Showing papers by "Stephen Barlow published in 2008"
TL;DR: Two-photon absorption has been extensively studied from both fundamental and application points of view as discussed by the authors, and a survey of the properties of these classes of materials can be found in this paper.
Abstract: Two-photon absorption, the process by which an excited molecule or material is produced by the simultaneousabsorption of two photons, has been studied extensively in recent years, from both fundamental and applicationpoints of view. On one side, the field has been expanded with the measurement of two-photon absorptionspectra and cross sections for a wide range of conjugated molecules. In this contribution, we willreview the two-photon properties of some of these classes of materials, and we will discuss structure/propertyrelationships that have been developed from these investigations. On the other side, two-photon absorptionhas been exploited as a means to activate a variety of chemical and physical processes with sub-diffraction–limitedresolution, because three-dimensional spatial confinement of the excitation volume in a material irradiatedby a tightly focused laser beam can be achieved via two-photon or, more generally, multi-photon absorptionprocesses. This characteristic has led to applications in a number of technological fields, such asmicrofabrication and laser scanning fluorescence microscopy. Here, we will survey material systems thathave been developed to activate radical or cationic polymerization reactions, deprotection of functionalgroups, and singlet oxygen generation via two-photon excitation of one of the components in the system.
152 citations
TL;DR: Both experimental and computational results suggest that the aromaticity of the dihydroheteroacenes is reduced, and this results validates the discussion of their aromaticity.
Abstract: The synthesis and X-ray characterization of two new dialkynylated diazatetracenes and the corresponding N,N-dihydrodiazatetracenes are reported. The dialkynylated heteroacenes are packed in a brick-wall motif that enforces significant overlap of their π-faces. Cyclic voltammetry indicates that the dehydrogenated forms are easily reduced to their radical anions in solution. The planarity of these species validates the discussion of their aromaticity. Nucleus Independent Chemical Shift (NICS) computations demonstrate that both of these 20 π and 24 π electron systems are aromatic. Both experimental and computational results suggest that the aromaticity of the dihydroheteroacenes is reduced.
145 citations
TL;DR: A narrow band-gap alternating copolymer of perylene diimide and bis(dithienothiophene) (2) and a polythiophene derivative substituted by a tris(thienylenevinylene) conjugated side chain (4) are used as acceptor and donor, respectively, in all-polymer solar cells.
Abstract: A narrow band-gap alternating copolymer of perylene diimide and bis(dithienothiophene) (2) and a polythiophene derivative substituted by a tris(thienylenevinylene) conjugated side chain (4) are used as acceptor and donor, respectively, in all-polymer solar cells (SCs). The optimized device based on the blend of 4 and 2 in the ratio 3:1 (w/w) gives a short circuit current (Jsc) of 5.02 mA cm−2 and a power conversion efficiency of 1.48%, under simulated AM 1.5 illumination at 100 mW cm−2. These values are among the highest values reported for all-polymer SCs.
129 citations
TL;DR: In this article, an organic p-i-n CuPc homojunction was constructed using F4-TCNQ and decamethylcobaltocene (DMC) as p-and n-dopants.
101 citations
TL;DR: The crystal structure of a cyanine dye Rotaxane shows that the cyclodextrin is tightly threaded round the polymethine bridge of the dye, making it possible to observe the rotaxane radical dication by ESR and UV-vis-NIR spectroscopy.
75 citations
TL;DR: In this article, a high-spectral-irradiance, high-quality continuum was generated by weakly focusing femtosecond pulses in Kr gas, and this continuum was used as a source for rapid Z-scan measurements of the degenerate nonlinear absorption spectrum and the associated dispersion of the nonlinear refraction in optical materials throughout the visible.
