Influence of vibronic interaction of charge transfer excitons in PTB7/BTA-based nonfullerene organic solar cells.

We studied photoinduced charge transfer (CT) states and their dissociation processes at the donor/acceptor (D/A) interface of PTB7/BTAx (x = 1 and 3) nonfullerene organic thin-film solar cells using density functional theory (DFT) and time-dependent DFT calculations. We focused on the CT distances and electron coupling in the CT state generated by photoexcitation and the Huang-Rhys (HR) factors that describe the nonadiabatic processes associated with vibronic interactions. The PTB7/BTA3 system with a large short-circuit current density (JSC) exhibited a large charge CT distance and electronic coupling. Contrastingly, the PTB7/BTA1 system with a low JSC has a large HR factor because of the low-wavenumber vibrational modes in the CT state of the D/A complex and is prone to nonadiabatic relaxation to the ground state. Systematic theoretical analysis of the excitonic states in the D/A complex has provided insight into the control of CT exciton dynamics, namely JSC and electron-hole recombination.

Laser-induced fluorescence of the trans-CHBrCHO radical.

A new laser-induced fluorescence spectrum was observed in the region of 350 nm-360 nm. The spectrum was observed in the reaction between the CHBrCHBr and OH radicals and in the reaction of CHBrCHBr and CH2CHBr with atomic oxygen O(3P). The spectrum was assigned to the B̃--X̃ transition of the trans-CHBrCHO (trans-2-bromovinoxy) radical. The B̃--X̃ electronic transition energy (T0) was 28 542 cm-1, which was 242 cm-1 lower than that of the unsubstituted vinoxy radical (CH2CHO). From an analysis of the laser-induced single vibronic level fluorescence aided by ab initio calculations, some of the vibrational frequencies were assigned to the ground electronic state ν3 (C-O str.) = 1581 cm-1, ν6 (C-C str.) = 1130 cm-1, and ν8 (C-C-O bend.) = 409 cm-1. The fluorescence lifetimes of the excited B̃ state were 35 ns-75 ns, depending on the excited vibrational modes, implying that predissociation had accelerated as the energy level (v') increased.

Laser-induced fluorescence of the CHFCHO radical and reaction of OH radicals with halogenated ethylenes.

A new laser-induced fluorescence spectrum of the 2-fluorovinoxy (CHFCHO) radical was first observed around 335 nm. The radical was produced in the reaction of an OH radical with 1,2-difluoroethylene (CHF=CHF). A single weak band was observed, which was assigned to the 00 0 band of the B̃-X̃ transition of the trans-CHFCHO radical. The B̃←X̃ electronic transition energy (T0) for trans-CHFCHO was 29 871 cm-1, which was just 3 cm-1 lower than that of its isomer, the 1-fluorovinoxy (CH2CFO) radical. The fluorescence lifetime at 29 871 cm-1 was shorter than 20 ns. This means that strong predissociation is probable at v' = 0 in the excited B̃ state of trans-CHFCHO. From an analysis of the dispersed fluorescence spectrum, some of the vibrational frequencies can be assigned for the ground electronic state: ν3 = 1557 cm-1 (C-O stretch), ν7 = 1162 cm-1 (C-C stretch), and ν8 = 541 cm-1 (CCO bend). These vibrational assignments were supported by ab initio calculations. The structure of the C-C-O skeleton and the spectroscopic character of trans-CHFCHO were close to those of CHClCHO and CH2CHO than those of CH2CFO. For the reaction of CH2=CHF with O(3P), the formation of both the regioisomeric radicals, i.e., 1- and 2-fluorovinoxy radicals, was confirmed. The regioselectivity of the oxygen atom added to the double bond of monofluoroethylene is discussed.

Investigations on the charge transfer mechanism at donor/acceptor interfaces in the quest for descriptors of organic solar cell performance.

Herein, we theoretically and experimentally investigated the mechanisms of charge separation processes of organic thin-film solar cells. PTB7, PTB1, and PTBF2 have been chosen as donors and PC71BM has been chosen as an acceptor considering that effective charge generation depends on the difference between the material combinations. Experimental results of transient absorption spectroscopy show that the hot process is a key step for determining external quantum efficiency (EQE) in these systems. From the quantum chemistry calculations, it has been found that EQE tends to increase as the transferred charge, charge transfer distance, and variation of dipole moments between the ground and excited states of the donor/acceptor complexes increase; this indicates that these physical quantities are a good descriptor to assess the donor-acceptor charge transfer quality contributing to the solar cell performance. We propose that designing donor/acceptor interfaces with large values of charge transfer distance and variation of dipole moments of the donor/acceptor complexes is a prerequisite for developing high-efficiency polymer/PCBM solar cells.

Reparametrization approach of DFT functionals based on the equilibrium temperature of spin-crossover compounds.

