Kari A. McGee

Inefficient Crystal Packing in Chiral [Ru(phen)3](PF6)2 Enables Oxygen Molecule Quenching of the Solid-State MLCT Emission

The molecular oxygen quenching of the solid-state emission from pure crystals of Delta-Ru(phen)(3)(PF(6))(2), Lambda-Ru(phen)(3)(PF(6))(2), and racemic Ru(phen)(3)(PF(6))(2) (phen = 1,10-phenanthroline) was studied by emission spectroscopy. Crystals of the pure enantiomers exhibit significant and nearly identical emission-intensity quenching [0.36(2) and 0.33(2), respectively] in the presence of air [where the fraction quenched is (I(nitrogen) - I(air))/I(nitrogen)]; in comparison, the racemic compound shows a much lower value [0.05(2)]. The large difference in the quenching behavior is a result of major structural differences between the two chiral salts and the racemic salt. The chiral compounds crystallize in the space groups P4(1) and P4(3), respectively, with toluene and acetonitrile molecules in the lattice that can be partially removed to create void-space channels. These open channels allow the diffusion of oxygen molecules within the crystals and enable efficient emission quenching that is not possible in the closely packed racemic salt. Lifetime measurements, thermal gravimetric analysis, and single-crystal X-ray structure determinations support these conclusions.

Concurrent sensing of benzene and oxygen by a crystalline salt of tris(5,6-dimethyl-1,10-phenanthroline)ruthenium(II)

The complex [Ru(5,6-Me2Phen)3]tfpb2 has been examined as a solid-state benzene and oxygen sensor. The crystalline solid undergoes a reversible vapochromic shift of the emission lambda max to higher energy in the presence of benzene. Additionally, in the presence of oxygen the solid exhibits linear Stern-Volmer quenching behavior. When simultaneously exposed to benzene vapor and oxygen the crystals uptake benzene which inhibits the diffusion of oxygen in the lattice; very little quenching is observed. However, when benzene is removed from the carrier gas, partial loss of benzene occurs and oxygen diffusion is restored resulting in quenching of the emission. The practicality of this crystalline solid as a benzene sensor was investigated by examination of a lower concentration of benzene vapor (0.76%).

Comparison of Thiophene–Pyrrole Oligomers with Oligothiophenes: A Joint Experimental and Theoretical Investigation of Their Structural and Spectroscopic Properties

We have prepared a new series of mixed thiophene-pyrrole oligomers to investigate the electronic benefits arising from the combination of these two heterocycles. The oligomers are functionalized with several hexyl and aryl groups to improve both processability and chemical robustness. An analysis of their spectroscopic (absorption and emission), photophysical, electrochemical, solid state, and vibrational properties is performed in combination with quantum-chemical calculations. This analysis provides relevant information regarding the use of these materials as organic semiconductors. The balance between the high aromatic character of pyrrole and the moderate aromaticity of thiophene allows us to address the impact of the coupling of these heterocycles in conjugated systems. The data are interpreted on the basis of the aromaticity, molecular conformations, ground and excited electronic state structures, frontier orbital topologies and energies, oxidative states, and quinoidal versus aromatic competition.

Understanding optoelectronic properties of cyano-terminated oligothiophenes in the context of intramolecular charge transfer

In this paper we have prepared a new series of oligothiophenes capped with hexyl groups and a variety of strong acceptors, mainly cyanovinyl moieties. An exhaustive analysis of the absorption, photophysical, electrochemical, solid state, nonlinear optical and vibrational properties has been presented guided by theoretical calculations. The investigation is centered on the efficiency of the intramolecular charge transfer (i.e., chain length and acceptor dependence) and its impact on all the relevant electronic, structural, optical, and vibrational properties. The most significant features imparted by the acceptors through the π-conjugated oligothiophene path are (i) intense visible electronic absorptions, (ii) tuned fluorescence wavelength emissions, (iii) solid state π-stacking, (iv) ambipolar redox behavior, (v) S(1) ⇝ S(0) internal conversion as being the major route for the deactivation of the excited state, and (vi) large electronic and vibrational contributions to their nonlinear optical response (hyperpolarizability). The analysis establishes connections between the different properties of the materials and structure-function relationships useful in organic electronics.