Yagnaseni Ghosh

Electron delocalization in the S1 and T1 metal-to-ligand charge transfer states of trans-substituted metal quadruply bonded complexes

The singlet S1 and triplet T1 photoexcited states of the compounds containing MM quadruple bonds trans-M2(TiPB)2(O2CC6H4-4-CN)2, where TiPB = 2,4,6-triisopropylbenzoate and M = Mo (I) or M = W (I′), and trans-M2(O2CMe)2((N[i Pr ])2CC ≡ CC6H5)2, where M = Mo (II) and M = W (II′), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I′ but localized for II and II′ involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMoδδ* for both I and II but delocalized 3MLCT for I′ and localized 3MLCT for II′. These differences arise from consideration of the relative orbital energies of the M2δ or M2δ* and the ligand π∗ as well as the magnitudes of orbital overlap.

The remarkable influence of M2δ to thienyl π conjugation in oligothiophenes incorporating MM quadruple bonds

Oligothiophenes incorporating MM quadruple bonds have been prepared from the reactions between Mo2(TiPB)4 (TiPB = 2,4,6-triisopropyl benzoate) and 3′,4′-dihexyl-2,2′-:5′,2″-terthiophene-5,5″-dicarboxylic acid. The oligomers of empirical formula Mo2(TiPB)2(O2C(Th)-C4(n-hexyl)2S-(Th)CO2) are soluble in THF and form thin films with spin-coating (Th = thiophene). The reactions between Mo2(TiPB)4 and 2-thienylcarboxylic acid (Th-H), 2,2′-bithiophene-5-carboxylic acid (BTh-H), and (2,2′:5′,2″-terthiophene)-5-carboxylic acid (TTh-H) yield compounds of formula trans-Mo2(TiPB)2L2, where L = Th, BTh, and TTh (the corresponding thienylcarboxylate), and these compounds are considered as models for the aforementioned oligomers. In all cases, the thienyl groups are substituted or coupled at the 2,5 positions. Based on the x-ray analysis, the molecular structure of trans-Mo2(TiPB)2(BTh)2 reveals an extended Lπ-M2δ-Lπ conjugation. Calculations of the electronic structures on model compounds, in which the TiPB are substituted by formate ligands, reveal that the HOMO is mainly attributed to the M2δ orbital, which is stabilized by back-bonding to one of the thienylcarboxylate π* combinations, and the LUMO is an in-phase combination of the thienylcarboxylate π* orbitals. The compounds and the oligomers are intensely colored due to M2δ–thienyl carboxylate π* charge transfer transitions that fall in the visible region of the spectrum. For the molybdenum complexes and their oligomers, the photophysical properties have been studied by steady-state absorption spectroscopy and emission spectroscopy, together with time-resolved emission and transient absorption for the determination of relaxation dynamics. Remarkably, THF solutions the molybdenum complexes show room-temperature dual emission, fluorescence and phosphorescence, originating mainly from 1MLCT and 3MM(δδ*) states, respectively. With increasing number of thienyl rings from 1 to 3, the observed lifetimes of the 1MLCT state increase from 4 to 12 ps, while the phosphorescence lifetimes are ≈80 μs. The oligomers show similar photophysical properties as the corresponding monomers in THF but have notably longer-lived triplet states, ≈200 μs in thin films. These results, when compared with metallated oligothiophenes of the later transition elements, reveal that M2δ–thienyl π conjugation leads to a very small energy gap between the 1MLCT and 3MLCT states of <0.6 eV.

Quadruply Bonded Dimetal Units Supported by 2,4,6-Triisopropylbenzoates MM(TiPB)4 (MM ) Mo2, MoW, and W2): Preparation and Photophysical Properties

The preparation and characterization (elemental analysis, (1)H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4), where MM = MoW and W(2) and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo(2)(TiPB)(4), previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMdelta to arylcarboxylate pi* transitions, (1)MLCT. Each compound also shows luminescence from two excited states, assigned as the (1)MLCT and (3)MMdeltadelta* states. The energy of the emission from the (1)MLCT state follows the energy ordering MM = Mo(2) > MoW > W(2), but the emission from the (3)MMdeltadelta* state follows the inverse order: MM = W(2) > MoW > Mo(2). Evidence is presented to support the view that the lower energy emission in each case arises from the (3)MMdeltadelta* state. Lifetimes of the (1)MLCT states in these systems are approximately 0.4-6 ps, whereas phosphorescence is dependent on the MM center: Mo(2) approximately 40 micros, MoW approximately 30 micros, and W(2) approximately 1 micros.

