NO2 Acceptor Research Articles

Electric-Field-Induced Second Harmonic Generation Nonlinear Optic Response of A4 β-Pyrrolic-Substituted ZnII Porphyrins: When Cubic Contributions Cannot Be Neglected.

In this work, we have prepared a series of A4 ZnII porphyrins, carrying in the β-pyrrolic-position one or two π-delocalized ethynylphenyl moieties with a −NO2 acceptor or a −NMe2 donor pendant, and measured their second-order NLO response in CHCl3 solution at 1907 nm via the electric-field-induced second harmonic generation (EFISH) technique. For some of these compounds, we have recorded an unexpected sign and/or absolute value of μβ1907. Since their sterically hindered A4 structure should ensure the lack of significant aggregation processes in solution, we explain such anomalous EFISH results by invoking a non-negligible contribution of the electronic cubic term γ(−2ω; ω, ω, 0) to γEFISH, as supported by a qualitative evaluation of the third-order response through the measure of the cubic hyperpolarizability (γTHG) and by computational evidence.

New cyanoacetanilides based dyes as effective co-sensitizers for DSSCs sensitized with ruthenium (II) complex (HD-2)

The synthesis of six novel 2-cyanoacetanilide-derived metal-free organic dyes, named SA11-16, containing 4-dimethylaminophenyl, 4-anisyl or 2,4,5-trimethoxyphenyl as electron donating moieties, and carboxyl (–COOH) or nitro (–NO2) groups as anchoring moieties was reported along with their optical properties, theoretical geometrical studies and spectroscopic characterization. From the UV–Vis absorption spectra, co-sensitizers SA11-14 carrying –NO2 as an anchoring moiety are found to be more red-shifted than other co-sensitizers having –COOH as an anchoring group, this may be due to the strong electron withdrawing nature of –NO2 acceptor unit. Further, the co-sensitization technique was applied by combining the newly prepared organic dyes with a previously published HD-2 ruthenium-based complex for dye-sensitized solar cells (DSSCs) and the corresponding effect on the photovoltaic performance was studied. All the metal-free dyes exhibited improved photoconversion efficiencies (PCE, η) in the co-sensitized DSSCs compared to the one fabricated with HD-2 alone. Among the fabricated devices, SA12 as a co-sensitizer obtained the best photovoltaic performance with JSC of 19.46 mA/cm2, VOC of 0.69 V and the overall PCE of 8.69%. Finally, the calculated electron lifetimes obtained from electrochemical impedance spectroscopy (EIS) measurements coincided well with the photovoltaic measurements.

Organoboron donor-π-acceptor chromophores for small-molecule organic solar cells

We introduce the use of facilely synthesizable and low-bandgap boron chromophores as donors in planar heterojunction solar cells. We show that simple changes in the compositional properties of these molecules can improve the performance of the devices. A simultaneous grafting of NO2 acceptor and N(Et2) donor groups into the molecule core causes an increase in efficiency of almost 50%. Such enhanced efficiency is mainly due to a higher photocurrent. The origin of this phenomenon is investigated.

Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study.

Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS β0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ∼5 times greater than that of the DAS+ cation), but systems with no donor, or the -NO2 acceptor show no activity, in some cases, despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β0 values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β0/N3/2 (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (β0/N3/2 vs λmax) for planar organic systems. However, β0 values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue, N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms the involvement of the POM in the electronic transitions, whether donor groups are present or not, but Stark spectroscopy indicates that, in their absence, the transitions have little dipolar character (hence, NLO inactive), consistent with DFT-calculated frontier orbitals, which extend over both POM and organic group. Stark and DFT also suggest that β is enhanced in the short compounds because the extension of charge transfer (CT) onto the POM increases changes in the excited-state dipole moment. With extended π-systems, this effect does not increase CT distances, relative to a -NO2 acceptor, so β0 values do not exceed that of DMNPEA. Overall, our results show that (i) the organoimido-POM unit is an efficient acceptor for second-order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation.

