Π-conjugated Bridge Research Articles

High Molar Extinction Coefficient Ruthenium Sensitizers for Thin Film Dye-Sensitized Solar Cells

We present two sensitizers, Ru(4,4′-dicarboxyvinyl)-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2 (K9) and Ru(4,4′-dicarboxy(phenylethenyl)-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2 (K23), which were synthesized and characterized by analytical, spectroscopic, and electrochemical impedance measurements. The two sensitizers that contain hydrophobic alkyl chains and an extended π-conjugation bridge between carboxylic acid groups and the ruthenium chromophore center exhibit enhanced red response and higher molar extinction coefficients when compared to the Ru(4,4′-dicarboxy-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2 (Z907) sensitizer. The performances of the K9, K23, and Z907 sensitizers in nanocrystalline TiO2-based thin film solar cells were studied using transparent 2.6-μm-thick mesoporous layers, which show short-circuit photocurrent densities (Jsc) of 7.80, 8.31, and 7.16 mA/cm2, corresponding to overall conversion efficiencies of 4.14, 4.41, and 4.06%, respectively. The increase in the Jsc of the cells with K9 and K23 with respect to that with Z907 is due to the enhanced molar extinction coefficient of the new sensitizers. The incident monochromatic photon-to-current conversion efficiency for the K23 is plotted as a function of excitation wavelength show in the plateau region 79% and even at 700 nm 69%.

Theoretical study of proton encircling modes in proton sponges with tetraamido/diamino quaternized macrocycles: the role of π-conjugated and aliphatic bridges

Theoretical studies of the proton encapsulation within a series of tetraamido/diamino quaternized macrocycles consisting of -NH-CO- functionalities linked by different pi-conjugated (phenyl, pyridine, furan, thiophene, butadienyl, and ethylene) or aliphatic (propyl and butyl) bridges are carried out. In macrocycles containing pi-conjugated spacers, the protons are held by asymmetrical hydrogen bonds O-HO right harpoon over left harpoon OH-O with very small barriers for the proton transfer. In the case of macrocycles with aliphatic spacers, a symmetrical hydrogen bond (OHO) is formed in the propyl analogue, in contrast to the asymmetrical hydrogen bond in the butyl analogue. The energy-based fragmentation quantum chemistry method is employed to calculate the hydrogen bond energies of macrocycles. Taking advantage of the fragment-based calculations, contributions from individual bridges to the strength of hydrogen bonds are also revealed. A qualitative agreement is shown between fragmentation MP2/6-31G(d,p) calculation results, and the hydrogen bond energies estimated from the empirical equation on the basis of O-H stretching frequency. The close correlation between the hydrogen bond energy with electrostatic interactions within the hydrogen bond framework, and the proton affinity of macrocycles, indicate that the intramolecular hydrogen bonds are mostly electrostatic. The calculations show that the modifications of the bridges linking the -NH-CO- functionalities and hence the OO distance between the proton acceptor and donor determine the proton binding modes.

Porphyrin dimers: A theoretical understanding of the impact of electronic coupling strength on the two-photon absorption properties

Many porphyrin oligomers exhibit large two-photon absorptions (TPA) at fundamental photon energies around 1.5 eV, while the corresponding monomers have negligible TPA cross-sections in that energy range. In general, our understanding of nonlinear absorption in these compounds is rather limited compared to that of linear absorption. Here, we seek to provide insight into this issue by examining various structural aspects of porphyrin dimers and analyzing how they lead to either “pure” or double resonance-enhanced TPA cross-sections. To do so, we have carried out highly correlated quantum-chemical calculations on model chromophores which differ by their central π-conjugated bridges or acceptor moieties. In a number of such dimers, the calculated energies of the lowest two-photon active states are stabilized and display significant cross-sections as the electronic coupling strength between the two porphyrin moieties becomes significant; in these instances, the lowest electronic TPA-active state is located in an energy range preventing contributions from double resonance effects. On the other hand, in dimers in which the porphyrin moieties are either very strongly or very weakly electronically coupled, the TPA cross-sections are mainly due to double resonance effects.