The required approach to investigate the electronic properties of spin-crossover (SCO) compounds needs to be able to provide a reliable estimate of high-spin/low-spin (HS/LS) energy gaps while retaining an accurate and efficient computation of the ground-state energy. We propose a reparametrization approach of the density functional theory (DFT) functionals to adjust the exact exchange admixture that governs the HS/LS energy splitting. Through the investigation of the thermodynamic properties of two typical SCO compounds, we demonstrate that the computed equilibrium temperature depends linearly, like the HS/LS energy gap, on the coefficient of the exact exchange admixture. We show that by taking the experimental value of the equilibrium temperature of the studied SCO compound as a reference, different hybrid functionals converge to comparable and realistic HS/LS energy gaps as well as enthalpy and entropy differences that agree well with the prior experimental investigations.

Raman spectroscopy of optically levitated supercooled water droplet.

By use of an optical trap, we can levitate micrometer-sized drops of purified water and cool them below the melting point free from contact freezing. Raman spectra of the OH stretching band were obtained from those supercooled water droplets at temperatures down to -35 °C. According to the two-state model, an enthalpy change due to hydrogen-bond breaking is derived from temperature dependence of the spectral profile. The isobaric heat capacity calculated from the enthalpy data shows a sharp increase as the temperature is lowered below -20 °C in good agreement with conventional thermodynamic measurements.

Redox-responsive molecular helices with highly condensed π-clouds.

Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a highly condensed π-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion. In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a long-lasting chiroptical memory.

Structural evolution of the [(CO2)n(H2O)]- cluster anions: quantifying the effect of hydration on the excess charge accommodation motif.

The [(CO2)n(H2O)]- cluster anions are studied using infrared photodissociation (IPD) spectroscopy in the 2800-3800 cm(-1) range. The observed IPD spectra display a drastic change in the vibrational band features at n = 4, indicating a sharp discontinuity in the structural evolution of the monohydrated cluster anions. The n = 2 and 3 spectra are composed of a series of sharp bands around 3600 cm(-1), which are assignable to the stretching vibrations of H2O bound to C2O4- in a double ionic hydrogen-bonding (DIHB) configuration, as was previously discussed (J. Chem. Phys. 2005, 122, 094303). In the n > or = 4 spectrum, a pair of intense bands additionally appears at approximately 3300 cm(-1). With the aid of ab initio calculations at the MP2/6-31+G* level, the 3300 cm(-1) bands are assigned to the bending overtone and the hydrogen-bonded OH vibration of H2O bound to CO2- via a single O-H...O linkage. Thus, the structures of [(CO2)n(H2O)]- evolve with cluster size such that DIHB to C2O4- is favored in the smaller clusters with n = 2 and 3 whereas CO2- is preferentially stabilized via the formation of a single ionic hydrogen-bonding (SIHB) configuration in the larger clusters with n > or = 4.

Structures of water-CO2 and methanol-CO2 cluster ions: [H2O x (CO2)n]+ and [CH3OH x (CO2)n]+ (n = 1-7).

Infrared photodissociation (IRPD) spectra of [H(2)O x (CO(2))(n)](+) and [CH(3)OH x (CO(2))(n)](+) (n=1-7) are measured in the 1100-3800 cm(-1) region. At the same time, the solvation characteristics in the clusters are investigated theoretically; the geometry optimization and the vibrational analysis are carried out for the [H(2)O x (CO(2))(n)](+) (n=1-4) and the [CH(3)OH x (CO(2))(n)](+) (n=1-3) ions at the MP2/6-31+G(*) level of theory. The IRPD spectrum of the [H(2)O x (CO(2))(1)](+) ion shows the free OH and the hydrogen-bonded OH stretching bands of the H(2)O(+) ion core and the antisymmetric CO stretching band of the solvent CO(2) molecule, indicating that the solvent CO(2) molecule is preferentially solvated to the H(2)O(+) ion core via the O-H...OCO hydrogen bond. In [H(2)O x (CO(2))(2)](+), the free OH stretching band is not observed; both of the OH groups of the H(2)O(+) ion core are hydrogen bonded to the solvent CO(2) molecules. Spectral features of the IRPD spectra of [H(2)O x (CO(2))(n)](+) (n=3-7) suggest that the third and the fourth CO(2) molecules are bound to the oxygen atom of the H(2)O(+) ion core, and that the first solvation shell of the H(2)O(+) ion core becomes filled with four CO(2) molecules. All the IRPD spectra of the [CH(3)OH x (CO(2))(n)](+) (n=1-7) ions display the hydrogen-bonded OH stretching band of the CH(3)OH(+) ion core, meaning that the solvent CO(2) molecule is preferentially bonded to the OH group of the CH(3)OH(+) ion core, similar to the case of [H(2)O x (CO(2))(n)](+). Quantum chemical calculations for the [CH(3)OH x (CO(2))(1-3)](+) ions demonstrate that the second and the third solvent CO(2) molecules are bonded to the oxygen atom of the CH(3)OH(+) ion core.

An IR study of (CO2)(+)n (n=3-8) cluster ions in the 1000-3800 cm-1 region.