Intriguing Influence of -COOH-Driven Intermolecular Aggregation and Acid-Base Interactions with N,N-Dimethylformamide on the Second-Order Nonlinear-Optical Response of 5,15 Push-Pull Diarylzinc(II) Porphyrinates.

A series of 5,15 push-pull meso-diarylzinc(II) porphyrinates, carrying one or two -COOH or -COOCH3 acceptor groups and a -OCH3 or a -N(CH3)2 donor group, show in N,N-dimethylformamide and CHCl3 solutions a negative and solvent-dependent second-order nonlinear-optical (NLO) response measured by the electric-field-induced second-harmonic generation (EFISH) technique, different from the structurally related zinc(II) porphyrinate carrying a -N(CH3)2 donor group and a -NO2 acceptor group, where a still solvent-dependent but positive EFISH second-order response was previously reported. Moreover, when a -N(CH3)2 donor group and a -COOH acceptor group are part of a sterically hindered 2,12 push-pull β-pyrrolic-substituted tetraarylzinc(II) porphyrinate, the EFISH response is positive and solvent-independent. In order to rationalize these rather intriguing series of observations, EFISH measurements have been integrated by electronic absorption and IR spectroscopic investigations and by density functional theory (DFT) and coupled-perturbed DFT theoretical and 1H pulsed-gradient spin-echo NMR investigations, which prompt that the significant concentration effects and the strong influence of the solvent nature on the NLO response are originated by a complex whole of different aggregation processes induced by the -COOH group.

Study on Pyrrole 4-Pyrazoline Derivatives: Experimental and Quantum Chemical Approaches

As part of a study on pyrrole derivatives we report here a combined experimental and quantum chemical study of pyrrole 4-pyrazoline biheterocyclic derivatives. The structure of the synthesised compounds have been studied using experimental IR, UV, 1H and 13C NMR spectroscopic analyses along with density functional theory (DFT) calculations using the B3LYP functional with 6–311+G (d,p) basis set. The global, local reactivity, and thermodynamic parameters support the analysis. All the experimental vibrational bands have been discussed and assigned to normal modes on the basis of our calculations. In addition, the computed 1H and 13C NMR data, obtained by DFT calculations, are found to be in good agreement with the experimental data and serve as valuable tools in identifying the products. The vibrational analysis shows red shifts in vN–H and vC=O stretching vibrations as a result of dimer formation. The theoretical electronic absorption spectra have been calculated by using time dependent-DFT methods. The static first hyperpolarizability (β0) values for the synthesized pyrrole–pyrazoline derivatives 4A–D are calculated as 16.97 × 10–30, 47.64 × 10–30, 65.40 × 10–30, 65.39 × 10–30 esu, respectively, and increase from 4A to 4C as a result of the addition of an –NO2 acceptor in 4B and two –NO2 group acceptors in 4C. However, an additional –Cl group in 4D on the phenyl ring attached to the pyrazoline moiety does not result in a clear change from 4C. The calculated static and dynamic hyperpolarizability results show that the investigated molecules might be used as non-linear optical materials.

Electrically tunable molecular doping of graphene

Electrical tunability of molecular doping of graphene has been investigated using back-gated field effect transistors. Variation of the gate voltage from positive to negative values resulted in reduced p-type doping by NO2, which decreased below detection limit at −45 V. A reverse trend was observed for NH3, where its n-type doping increased with more negative gate voltage, becoming undetectable at 5 V. Our results indicate that adsorption induced molecular doping of graphene could not be detected when the Fermi level coincides with the adsorption induced defect states, which yields NO2 acceptor energy level of ∼320 meV below the Dirac point.