Extremely long-distance electron transfer in porphyrin or phthalocyanine systems directly functionalized by an oligo(phenyleneethynylene) spacer

This review focuses on a new strategy to design covalently linked and electronically coupled porphyrin systems to achieve long-range photoinduced electron transfer. We show that the attachment of a π-conjugated bridge directly on the meso position of the porphyrin via an ethynyl or ethenyl linkage affords significant electronic coupling both on the ground- and excited-states and that oligophenyleneethynylenes represent suitable spacers to assist charge separation between porphyrins. They can indeed exhibit wire-like behaviour and promote fast interchromophore charge separation over distances up to 4.5 nm. These recent electronically coupled dyads represent attractive photochemical systems to build more complex arrays for solar energy conversion or for molecular electronic applications.

Application of Triphenylamine-Based Sensitizers with Two Carboxylic Acid Groups to Dye-Sensitized Solar Cells

Two triphenylamine-based dyes (TPAR3 and TPAR6) containing two carboxylic acid groups with different conjugated lengths were synthesized and characterized with regard to their photophysical and photoelectrochemical properties. Experimental results showed that the λ max of TPAR6 either in methanol solution or on TiO 2 film was red-shifted and broadened by extending the π-conjugated bridge. However, the performance of TPAR3-based dye-sensitized solar cell (DSC) was superior to that of TPAR6. This reason was due to the serious self-quenching of the electronically excited state in TPAR6 molecule, resulting from its cis-trans isomerization. The effects of addition of guanidinium thiocyanate (GT) in the electrolyte on the performance of DSCs based on TPAR3 were also investigated, revealing a maximum energy conversion efficiency of 4.02% at 0.100 mol·L −1 GT.

Theoretical Study on Photophysical Properties of Multifunctional Electroluminescent Molecules with Different π-Conjugated Bridges

The photophysics of a series of molecular organic light-emitting diodes (OLEDs) has been studied by theoretical calculation. These molecular OLEDs have been integrated by an electron- and hole-transporting components as well as an emitting components into the donor-pi-acceptor (D-pi-A) structures: 2-carbazolyl-7-dimesitylboryl-9,9-diethylfluorene (1), trans-4'-N-carbazolyl-4-dimesitylborylstilbene (2), and trans-2-[(4'-N-carbazolyl)styryl]-5-dimesitylborylthiophene (3). To reveal the relationship between the structures and properties of these multifunctional electroluminescent materials, the ground- and excited-state geometries were optimized at the B3LYP/6-31G(d), HF/6-31G(d), and CIS/6-31G(d) levels, respectively. The ionization potentials and electron affinities were computed. The mobilities of hole and electron in these compounds were studied computationally based on the Marcus electron transfer theory. The lowest excitation energies (E(g)) and the maximum absorption and emission wavelengths of these compounds were calculated by time-dependent density functional theory methods. The solvent effect on the emission spectra of these compounds was considered by a polarizable continuum model. As a result of these calculations, it was concluded that the electron injections of these compounds are much easier than Mes(2)B[p-4,4'-biphenyl-NPh(1-naphthyl)], and the diethylfluorene-based compound has higher electron mobility and better equilibrium properties as compared to the stilbene-based and styrylthiophene-based compounds.

Molecular Design of Coumarin Dyes for Stable and Efficient Organic Dye-Sensitized Solar Cells

A series of coumarin dyes (NKX-2593, NKX-2807, and NKX-2883) with one or two -CN groups as electron acceptors were synthesized and applied as dye sensitizers for dye-sensitized solar cells. Compared with the dye containing one -CN group, linking one more -CN group to the π-conjugation bridge positively shifts the lowest unoccupied molecular orbital and thus red-shifts the maximum absorption band, harvesting more photons in the long-wavelength region for photoelectric conversion. Among the three dyes studied, NKX-2883 showed the best photovoltaic performance, yielding 7.6% power conversion efficiency using a volatile electrolyte and demonstrating good photostability under visible light soaking with 6% of power conversion efficiency for 1000 h using a nonvolatile electrolyte.

