S ₁ and T ₁ metal-to-ligand charge transfer states of trans -substituted metal quadruply bonded complexes

Abstract

Electron delocalization in the S 1 and T 1 metal-toligand charge transfer states of trans-substituted metal quadruply bonded complexes . These differences arise from consideration of the relative orbital energies of the M 2 δ or M 2 δ* and the ligand π Ã as well as the magnitudes of orbital overlap.excited state mixed valency | vibrational spectroscopy | potential energy surface | cyano group | ethynyl group C onjugated organic polymers have been intensely studied over the past two decades because of their fascinating optoelectronic properties. Aside from the sheer scientific curiosity that they aroused, it soon was recognized that a plastic electronics industry was possible and a good deal of this expectation has already been realized. Conjugated organic polymers find commercial applications as field-effect transistors (1), light emitting diodes (2), and photovoltaic devices (3, 4). As an extension of this field there is considerable interest in incorporating metal ions into conjugated organic systems and numerous reports are to be found in the literature concerning the role of metal ions in tailoring the optoelectronic properties of the organic conjugated systems. For example, the attachment of π-conjugated ligands to Ir(III) has allowed the luminescence to cover the entire region of the visible spectrum (5, 6) and the incorporation of Pt(II) into conjugated ethynylthiophenes has led to a significant enhancement of the efficiency of a bulk heterojunction photocell (7).Knowledge of electronic structure is key to the understanding of these properties and the ability to manipulate electronic structure by selection of metal-organic components holds the promise of custom design. The MM quadruply bonded unit (MM ¼ Mo 2 , MoW, or W 2 ) has many attractive features due to the tunability of the energy of the M 2 δ orbital by selection of the metal and its attendant ligands and due to M 2 δ to organic π-conjugation. In this report, we show how this can influence the charge dynamics and delocalization of singlet and triplet photo-excited states that may be delocalized or localized (valence trapped) metal-to-ligand charge transfer (MLCT) states or MMδδ* states. To achieve this we have prepared the trans-substituted compounds M 2 ðT i PBÞ 2 ðO 2 CC 6 H 4 -4-CNÞ 2 , where T i PB ¼ 2,4,6-triisopropylbenzoate and M ¼ Mo (I) or W (I 0 ), and M 2 ðO 2 CMeÞ 2 ððN½ i PrÞ 2 CC≡ CC 6 H 5 Þ 2 , where M ¼ Mo (II) or W (II 0 ), whose structures are shown in Fig. 1. Here, the bulky T i PB and amidinate ligands favor the trans-substitution and allow the extended conjugation of the two trans-ligands via Lπ-M 2 δ-Lπ conjugation. Evidence of this is seen in the molecular structures found in the solid-state that reveal the near coplanarity of the aryl groups of the p-cyanobenzoate and amidinate ligands in I and II 0 , respectively.The ground state geometry and symmetry of these discrete compounds presents a situation where added charge could reside on either of two interchangeable redox active ligand sites. This situation is, by definition,...

Tuning the electronic structure of Mo–Mo quadruple bonds by N for O for S substitution

“…16,17 Investigations by Chisholm and coworkers on the photoexcited states of trans-[M 2 (TiPB) 2 (O 2 CR) 2 ] (R = 2-thiophene 18,19 or C 6 H 4 -4-CN; TiPB = 2,4,6-triisopropylbenzoate) 20 show emission from T 1 states that are about the dimetal core also makes MM multiply bonded paddlewheel compounds good candidates for incorporation into materials with potentially interesting optoelectronic properties. [21][22][23][24][25][26][27] This was highlighted in a detailed study by Zhou and co-workers which demonstrated that the shape and size of molecular architectures formed using dicarboxylate ligands to bridge Mo 2 4+ units can be controlled by tuning the bridging angle and size of the bridging dicarboxylate ligand.…”

Mixed valency and metal–metal quadruple bonds

“…The situation is quite different for the amidinate complexes trans M 2 (O 2 CMe) 2 [(Pr i N) 2 CC C-tolyl] 2 which show two C C stretches in the 1 MLCT states, one shifted ∼200 cm −1 and the other ∼40 cm −1 [29]. See Fig.…”

“…9 [29]. Both compounds, M 2 = Mo 2 and W 2 show an excited state C N stretch shifted ∼65 cm −1 to lower energy relative to the ground state IR spectrum.…”

“…Rather interestingly most Mo 2 -containing complexes show solvent independent phosphorescence but this is not from an MLCT state but rather from the 3 MoMo␦␦* Reproduced from Ref. [29] with permission. state.…”

“…Time-resolved infrared spectra of the compounds trans-M2(O2CMe)2[(Pr i N)2CC CC6H5]2 showing the C C stretching modes of their MLCT states.Reproduced from Ref [29]. with permission.…”