Theoretical investigation of the dynamic first hyperpolarizability of DHA–VHF molecular switches

The contrast of second-order nonlinear optical response in the dihydroazulene (DHA)-vinylheptafulvene (VHF) equilibrium has been investigated as a function of the nature of the substituent (R) on the phenyl ring by means of quantum chemistry calculations including electron correlation, frequency dispersion, and solvent effects. By considering the hyper-Rayleigh scattering (HRS) response, the contrast for R = H and R = CH3 between the DHA and VHF forms is larger than 5 while the contrast between the cis and transVHF forms is close to 1. Adding the NH2 donor group in para position of the phenyl leads to a substantial increase of the HRS first hyperpolarizability of the three forms, which is detrimental to the contrast. Then, in the case of the NO2 acceptor group, a contrast is recovered because the HRS first hyperpolarizability of the DHA form is about 2–3 times larger than for both VHF forms. These variations of first hyperpolarizability as a function of the substituents as well as the associated contrasts have been explained in terms of donor/acceptor strengths and geometrical parameters.

Structure optimization of azobenzene oligomers for holography

Azobenzene oligomer layers with side chain chromofore groups (ChG) has been experimentally studied to optimize scalar and vector hologram recording at 633 nm. ChG with different acceptors (Br, NO2, H, CN) and different bonding type to the matrix (dispersed without bonding or with covalent bonding) were used. The influence of oligomer matrix (polyvinylbuthirol, tolyle, hexamethylene, polyvinylpirolidone, double matrix) and spacers was also studied. The best results (the maximal diffraction efficiency of 7.9% and the minimal specific recording energy of 0.38 J/(cm2 %)) are obtained for ChG with one NO2 acceptor group covalently bonded to tolyle matrix. Double matrix considerably decreases the recording energy and the hologram lifetime, and enables more efficient vector recording.The conclusion is made that ChG photoorientation accompanied by other processes is responsible for 633 nm recording. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Third-order Nonlinear Optical Properties of Organoboron Compounds: Molecular Structures and Second Hyperpolarizabilities

The third-order nonlinear optical properties (second molecular hyperpolarizabilities, γ), of a series of organoboron compounds containing dimesitylboron moieties have been investigated by third harmonic generation (THG) measurements at 1.907 μm. Symmetric systems with B(mes)2 groups (mes=2,4,6–Me3C6H2) at both ends, e.g. (mes)2B–Y–B(mes)2 where Y=(C6H4)n (n=1,2) or trans-trans-CH=CH–(C6H4)n–CH=CH– (n=1,2), were prepared as well as push– pull systems of the form D–Y–B(mes)2 where D=MeS, Me2N, H2N, etc., and Y=C6H4, C6H4–C≅C–, C6H4–CH=CH–, C6H4–CH=CH–C6H4. The B(mes)2 group compares favorably with the more commonly used NO2 acceptor function for enhancing γ. Values of γ increase significantly with increasing length of π-conjugation for both symmetric and unsymmetric molecules. The MeS group is much more efficient than MeO for enhancing γ. Values as high as 22910–36 esu are reported for trans-trans-(mes)2-B–CH=CH–(C6H4)2–CH=CH–B(mes)2 which has a lmax for absorption of 370 nm. Crystal and molecular structures of the bis(dimesitylboryl) benzene and anthracene compounds 1,4-C6H4-{B(mes)2}2 and 9,10-C14H8{B(mes)2}2 are reported, as are fluorescence maxima for the symmetric molecules.

The effects of humidity on phthalocyanine NO2 and NH3 sensors

The effect of humidity on the behaviour of metal phthalocyanine (MPc) gas sensors has been studied for an electron donor gas (NH3) and an acceptor (NO2). Water vapour acts as an effective donor even in the presence of a strong donor adsorbate such as NH3. In this case the water vapour enhances device sensitivity and reduces response and recovery times. NiPc is found to display much greater humidity sensitivity than the other metal Pc's studied. For the PbPc-NO2 system at room temperature, the water overcompensates the NO2 acceptor adsorbate. The results with PbPc support previous observations that a PbPc NO2 sensor is only viable at elevated operating temperatures.