Infrared photodissociation (IRPD) spectra of carbon dioxide cluster ions, (CO(2))(n) (+) with n=3-8, are measured in the 1000-3800 cm(-1) region. IR bands assignable to solvent CO(2) molecules are observed at positions close to the vibrational frequencies of neutral CO(2) [1290 and 1400 cm(-1) (nu(1) and 2nu(2)), 2350 cm(-1) (nu(3)), and 3610 and 3713 cm(-1) (nu(1)+nu(3) and 2nu(2)+nu(3))]. The ion core in (CO(2))(n) (+) shows several IR bands in the 1200-1350, 2100-2200, and 3250-3500 cm(-1) regions. On the basis of previous IR studies in solid Ne and quantum chemical calculations, these bands are ascribed to the C(2)O(4) (+) ion, which has a semicovalent bond between the CO(2) components. The number of the bands and the bandwidth of the IRPD spectra drastically change with an increase in the cluster size up to n=6, which is ascribed to the symmetry change of (CO(2))(n) (+) by the solvation of CO(2) molecules and a full occupation of the first solvation shell at n=6.

Structures of [(CO2)n(CH3OH)m](-) (n = 1-4, m = 1, 2) cluster anions.

The infrared photodissociation spectra of [(CO 2) n (CH 3OH) m ] (-) ( n = 1-4, m = 1, 2) are measured in the 2700-3700 cm (-1) range. The observed spectra consist of an intense broad band characteristic of hydrogen-bonded OH stretching vibrations at approximately 3300 cm (-1) and congested vibrational bands around 2900 cm (-1). No photofragment signal is observed for [(CO 2) 1,2(CH 3OH) 1] (-) in the spectral range studied. Ab initio calculations are performed at the MP2/6-311++G** level to obtain structural information such as optimized structures, stabilization energies, and vibrational frequencies of [(CO 2) n (CH 3OH) m ] (-). Comparison between the experimental and the theoretical results reveals the structural properties of [(CO 2) n (CH 3OH) m ] (-): (1) the incorporated CH 3OH interacts directly with either CO 2 (-) or C 2O 4 (-) core by forming an O-HO linkage; (2) the introduction of CH 3OH promotes charge localization in the clusters via the hydrogen-bond formation, resulting in the predominance of CO 2 (-).(CH 3OH) m (CO 2) n-1 isomeric forms over C 2O 4 (-).(CH 3OH) m (CO 2) n-2 ; (3) the hydroxyl group of CH 3OH provides an additional solvation cite for neutral CO 2 molecules.

Structures of [(CO2)n(H2O)m]- (n=1-4, m=1,2) cluster anions. I. Infrared photodissociation spectroscopy.

The infrared photodissociation spectra of [(CO(2))(n)(H(2)O)(m)](-) (n=1-4, m=1, 2) are measured in the 3000-3800 cm(-1) range. The [(CO(2))(n)(H(2)O)(1)](-) spectra are characterized by a sharp band around 3570 cm(-1) except for n=1; [(CO(2))(1)(H(2)O)(1)](-) does not photodissociate in the spectral range studied. The [(CO(2))(n)(H(2)O)(2)](-) (n=1, 2) species have similar spectral features with a broadband at approximately 3340 cm(-1). A drastic change in the spectral features is observed for [(CO(2))(3)(H(2)O)(2)](-), where sharp bands appear at 3224, 3321, 3364, 3438, and 3572 cm(-1). Ab initio calculations are performed at the MP2/6-311++G(**) level to provide structural information such as optimized structures, stabilization energies, and vibrational frequencies of the [(CO(2))(n)(H(2)O)(m)](-) species. Comparison between the experimental and theoretical results reveals rather size- and composition-specific hydration manner in [(CO(2))(n)(H(2)O)(m)](-): (1) the incorporated H(2)O is bonded to either CO(2) (-) or C(2)O(4) (-) through two equivalent OH...O hydrogen bonds to form a ring structure in [(CO(2))(n)(H(2)O)(1)](-); (2) two H(2)O molecules are independently bound to the O atoms of CO(2) (-) in [(CO(2))(n)(H(2)O)(2)](-) (n=1, 2); (3) a cyclic structure composed of CO(2) (-) and two H(2)O molecules is formed in [(CO(2))(3)(H(2)O)(2)](-).

Very slow relaxation observed in the surface tension of surfactant solutions.

A light scattering study was made on ripplons generated over the surface of CnTAB (n-cetyltrimethyl ammonium bromide) solutions (in the present case n=14 and 16). After preparing the sample solution we measured its surface tension for 3 h and confirmed that the tension did decrease with time exponentially but very slowly. The relaxation time depended on the surfactant concentration of solutions and increased sensitively with the progress of dilution, e.g., 3 min at 10(-2) mol/l to 80 min at 10(-4) mol/l for C16TAB. The results obtained were correlated with surface adsorption densities estimated from the observed surface pressure vs concentration curves using Langmuir's adsorption equation. Two mechanisms were proposed to interpret such a slow relaxation process: One is a model in which reorientation or redistribution of the surfactant molecules adsorbed on the solution is considered, while the other is a two-state model that assumes the existence of a high energy barrier for surfactant molecules between those on the surface and those in the subsurface. A question still remains as to which of the two is more probable.