Geoffrey J. Ashwell

Langmuir–Blodgett alignment of zwitterionic optically non-linear D–π–A materials

The synthesis, Langmuir–Blodgett (LB) deposition and charge-transfer spectra of Z-β-(1-alkyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (R-Q3CNQ where R = C6H13 to C20H41) and four substituted analogues are described. The deposition is Z-type and the properties are dependent upon the hydrophobic chain length. C6H13-Q3CNQ to C14H29-Q3CNQ form green LB films (λmax= 614 ± 4 nm; HWHM = 37 ± 2 nm) and occupy 28–34 A2 per molecule at 25 mN m–1. In contrast, C15H31-Q3CNQ to C20H41-Q3CNQ occupy 40–50 A2 per molecule; their films are purple with λmax= 565 ± 4 nm and HWHM = 22 ± 1 nm. The shift is attributed to a change in molecular tilt which causes the transition to alter from an intermolecular to an intramolecular process. Multilayer films of C16H33-Q3CNQ exhibit strong second-harmonic generation (SHG) and the intensity increases quadratically with the number of LB layers.

Molecular rectification: self-assembled monolayers of a donor–(π-bridge)–acceptor chromophore connected via a truncated Au–S–(CH2)3 bridge

Bis-[1-{(3-propyl)-4-(2-(4-dimethylaminophenyl)vinyl)quinolinium}]disulfide diodide interacts with gold to form self-assembled monolayers, the thickness and contact area being 2.25 ± 0.05 nm and 0.30 ± 0.03 nm2 molecule−1 respectively. The films exhibit asymmetric current–voltage characteristics with a rectification ratio of ca. 12 at ±1 V. These become symmetrical when the intramolecular charge-transfer axis is suppressed by protonation but the electrical asymmetry is restored when the films are exposed to base. Molecular rectification is intrinsic to the bridged donor–acceptor combination.

Photochromic and non-linear optical properties of C16H33P3CNQ and C16H33Q3CNQ Langmuir-Blodgett films

Z-;-(1-hexadecyl-4-pyridinium)-α-cyano-4-styryldicyanomethanide (C16H33 P3CNQ) and the quinolinium analogue, Z-s-(1-hexadecyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (C16H33 Q3CNQ), are photochromic. Their Langmuir-Blodgett (LB) films have narrow, switchable, charge transfer bands at 495 nm and 565 nm respectively with halfwidths at half-maximum of 27 and 22 nm and absorbances of 0.020 monolayer−1. At wavelengths which correspond to the absorption maxima there is little overlap of the bands and thus they are suitable for use in a multifrequency optical memory. The aligned LB films also exhibit second harmonic generation and have promising non-linear optical properties.

Au-S-CnH2n-Q3CNQ: self-assembled monolayers for molecular rectification

Self-assembled monolayers (SAMs) formed from Z-β-[N-(ω-acetylthioalkyl)-4-quinolinium]-α-cyano-4-styryldicyanomethanide (1) on gold-coated highly oriented pyrolytic graphite exhibit asymmetric current–voltage characteristics that are independent of the length of the linking group [Au-S-CnH2n-Q3CNQ where 3 ≤ n ≤ 12]. The contribution to the electrical asymmetry arising from the chromophore being asymmetrically located between the electrodes is negligible compared to that induced by the non-planar donor–(π-bridge)–acceptor moiety. Current rectification ratios of 30 at ±1 V are typical (cf. 8 for C16H33-Q3CNQ Langmuir–Blodgett films) but also fall within the range 10 to 80 at ±1 V. Molecular dimensions within the film are consistent with the D–π–A chromophores being upright and closely packed: the thickness increases with the length of the linking CnH2n group and areas of 0.33 ± 0.03 nm2 molecule−1 approximate to the van der Waals cross-section of the chromophore.