Efficient π-electron conjugated push–pull nonlinear optical chromophore 1-(4-methoxyphenyl)-3-(3,4-dimethoxyphenyl)-2-propen-1-one: A vibrational spectral study

As part of the efforts for the design new organic nonlinear optical (NLO) materials with high efficiency for present day technological requirements, a comprehensive investigation on the intramolecular charge transfer (ICT) of an efficient π-conjugated potential push–pull NLO chromophore, 1-(4-methoxyphenyl)-3-(3,4-dimethoxyphenyl)-2-propen-1-one to a strong electron acceptor group through the π-conjugated bridge has been carried out from their vibrational spectra. The NIR FT-Raman and FT-IR spectra supported by the density functional theory (DFT) quantum chemical computations have been employed to analyze the effects of intramolecular charge transfer on the geometries and the vibrational modes contributing to the linear electro-optic effect of the organic NLO material. The calculated first hyperpolarizability of DMMC is 6.650 × 10 −30 esu, which is 25 times that of urea. The simultaneous IR and Raman activation of the phenyl ring modes of ν(C C/C–C) mode, ring C C stretching modes, in-plane deformation modes and the umbrella mode of methyl groups also provide evidences for the charge transfer interaction between the donors and the acceptor group through the π-system. Vibrational analysis indicates the electronic effects such as induction and back-donation on the methyl hydrogen atoms causing the lowering of stretching wavenumbers have also been analyzed in detail. The planar conformations would give an enhanced NLO activity where as any deviations from planarity would decrease the mobility of electrons within the π-conjugated molecular system, resulting a reduction in NLO activity.

Site-Isolated Electro-optic Chromophores Based on Substituted 2,2′-Bis(3,4-propylenedioxythiophene) π-Conjugated Bridges

Application of bis(propylenedioxythiophene) (bis(ProDOT)) π-conjugated bridges bearing alkyl or aryl substituents in electro-optic (EO) chromophores is presented. A series of three bis(ProDOT)-based chromophores and a bithiophene-based control chromophore were prepared and fully characterized with regard to EO applications. The highly planar bis(ProDOT) bridge results in slightly larger (∼10%) molecular hyperpolarizability (β) values as compared to the bithiophene bridge, as measured by hyper-Rayleigh scattering at a variety of wavelengths. In amorphous polycarbonate guest–host films, however, the bulky substituents on the bis(ProDOT) bridge result in significantly larger (∼70%) poling-induced EO coefficient (r33) values, as measured by simple reflection ellipsometry at 1310 nm. This can be attributed to a roughly 2-fold enhancement in poling efficiency due to reduced intermolecular dipole–dipole interactions. This chromophore architecture also exhibits excellent temporal alignment stability and photochem...

New Triphenylamine-Based Dyes for Dye-Sensitized Solar Cells

A series of new conjugated metal−free organic dyes (TC1, TC2, TC3, and TC4) comprising triphenylamine (TPA) moieties as the electron donor and cyanoacetic acid moieties as the electron acceptor/anchoring groups were designed at the molecular level and developed for the use in dye-sensitized solar cells (DSCs). Quantum chemical calculations have been performed to gain insight into structural, electronic, and optical properties of the as-synthesized sensitizers. The time-dependent density functional theory calculations allowed assignment of the experimental spectroscopic data. It is found that the photovoltaic performance of the DSCs with the as-synthesized dyes can be improved by enhancing the electron-donor ability and extending the π-conjugated bridge of the dyes. In particular, the DSCs based on 2-cyano-5-(4-(phenyl(4-vinylphenyl)amino)phenyl) penta-2,4-dienoic acid dye (TC4) showed an open circuit voltage of 652 mV, a short circuit photocurrent density of 11.5 mA cm-2, and a fill factor of 0.64, corresponding to an overall light to electricity conversion efficiency of 4.82% under AM 1.5 irradiation (100 mW cm-2). This result reveals that efficient electron injection from the excited sensitizer to the conduction band of titania film occurs, owing to the more delocalizing electrons of the bridge and donor part of the dyes.

