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.
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.
We report tensile testing and in situ X-ray scattering measurements of a homologous series of precise poly(ethylene-co-acrylic acid) copolymers (pxAA). The number of backbone carbons (x) between pendant acrylic acid groups along the polyethylene chain (x = 9, 15, 21) has a pronounced effect on both their tensile properties as well as their morphologies during deformation. The semicrystalline precise copolymer (p21AA) displays yielding behavior similar to polyethylene. Also, strain hardening in p21AA coincides with the originally isotropic acid-rich layered structures strongly aligning with acid layers perpendicular to the strain direction, demonstrating the facile nature of the H-bonding within the acid aggregates. When the alkyl spacer is only nine carbons (p9AA), the precise copolymer withstands strains of >1000% without failing, because the liquid-like assembly of acid aggregates permits the acid groups to exchange without developing substantial anisotropy in the structure. Both p21AA and p9AA maintain..
The quest for cyaphide, the phosphorus equivalent of cyanide, has been a continuous struggle for many years. Potential applications arising from the use of C P as a bridging ligand between two metals, the incorporation of CP into coordination polymers and new materials, and the basic synthetic challenges of making C P have provided inspiration for decades. While phosphaalkynes (R À C P) have been known for some time, [1][2][3] the terminal MÀCP has only been reported as a transient species. [4,5] Other C-functionalized XÀCP compounds (X = R 3 Si, [6] R 2 N, [7,8] RO, [8] F, [9] Cl, [10] ), anionic species [XÀCP] À (X = R 3 B, [11] RN, [8] O, [12] S [13] ), and the cationic phosphonio phosphaalkyne [R 3 P À C P] + , [14] have been synthesized, but the vast majority of reports deal with tert-butyl, [15] adamantyl, [16] 2,4,6-trimethylphenyl [17] and 2,4,6-tri-tert-butylphenyl [18] phosphaalkynes. Recently, we devised a very simple method for accessing the kinetically stable crystalline triphenylmethyl (trityl)-substituted phosphaalkyne, Ph 3 CC P (1), which allowed for the economical metal-promoted synthesis of phosphorus heterocycles. However, our ultimate goal and incentive for synthesizing the trityl-substituted phosphaalkyne was to find an adequate leaving group that would lead us to cyaphide. [19] Treatment of 1 or its complexes [MH(dppe) 2 (Ph 3 C-CP)]OTf (M = Fe or Ru; dppe = bis(1,2-diphenylphosphinoethane); OTf = trifluoromethanesulfonate) with nucleophiles (Nu) did not furnish the desired cyaphide, C P À , or its complexes [Eq. (1); electrophile (E) = C].Therefore, we reasoned that silicon would be more susceptible to nucleophilic attack (E = Si) and applied an analogous synthetic procedure to access the higher homologue Ph 3 SiCP (3).Conversion of Ph 3 SiCH 2 Cl into the Grignard reagent followed by treatment with PCl 3 produced the silyl-substituted alkyl phosphonous dichloride, Ph 3 SiCH 2 PCl 2 (2), in over 87 % yield (Scheme 1). Employing 2.2 equiv of DABCO (1,8-diazabicyclo[2.2.2]octane) effected the dehydrohalogenation reaction, and 2 was fully transformed into the triphenylsilyl-substituted phosphaalkyne 3. The reaction occurred at ambient temperature and in multiple solvents in less than one hour. (In contrast, the reaction of DABCO with Ph 3 CCH 2 PCl 2 to furnish Ph 3 CC P required a 10-fold excess of base, elevated temperatures, and proceeded best in acetonitrile.[19] ) While DABCO alone was effective for the conversion of 2 into 3, we saw rapid decomposition of this new phosphaalkyne, which was dependent on the solvent. Qualitatively, the rate of decomposition of 3 followed the order Et 2 O % toluene < THF ! CH 3 CN. Suspecting that DABCO-HCl was acting as a soluble source of chloride anion in the more polar solvents that then attacked silicon to produce Ph 3 SiCl and the CP anion, which is seemingly unstable under the reaction conditions, we added AgOTf to the reaction mixture. When a solution of 2 in toluene was pretreated with 2.2 equiv of AgOTf for 5 min followed by addition of 2.2 equ...
Melt state dynamics for a series of strictly linear polyethylenes with precisely spaced associating functional groups were investigated. The periodic pendant acrylic acid groups form hydrogen-bonded acid aggregates within the polyethylene (PE) matrix. The dynamics of these nanoscale heterogeneous morphologies were investigated from picosecond to nanosecond timescales by both quasi-elastic neutron scattering (QENS) measurements and fully atomistic molecular dynamics (MD) simulations. Two dynamic processes were observed. The faster dynamic processes which occur at the picosecond timescales are compositionally insensitive and indicative of spatially restricted local motions. The slower dynamic processes are highly composition dependent and indicate the structural relaxation of the polymer backbone. Higher acid contents, or shorter PE spacers between pendant acid groups, slow the structural relaxation timescale and increase the stretching parameter (β) of the structural relaxation. Additionally, the dynamics of specific hydrogen atom positions along the backbone correlate structural heterogeneity imposed by the associating acid groups with a mobility gradient along the polymer backbone. At time intervals (<2 ns), the mean-squared displacements for the four methylene groups closest to the acid groups are up to 10 times smaller than those of methylene groups further from the acid groups. At longer timescales acid aggregates rearrange and the chain dynamics of the slow, near-aggregate regions and the faster bridge regions converge, implying a characteristic timescale for the passage of chains between aggregates. The characterization of the nanoscale chain dynamics in these associating polymer systems both provides validation of simulation force fields and provides understanding of heterogeneous chain dynamics in associating polymers.
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