Abstract: We generate a high-spectral-irradiance, high-quality continuum by weakly focusing femtosecond pulses in Kr gas. We use this continuum as a source for rapid Z-scan measurements of the degenerate nonlinear absorption spectrum and the associated dispersion of the nonlinear refraction in optical materials throughout the visible. We measure the degenerate two-photon absorption spectra and the dispersion of the nonlinear refractive index, n2, of two well-characterized semiconductors (ZnSe and ZnS) as reference samples for our method, along with dilute solutions of organic materials. The latter materials demonstrate application of the technique to samples with lower nonlinearities.
57 citations
TL;DR: Through implementation of Herzberg-Teller theory, the vibrational modes responsible for the low-energy TPA peak are identified and reproduced to reproduce, at least qualitatively, the experimental TPA spectra of several squaraines of this type.
Abstract: Many squaraines have been observed to exhibit two-photon absorption at transition energies close to those of the lowest energy one-photon electronic transitions Here, the electronic and vibronic contributions to these low-energy two-photon absorptions are elucidated by performing correlated quantum-chemical calculations on model chromophores that differ in their terminal donor groups (diarylaminothienyl, indolenylidenemethyl, dimethylaminopolyenyl, or 4-(dimethylamino)phenylpolyenyl) For squaraines with diarylaminothienyl and dimethylaminopolyenyl donors and for the longer examples of 4-(dimethylamino)phenylpolyenyl donors, the calculated energies of the lowest two-photon active states approach those of the lowest energy one-photon active (1Bu) states This is consistent with the existence of purely electronic channels for low-energy two-photon absorption (TPA) in these types of chromophores On the other hand, for all squaraines containing indolinylidenemethyl donors, the calculations indicate that there are no low-lying electronic states of appropriate symmetry for TPA Actually, we find that the lowest energy TPA transitions can be explained through coupling of the one-photon absorption (OPA) active 1Bu state with bu vibrational modes Through implementation of Herzberg–Teller theory, we are able to identify the vibrational modes responsible for the low-energy TPA peak and to reproduce, at least qualitatively, the experimental TPA spectra of several squaraines of this type
50 citations
TL;DR: Poly(glycidyl methacrylate) has been shown to be a useful material for fabrication of photonic crystal templates using multibeam interference lithography, since it exhibits lower shrinkage than conventional SU8 as discussed by the authors.
Abstract: Poly(glycidyl methacrylate) has been shown to be a useful material for fabrication of photonic crystal templates using multibeam interference lithography, since it exhibits lower shrinkage than conventional SU8.
42 citations
TL;DR: In this paper, the one-photon and twophoton absorption properties of cross-shaped chromophores consisting of four donor-substituted styryl branches linked to an aromatic core (benzene or pyrazine) were investigated and compared with those of linear analogues with only two branches (donor−π−donor distrylarenes).
Abstract: The one-photon and two-photon absorption properties of cross-shaped chromophores consisting of four donor-substituted styryl branches linked to an aromatic core (benzene or pyrazine) have been investigated and compared with those of linear analogues with only two branches (donor−π−donor distryrylarenes). The areas of the lowest energy two-photon absorption bands of the compounds with four branches were less than twice those of analogues with two branches. The spectral features observed in these chromophores suggest that electronic coupling between the branches is effective but does not lead to significant enhancement of the two-photon cross section when the branches extend in more than one dimension. In a chromophore with two donor-substituted and two acceptor-substituted branches the two-photon cross section is smaller than in the corresponding linear analogues. The main characteristics of both the one-photon and two-photon spectra of multibranched compounds of the type discussed here can be explained qu...
40 citations
TL;DR: In this paper, the effect of substitution pattern on the geometric, thermal, electronic, optical, and electroluminescence properties of a series of silole molecules with fluorenyl substituents at varying positions was investigated both experimentally and theoretically.