Efficient π electrons delocalization in prospective push–pull non-linear optical chromophore 4-[ N, N-dimethylamino]-4′-nitro stilbene (DANS): A vibrational spectroscopic study

A comprehensive investigation on the intramolecular charge transfer (ICT) of an efficient π-conjugated potential push–pull NLO chromophore, 4-[ N, N-dimethylamino]-4′-nitro stilbene (DANS), from a strong electron-donor group (dimethylamino- N(CH 3) 2) to a strong electron-acceptor group (nitro-NO 2) through the π-conjugated bridge ( trans-stilbene) has been carried out from their vibrational spectra. The NIR FT-Raman and FT-IR spectra supported by the density functional theory (DFT) quantum chemical computations have been employed to analyze the effects of intramolecular charge transfer on the geometries and the vibrational modes contributing to the linear electro-optic effect of the organic NLO material. It has been observed that the changes in the endocyclic and exocyclic angles result from the charge-transfer interaction of the phenyl ring and the amino group in the electron-donor side of the NLO chromophore. The strongest vibrational modes contributing to the electro-optic effect have been identified and examined from the concurrent IR and Raman activation of ν(C C/C–C) mode, ring C C stretching modes, in-plane deformation modes, nitro modes and the umbrella mode of methyl groups. Furthermore, the splitting of the vinyl stretching modes and the electronic effects such as hyperconjugation and backdonation on the methyl hydrogen atoms causing the decrease of stretching frequencies and infrared intensities have also been analyzed in detail. The effect of frontier orbitals transition of electron density transfer and the influence of planarity between the phenyl rings of the stilbene moiety on the first hyperpolarizability have also been discussed.

Synthesis and characterization of 2,5-bis[4-(2-arylvinyl)phenyl]-1,3,4-oxadiazoles with two-photon fluorescence properties

Four novel 2,5-bis[4-(2-arylvinyl)phenyl]-1,3,4-oxadiazoles that exhibit strong two-photon absorption and enhanced two-photon excited fluorescence were designed and synthesized based on “push-core-pull-core-push” molecules built from embedding electron-transporting 1,3,4-oxadiazole in aromatic conjugated system through Wittig-Horner reaction. Their chemical structures were determined to show trans-vinylene character according to infrared (IR) and 1H nuclear magnetic resonance (NMR) spectra. A very effective energy transfer from the excited units to the π-conjugated bridging unit can enhance the two-photon absorption and two-photon fluorescence.

Theoretical study of one-photon and two-photon absorption properties for 2,1,3-benzothiadiazole-based red-fluorescent dyes

We have theoretically investigated a series of D-π-A-π-D type 2,1,3-benzothiadiazoles (BTD)-based fluorescent dyes by using DFT(B3LYP/6-31G(d)) and ZINDO/SOS methods. The geometrical and electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties have been studied in detail. The calculated results indicate that conjugation length, π-electron bridges and terminal electron-donating groups (D) are three important factors for influencing OPA and TPA properties. Both oscillator strength ( f) of one-photon absorption and two-photon absorption cross-section values increase by degrees with the strength of terminal electron donors from molecule 1a to 1e. When increasing π-conjugation length between the BTD unit and the amino groups, two-photon absorption cross-section values of molecules 2–6 are enhanced about 1.5–2.6 times relative to 1c. Thereinto, the molecule 5 with additional ethyne π-conjugated bridge shows a larger two-photon absorption cross-section (270 GM) in comparison with ethene, benzene, thiophene and phenylethene π-conjugated bridge. The possible reason is discussed in detail by three-level energy model. Finally, we design a molecule 5′ with additional ethyne π-conjugated bridge and asymmetrical electron-donating groups, its two-photon absorption cross-section (336 GM) is larger than any other molecules in this system because the asymmetry makes Δ μ 0 n increased obviously. The calculations also show low-energy transition arising from charge-transfer makes one-photon and two-photon absorption of these BTD derivatives locate in longer wavelength regions (420–550 nm for OPA and 690–880 nm for TPA). So our theoretical findings demonstrate that D-π-A-π-D type BTD derivatives are promising molecules that can be assembled to a series of materials with large TPA cross-section and emitting spectra in the red region.

