Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations.

The band structure and electronic properties in a series of vinylene-linked heterocyclic conducting polymers are investigated using density functional theory (DFT). In order to accurately calculate electronic band gaps, we utilize hybrid functionals with fully periodic boundary conditions to understand the effect of chemical functionalization on the electronic structure of these materials. The use of predictive first-principles calculations coupled with simple chemical arguments highlights the critical role that aromaticity plays in obtaining a low band gap polymer. Contrary to some approaches which erroneously attempt to lower the band gap by increasing the aromaticity of the polymer backbone, we show that being aromatic (or quinoidal) in itself does not ensure a low band gap. Rather, an iterative approach which destabilizes the ground state of the parent polymer toward the aromatic ↔ quinoidal level crossing on the potential energy surface is a more effective way of lowering the band gap in these conjugated systems. Our results highlight the use of predictive calculations guided by rational chemical intuition for designing low band gap polymers in photovoltaic materials.

Improved synthesis of bis(borano)hypophosphite salts.

A synthesis of the bis(borano)hypophosphite anion with various counterions has been developed to make use of more benign and commercially available reagents. This method avoids the use of potentially dangerous reagents used by previous methods and gives the final products in good yield. Details of the crystal structure determination of the sodium salt in space group Ama2 are given using a novel computational technique combined with Rietveld refinement.

Coumarin dyes for dye-sensitized solar cells: A long-range-corrected density functional study.

The excited-state properties in a series of coumarin solar cell dyes are investigated with a long-range-corrected (LC) functional which asymptotically incorporates Hartree-Fock exchange. Using time-dependent density functional theory (TDDFT), we calculate excitation energies, oscillator strengths, and excited-state dipole moments in each of the dyes as a function of the range-separation parameter mu. To investigate the acceptable range of mu and to assess the quality of the LC-TDDFT formalism, an extensive comparison is made between LC-BLYP excitation energies and approximate coupled-cluster singles and doubles calculations. When using a properly optimized value of mu, we find that the LC technique provides a consistent picture of charge-transfer excitations as a function of molecular size. In contrast, we find that the widely used B3LYP hybrid functional severely overestimates excited-state dipole moments and underestimates vertical excitation energies, especially for larger dye molecules. The results of the present study emphasize the importance of long-range exchange corrections in TDDFT for investigating the excited-state properties in solar cell dyes.

A synthetic cycle for the ruthenium-promoted formation of 1H-phosphindoles from phosphaalkynes.

Beginning with inexpensive and commercially available starting materials, a rational synthesis for the new phosphaalkyne Ph3C-C[triple bond]P (1) is presented. Coordination of 1 to group 8 transition metal centers furnishes the eta1-complexes [MH(dppe)2(Ph3CC[triple bond]P)]OTf, where M = Fe (3) or Ru (4) (dppe = bis-1,2-diphenylphosphinoethane). Treatment of 3 or 4 with a strong acid cyclizes the coordinated phosphaalkyne and is the first example of an electrophilic aromatic substitution reaction in which the electrophile is a low coordinate phosphorus. With the aid of DFT calculations, we were able to gain a more thorough understanding of the energetics and mechanism of this new cyclization reaction. Thermolysis of the iron-3,3-diphenyl-3H-phosphindole adduct (6) in CH3CN results in quantitative formation of the free 3H-phosphindole (7). Alternatively, when ruthenium-3,3-diphenyl-3H-phosphindole adduct (5) is irradiated, a photochemical rearrangement occurs furnishing 2,3-diphenyl-1H-phosphindole (9). Mechanistic work is presented that provides an explanation for this transformation. Compounds 1, 3, 5, and 9 have been characterized by single X-ray diffraction studies. Finally, a synthetic cycle for the conversion of 1 to 1H-phosphindole 9 has been developed that recycles the ruthenium cation [RuH(dppe)2]+.

Dissociation of carbanions from acyl iridium compounds: an experimental and computational investigation.

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp(PMe(3))Ir(H)[C(O)CF(3)] (1) leads to the quantitative formation of Cp(PMe(3))Ir(CO) (2) and CF(3)H. Kinetic experiments, isotope labeling studies, solvent effect studies, and solvent-inclusive DFT calculations support a mechanism that involves initial dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp(PMe(3))Ir(H)(CO)](+)[CF(3)](-). Further evidence for the ability of CF(3)(-) to act as a leaving group came from the investigation of the analogous methyl and chloride derivatives Cp(PMe(3))Ir(Me)[C(O)CF(3)] and Cp(PMe(3))Ir(Cl)[C(O)CF(3)]. Both of these compounds undergo a similar loss of trifluoromethyl anion, generating an iridium carbonyl cation and CF(3)D in CD(3)OD. Three additional acyl hydrides, Cp(PMe(3))Ir(H)[C(O)R(F)] (where R(F) = CF(2)CF(3), CF(2)CF(2)CF(3), or CF(2)(CF(2))(6)CF(3)) undergo R(F)-H elimination to give 2 at a faster rate than CF(3)H elimination from 1. Stereochemical studies using a chiral acyl hydride with a stereocenter at the beta-position reveal that ionization of the carbanion occurs to form a tight ion-pair with high retention of configuration and enantiomeric purity upon proton transfer from iridium.

Elimination of R-H from iridium acyl hydrides without loss of CO: evidence for the intervention of metal cation/carbanion pairs.

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp*(PMe3)Ir(H)[C(O)CF3] leads to the quantitative formation of Cp*(PMe3)Ir(CO) and CF3H. Kinetic experiments, isotope-labeling studies, solvent effect studies, and DFT calculations support a mechanism which involves dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp*(PMe3)Ir(H)(CO)]+[CF3]-. Further evidence for the ability of CF3 to act as a leaving group came from investigation of the analogous methyl and chloride derivatives Cp*(PMe3)Ir(Me)[C(O)CF3] and Cp*(PMe3)Ir(Cl)[C(O)CF3]. Both of these compounds undergo a similar loss of trifluoromethyl anion, generating an iridium carbonyl cation and CF3D in CD3OD.

Synthesis of mono-substituted 2,2'-bipyridines.

The rapid synthesis of 4-aryl-2,2'-bipyridines is described leading to some previously reported compounds in good yields in addition to some new functionalized bipyridines.