Abstract: A series of silole molecules with fluorenyl substituents at varying positions—1-(9,9-dimethylfluoren-2-yl)-1,2,3,4,5-pentaphenylsilole, 1-(fluoren-9-yl)-1,2,3,4,5-pentaphenylsilole, 1,1,3,4-tetraphenyl-2,5-bis(9,9-dimethylfluoren-2-yl)silole, and 1,1-diphenyl-2,3,4,5-tetrakis(9,9-dimethylfluoren-2-yl)silole—has been synthesized and compared to the previously reported compounds, 1,1,2,3,4,5-hexaphenylsilole and 1,1-bis(9,9-dimethylfluoren-2-yl)-2,3,4,5-tetraphenylsilole. The effect of fluorenyl substitution pattern on the geometric, thermal, electronic, optical, and electroluminescence properties was investigated both experimentally and theoretically. Analysis of the X-ray crystal packing diagrams for two new fluorenyl-substituted siloles indicates the presence of π–π stacking and CH⋯π interactions in the solid state. Across the series, excellent thermal and morphological stabilities are displayed. Photoelectron/inverse-photoelectron spectroscopy measurements and density functional theory (DFT) calculations suggest that increased conjugation length through substitution at the 2- and 5-positions plays a more significant role in tuning the ionization potentials and electron affinities of these siloles than do inductive effects through substitution of the silicon. The electronic structure (e.g., HOMO–LUMO gap) and, hence, the optical absorption and fluorescence properties are also sensitive to the positions at which the fluorenyl groups are introduced, with substitution at the 2,5-positions having the largest effect. Solution-processed electroluminescent devices fabricated with the fluorenyl-substituted siloles as the emissive layer have luminous efficiencies as high as 3.6 cd A−1.
37 citations
TL;DR: In this paper, the work function of the organic underlayer predeposited on different substrates was investigated by ultraviolet photo-emission spectroscopy, and the electronic structure of the OOH was found to depend on the work functions of the underlayer.
Abstract: This work focuses on organic-organic heterojunctions (OOHs) formed between two small-molecule, low band-gap, semiconductors, tris{2,5-bis(3,5-bis-trifluoromethyl-phenyl)-thieno}$[3,4\text{\ensuremath{-}}b,$$h,n]\text{\ensuremath{-}}1,4,5,$$8,9,12$ $\ensuremath{-}\text{hexaaza}\ensuremath{-}\text{triphenylene}$ (THAP), and copper phthalocyanine (CuPc). The organic layers are deposited on various substrates, and the energy level alignment between them is investigated by ultraviolet photoemission spectroscopy. The electronic structure of the OOH is found to depend on the work function of the organic underlayer predeposited on the different substrates. The vacuum level offset between THAP and CuPc, which consists of the sum of the interface dipole and the molecular level shift, varies from $0.26\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The interface dipole between the two organic films linearly changes with the work function of the organic underlayer.
TL;DR: In this paper, the electron-vibration and electronic interactions in a series of bisdimethylamino mixed-valence systems were investigated using gas-phase ultraviolet photoelectron spectroscopy, vis/NIR spectrographs, and electronic-structure calculations.
Abstract: By using gas-phase ultraviolet photoelectron spectroscopy, vis/NIR spectroscopy, and electronic-structure calculations, we have investigated the electron-vibration and electronic interactions in a series of bisdimethylamino mixed-valence systems: N,N,N′,N′-tetramethyl-p-phenylenediamine, N,N,N′,N′-tetramethylbenzidine, and N,N,N′,N′-tetramethyltolane-4,4′-diamine. Experiment and theory concur to indicate that the electron-vibration coupling in these systems is dominated by interactions with symmetric modes. The results reveal that the strength of both electronic and electron-vibration couplings decreases as the molecular bridge lengthens. The parameters derived for the present compounds have been compared to those of diarylamino-based structural analogs. This comparison underlines that the replacement of the methyl terminal groups with p-anisyl groups has a significant effect on the electronic and electron-vibrational interactions.
TL;DR: In this paper, a comparative study of hole-mobility in N,N′-diphenyl and 4,4′ diamine (TPD) using three independent measurement techniques is presented.