2-Thienylcarboxylato and 2-thienylthiocarboxylato ligands bonded to MM quadruple bonds (M = Mo or W): a comparison of ground state, spectroscopic and photoexcited state properties.

The compounds M(2)(TiPB)(2)(OSC-2-Th)(2) have been prepared from the reactions between M(2)(TiPB)(4) and Th-2-COSH (2 equiv) in toluene solution, where M = Mo (Mo(2)ThCOS) or W (W(2)ThCOS), TiPB = 2,4,6-triisopropylbenzoate and Th = thienyl. The molybdenum and tungsten compounds are pink and blue, air-sensitive, ether soluble solids that show M(+) ions in the mass spectrometer and metal and ligand based reversible oxidation and reduction waves, respectively, by cyclic voltammetry. Electronic structure calculations on the model compounds M(2)(O(2)CH)(2)(OSC-2-Th)(2) indicate that the highest occupied molecular orbital (HOMO) is principally M(2)delta and the lowest unoccupied molecular orbital (LUMO) is thienylthiocarboxylate pi* but with significant metal-sulfur mixing. The intense visible absorptions arise from (1)MLCT, M(2)delta to thienylthiocarboxylate. The photoexcited states of these molecules have been studied by transient absorption spectroscopy and steady state emission. These properties are compared with those of previously reported thienylcarboxylate compounds, M(2)(TiPB)(2)(O(2)C-2-Th)(2), where M = Mo (Mo(2)ThCO(2)) or W (W(2)ThCO(2)).

Furan- and selenophene-2-carboxylato derivatives of dimolybdenum and ditungsten (M[quadruple bond]M): a comparison of their chemical and photophysical properties.

From the reactions between M(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents each of 2-furan carboxylic acid, FuCO(2)H, and 2-selenophene carboxylic acid, SpCO(2)H in toluene, the new compounds trans-M(2)(T(i)PB)(2)(O(2)CFu)(2) (1a M = Mo, 2a M = W) and trans-M(2)(T(i)PB)(2)(O(2)CSp)(2) (1b M = Mo, 2b M = W) were formed. These new compounds have been characterized by (1)H NMR, steady-state UV-Vis-NIR absorption and emission spectroscopy, cyclic and differential pulse voltammetry, and fs and ns transient absorption spectroscopy. The compound Mo(2)(T(i)PB)(2)(O(2)CSp)(2) (1b) has been characterized by single crystal X-ray crystallography. These data are compared with those previously reported for related 2-thiophene carboxylate derivatives: M(2)(T(i)PB)(2)(O(2)CTh)(2). The physico-chemical data correlate well with electronic structure calculations performed on model compounds. All compounds have detectible S(1) photoexcited states with lifetimes that vary from ∼5 ps to < 1 ps. The molybdenum compounds have T(1) states with microsecond lifetimes that are assigned as MMδδ* whereas the T(1) states for tungsten are (3)MLCT with lifetimes on the order of nanoseconds. In all cases, shorter lifetimes were seen in complexes containing heavier atoms.

Efficient organic bulk heterojunction solar cells through near infrared absorbing metallated thiophene complexes

Recently novel metallo-organic (MO) complexes have been synthesized by the Chisholm group at Ohio State, leading to the potential for a MO complex:fullerene bulk heterojuction solar cell that overcomes the limitations of current polymer:fullerene BHJ solar cell by expanding the usable solar spectrum into the near infrared. These metallated oligimers provide for high absorption at longer wavelengths of solar energy and with very long lifetimes of created electrons for high collection efficiency, leading to a potential efficiency enhancement over conventional organic solar cells. However, many fundamental issues dealing with processability and optimum device topology need to be addressed for this new chemistry system to move from the Chemistry lab to a viable device platform. In this work, prerequisites and preliminary findings of MO complexes for efficient photovoltaic operation are mainly discussed.