Redox-switchable conjugated bimetallic ruthenium(II) complexes

The conjugated bimetallic ruthenium(II) complex composed of 1,4-phenylenediamine as a bridging ligand was synthesized by photo-irradiation to show redox-switching of the emission properties of the terminal Ru(II) units depending on the redox state of the π-conjugated bridging spacer.

First and second hyperpolarizabilities of donor–acceptor disubstituted diphenalenyl radical systems

Using hybrid density functional theory methods, the static first ( β) and second ( γ) hyperpolarizabilities of a donor–acceptor disubstituted singlet diradical system built from phenalenyl radicals linked by an acetylene π-conjugated bridge, DA-BPLE, is investigated and compared to an analogous closed-shell system, DA-BPRY, built from pyrene rings. DA-BPLE presents an intermediate diradical character and exhibits β and γ values about 14 and 7 times larger than those of DA-BPRY having a similar π-conjugation length, respectively. These results suggest the applicability of the structure-property relation in γ of symmetric diradical systems to β and γ of asymmetric diradical systems.

3,5-Bis[4-(dimethylamino)benzylidene]-1-propyl-4-piperidone: a prospective biophotonic material

In the title compound, C26H33N3O, the central heterocyclic ring exhibits a flattened boat conformation, with the N and the opposite C atom displaced from the plane of the other four C atoms. The dihedral angles between the planar part of the heterocycle and the two almost flat fragments which include the benzene rings and attached C atoms are 17.1 (1) and 14.8 (1)°. This conformation helps promote conjugation in the mol­ecule. Bond-length distributions in the π-conjugated bridges and p-dimethyl­amino­phenyl fragments show an alternation of single C—C and double C=C bond lengths. In the crystal structure, weak C—H⋯O hydrogen bonds link the mol­ecules into ribbons along the b axis.

Synthesis, characterization and magnetic properties of new homotrinuclear copper(II) complexes

Two new mononuclear bis(oxamato) complexes with the formula [ nBu 4N] 2[M(nabo)] M = Ni ( 4), Cu ( 5), with nabo = 2,3-naphthalene-bis(oxamato) have been synthesized as precursors for trinuclear oxamato-bridged transition metal complexes. Starting from 5 the homo-trinuclear complex [Cu 3(nabo)(pmdta) 2(BF 4)](BF 4) · MeCN · Et 2O ( 7), with pmdta = N, N, N′, N″, N″-pentamethyldiethylenetriamine, has been prepared. The central N, N′-2,3-naphthalene bridge of 7 is so far the most extended π-conjugated bridge of trinuclear bis(oxamato) type transition metal complexes. The goal of this work was to verify the N, N′-2,3-naphthalene bridge of 7 on its magnetic properties in comparison to the N, N′- o-phenylene bridge of the related homo-trinuclear complex [Cu 3(opba)(pmdta) 2(NO 3)](NO 3) · 2MeCN ( 6) (opba = o-phenylene-bis(oxamato)). The crystal structures of 4– 7 were solved. The magnetic properties of 6 and 7 were studied by susceptibility measurements versus temperature. For the intramolecular J parameter, values of −89 cm −1 ( 6) and −113 cm −1 ( 7) were obtained. The different J values are discussed based on the crystal structures of 6 and 7.