Abstract: A comparative study of hole-mobility in N,N′-diphenyl-N,N′-di(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) using three independent measurement techniques is presented. Time-of-flight (TOF), steady-state space charge-limited current (SCLC) experiments are carried-out in neat films of TPD, sandwiched between electrodes with different work functions. Organic field-effect transistors (OFET) are fabricated from TPD thin films and their electrical properties characterized. The comparative results show that all-electrical measurements that involve carrier injection from a metal electrode into the organic film yield effective mobility values that are one order of magnitude lower than those obtained from time-of-flight experiments in which carriers are created in the film optically. Similarly, SCLC and TOF experiments were used to measure electron-mobility in a discoid organic compound 2,3,8,9,14,15-hexakis-dodecylsulfanyl-5,6,11,12,17,18-hexaazatrinaphthylene (HATNA-(SC12H25)6).
TL;DR: Processable organic materials with large, ultrafast third-order susceptibilities, x, could enable all-optical signal processing (AOSP) applications through either fabrication of large-area thick films or integration into hybrid organic/ silicon photonic devices.
Abstract: Processable organic materials with large, ultrafast thirdorder susceptibilities, x, could enable all-optical signal processing (AOSP) applications through either fabrication of large-area thick films or integration into hybrid organic/ silicon photonic devices. Conjugated polymers, such as polyacetylene, have been shown to exhibit large optical nonlinearities upon application of intense electromagnetic fields. Despite these nonlinearities, many conjugated polymers are inadequate for photonic applications because they lack sufficient processability, or form films with significant crystallinity or inhomogeneous morphologies resulting in poor optical quality. For example, although films of unsubstituted polyacetylene have been found to possess large third-order susceptibilities, they also exhibit extensive crystallinity, resulting in poor optical quality and large scattering losses. A more processable soluble diester polyacetylene derivative has been reported; however, only thin films have been produced from this material and the bulky side groups can be anticipated to lead to a reduced nonlinear
TL;DR: It is reported on photoinduced charge separation in solid films of two perylene diimides; intramolecular charge separation and recombination is correlated with a reduction in the yield of long-lived, intermolescular charge-separated species.
TL;DR: In this article, the electron transport material tris 2,5-bis 3,5bis-trifluoromethyl-phenyl -thieno 3,4-b,h,n -1,4,5,8,9,12-hexaazatriphenylene THAP on Au 111 is probed by scanning tunneling microscopy STM.
Abstract: Layer-by-layer growth of the electron-transport material tris 2,5-bis 3,5-bis-trifluoromethyl-phenyl -thieno 3,4-b ,h ,n -1,4,5,8,9,12-hexaazatriphenylene THAP on Au 111 is probed by scanning tunneling microscopy STM . A relative of discotic liquid crystalline molecules, THAP is shown to grow in commensurate ordered planes from the first to fourth monolayers. The four monolayers all show a concordant ordered structure in which the molecules arrange parallel to the substrate in a hexagonal close-packed lattice with a herringbone pattern defined by alternating rows of molecules with antiparallel orientation. The unit cell is rectangular with two molecules per cell and is nearly equivalent for each layer. The spatial broadening of the local density of states due to the metallic substrate is appreciably diminished in upper layers, as expected and as evidenced by the localization of states seen in STM. There is good agreement between the highest occupied molecular orbital obtained in density functional theory calculations for a single molecule and STM images of the upper layers, in accord with the localized nature of electronic states on molecules under minimal substrate influence.
TL;DR: In this paper, the two-photon absorption (2PA) properties of donor-acceptor dipolar chromophores, the conjugated backbones of which contain two five-membered heterocyclic groups which may be electron-rich (thiophene 2,5-diyl) and/or electron-deficient (thiazole-2,5)-diyl), were investigated.
Abstract: We have investigated computationally the two-photon absorption (2PA) properties of donor-acceptor dipolar chromophores, the conjugated backbones of which contain two five-membered heterocyclic groups which may be electron-rich (thiophene-2,5-diyl) and/or electron-deficient (thiazole-2,5-diyl). Quantum-chemical calculations (INDO/MRDCI/S-tensor and Sum-Over-States calculations based on DFT-optimized geometries) indicate that the two-photon cross-sections into the lowest two excited states S 1 and S 2 can be tuned by more than an order of magnitude by varying the nature, order, and, in the case of thiazole, orientation of the heterocycles. Going from one thiazole regioisomer to the other has the strongest impact on the 2PA spectra and can even invert the ratio between the 2PA cross-sections of S 1 and S 2 . An essential-state analysis reveals that different channels dominate 2PA into S 1 and S 2 . The sensitivity of 2PA into S 1 towards the orientation of the thiazole ring stems from a local modulation on the thiazole ring of the change in state dipole moment upon excitation to S 1 , Δμ 01 , whereas the dominant essential parameter through which the thiazole orientation affects 2PA into S 2 is the transition dipole moment between S 1 and S 2 , μ 12 .
TL;DR: In this article, the authors used scanning tunneling microscopy to study the structure of monolayers of tris{2,5-bis(3,5bis-trifluoromethyl-phenyl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaaza-triphenylene adsorbed on a clean Ag(110) single-crystal surface.
Abstract: Scanning tunneling microscopy is used to study the structure of monolayers of tris{2,5-bis(3,5-bis-trifluoromethyl-phenyl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaaza-triphenylene adsorbed on a clean Ag(110) single-crystal surface. The molecules adsorb flat on the surface, forming two-dimensional islands within which intermolecular interactions constrain the molecules to form rows of molecular pairs. The balance between intermolecular and molecule–substrate interactions leads to two specific orientations of these rows at +11° and −11° from the [001] direction of the Ag(110) surface. A comparison between observed and calculated local densities of states shows an antiparallel azimuthal orientation of the molecules within the closely associated pairs. The structure of the full monolayer shows that the intermolecular interaction overcomes the interaction with the substrate, resulting in a reorientation of the rows along various directions.
TL;DR: The diketones 4,4′-di[3,4-bis(n-dodecyloxy)styryl]benzil and 4, 4′-dibromobenzil have been synthesized using Heck coupling with styrene and Sonogashira coupling with the appropriate phenylalkyne, respectively as discussed by the authors.
Abstract: The diketones 4,4′-di[3,4-bis(n-dodecyloxy)styryl]benzil and 4,4′-di[3,4-bis(n-dodecyloxy)phenylethynyl]benzil have been synthesized from 4,4′-dibromobenzil using Heck coupling with the appropriate styrene and Sonogashira coupling with the appropriate phenylalkyne, respectively. Treatment of the distyrylbenzil derivative with phosphorus pentasulfide followed by nickel(II) chloride gave the corresponding extended nickel bis(dithiolene) derivative which shows strong near-infrared two-photon absorption with a cross-section of over 440 GM (1 GM = 10−50 cm4 s photon−1 molecule−1) throughout the telecommunications range and a peak cross-section of 5300 GM at ca. 1.2 μm.
TL;DR: In this paper, photoemission spectroscopy is used to probe the electronic properties, reactivity and energy level alignment at interfaces of tris{2,5-bis(3,5bis-trifluoromethyl-phenyl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenylene (THAP) deposited on both medium and high work function metallic substrates.
TL;DR: In this paper, double pump-probe experiments were performed to study the intra-molecular dynamics of several nonlinear organic dye molecules, allowing for characterization of triplet states yield and cross-section.
Abstract: We performed double pump-probe experiments to study the intra-molecular dynamics of several nonlinear organic dye molecules. The method allows for characterization of triplet states yield and cross-section. Several special cases of molecular dynamics are presented.