Syntheses of novel 1,3-diazaazulene derivatives and their nonlinear optical characterization

We have synthesized three new 1,3-diazaazulene derivatives, namely, 2-(4′-N,N-dimethylaminophenyl)-6-nitro-1,3-diazaazulene (18, DMAPNA), 2-(4′-aminophenyl)-6-nitro-1,3-diazaazulene (19, APNA) and 2-[4′-N-(2-hydroxyethyl)aminophenyl]-6-nitro-1,3-diazaazulene (20, HEAPNA), each of them contains an amino substitute as the electron donor (D), 2-phenyl-1,3-diazaazulene as the π-conjugated bridge and a nitro as the electron acceptor (A). Our theoretical results have predicted that these D–π–A type chromophores possess low ground-state dipole moment (μg) and large first-order hyperpolarizability (β), which may facilitate a low degree of aggregation for the chromophores dispersed in a polymeric matrix as well as a large nonlinear optical (NLO) response. The expected NLO performance has been confirmed by the experimental β and μg values, e.g., for 18, 407.8 × 10−30 esu and 4.7 D, respectively. The origins of large β and low μg are explained in terms of a two-state quantum model. The DMAPNA (18)-doped and poled polymethylmethacrylate film exhibits a large second harmonic generation (SHG) coefficient of d33 = 10.9 pm V−1 with excellent thermal stability (above 70% of the maximal SHG coefficient remains at ∼100 °C).

Synthesis and two-photon absorption properties of 2,5-bis[4-(2-arylvinyl)phenyl]-1,3,4-oxadiazoles

Two symmetrical 2,5-bis[4-(2-arylvinyl)phenyl]-1,3,4-oxadiazoles that exhibit strong two-photon absorption and enhanced two-photon excited fluorescence were designed and synthesized based on “push-core-pull-core-push” molecules built from embedding electron-transporting 1,3,4-oxadiazole in aromatic conjugated system through Wittig–Horner reaction. Pumped by nanosecond laser at 800 nm, strong up-conversion emissions with the central wavelength at 507 nm (green) of 2,5-bis[4-(2- N, N-diphenylaminostyryl)phenyl]-1,3,4-oxadiazole and 475 nm (blue) of 2,5-bis[4-{2-(3- N-ethylcarbazolyl)vinyl}phenyl]-1,3,4-oxadiazole in the solution of CHCl 3 have been observed. Their two-photon absorption cross-sections obtained by nonlinear transmission method are 107 × 10 −48 cm 4 s photon −1 and 66 × 10 −48 cm 4 s photon −1. A very effective energy transfer from the excited terminal units to the π-conjugated bridging units of the 2,5-bis[4-(2-arylvinyl)phenyl]-1,3,4-oxadiazoles is the dominant contribution to the two-photon absorption.

THEORETICAL STUDIES OF ONE- AND TWO-PHOTON ABSORPTION PROPERTIES FOR SYMMETRIC MOLECULES BASED ON BIS(STILBENE)DIETHYLENE

In order to investigate the substituting effects on the two-photon absorption (TPA) cross-section and the position of the TPA peak, the one- and two-photon absorption properties of a series of symmetric molecules with bis(stilbene)diethylene π-conjugated bridge have been studied using ZINDO-SOS method. The transition moments from the ground state to the lowest excited state (M0k) and from the lowest excited state to the TPA final state (Mkn), together with the detuning energy ΔE = E0k - Ekn/2, were calculated using the INDO1 method included in the ZINDO program. The results indicate that bis-donor and bis-acceptor substitutions mainly affect the transition dipole moments M0k and Mkn, and that donor or acceptor side groups have a strong influence on the detuning energy. Meanwhile, it was found that the molecules with the bis-donor structure possess enhanced TPA cross sections, and the molecules with cyano or methoxy side groups have a large red-shift with respect to molecules without side groups.