Langmuir-Blodgett films: molecular engineering of non-centrosymmetric structures for second-order nonlinear optical applications

Optical second-harmonic generation (SHG) is conditional upon the structure being non-centrosymmetric and consequently, the Langmuir-Blodgett (LB) technique is of interest because it permits control at the molecular level. However, the criteria for alignment at the air/water interface, >i.e. a hydrophilic head and a hydrophobic tail, tend to impede the required packing arrangement within the multilayer. Most show inversion symmetry with the interfaces being alternately hydrophobic (tail-to-tail) and hydrophilic (head-to-head). This has been overcome by interleaving the layers with inactive spacers and, in such films, the long-range structural order is controlled by utilising compatible component molecules, interdigitating arrangements (‘molecular zips’) and interlayer hydrogen-bonding. Furthermore, the employment of optically nonlinear chromophores with two hydrophobic end groups eliminates the need for inactive spacers. The molecules form stable non-centrosymmetric structures because, unlike above, the interfaces are invariably hydrophobic. In this review, the molecular requirements for LB deposition are discussed together with the film-forming behaviour and properties of a variety of optically nonlinear materials.

Induced rectification from self-assembled monolayers of sterically hindered π-bridged chromophores

Self-assembled monolayers (SAMs) obtained via the chemisorption of bis-[1-(10-decyl)-4-{2-(4-methoxynaphthalen-1-yl)vinyl}quinolinium]disulfide diiodide (1) on gold-coated highly oriented pyrolytic graphite (HOPG) substrates exhibit asymmetric current–voltage characteristics with rectification ratios of ca. 30 at ±1 V when contacted by PtIr or Au probes. The ratio decreases to ca. 10 at ±1 V for pentanethiolate-coated gold probes, which locate the donor–(π-bridge)–acceptor moiety approximately midway between the electrodes: Au–S–C5H11//D–π–A–C10H20–S–Au. The I–V characteristics are different but the current versus electric field dependence is indistinguishable for both types of electrode, the field being calculated for a monolayer thickness of 2.4 ± 0.1 nm and an assumed bilayer thickness of 3.2 nm (i.e. 0.8 nm for pentanethiolate). SAMs obtained from bis-[1-(10-decyl)-4-{2-(4-methoxyphenyl)vinyl}pyridinium]disulfide diiodide (2) exhibit symmetrical I–V characteristics and, unlike the bulkier rectifying analogue above, its D–π–A chromophore is almost planar. This structure–property relationship highlights the significance of steric hindrance for rectification from π-bridged donor–acceptor molecules and consistent I–V characteristics have been obtained from SAMs investigated by scanning tunnelling spectroscopy (STS) as well as electrometer-based techniques with an alkanethiolate-coated mercury droplet electrode.

Molecular rectification: dipole reversal in a cationic donor–(π-bridge)–acceptor dye

The cationic donor–(π-bridge)–acceptor dye, E-4-[(N-alkyl-5,6,7,8-tetrahydroisoquinolinium-5-ylidene)methyl]-N,N-dibutylaniline octadecyl sulfate is a molecular rectifier. Its Langmuir–Blodgett films exhibit asymmetric current–voltage characteristics but the dodecyl and octadecyl analogues behave differently. Rectification occurs in opposite quadrants of the I–V plot as a consequence of anion-induced dipole reversal: the aromatic form (D–π–A+–CnH2n + 1) exists when the negatively charged sulfate is adjacent to the heterocycle and the quinonoid form (D+πA′–CnH2n + 1 where A′ is the donor) when it is adjacent to the amino group. This is supported by the fact that mixed monolayers of the two dyes do not exhibit second-harmonic generation whereas films of the individual dyes have high second-order susceptibilities (e.g.χ(2)zzz = 100 pm V−1 at 1.064 µm for the dodecyl analogue). The monolayer structure is non-centrosymmetric and, therefore, the nonlinear optical behaviour can only be interpreted as above. Theoretical modelling has also verified the transition as the counterion is relocated. MNDO, AM1 and PM3 calculations indicate changes of ca. 0.08 A in the C–C bonds of the central bridging unit as well as the exocyclic C–N bond. It is the first time that anion-induced dipole reversal has been observed.

Molecular rectification: asymmetric current–voltage curves from self-assembled monolayers of a donor–(π-bridge)–acceptor dye

Thioacetic acid S-(10-{4-[2-cyano-2-(4-dicyanomethylenecyclohexa-2,5-dienylidene)ethylidene]-4H-quinolin-1-yl}decyl) ester (1a) forms self-assembled monolayers on gold with loss of the acetyl group, the thickness and contact area being 2.25 ± 0.05 nm and 0.36 ± 0.03 nm2 molecule−1 respectively. The films exhibit asymmetric current–voltage characteristics, which have been unambiguously assigned to molecular rectification.

Linear and non-linear optical properties of the different Langmuir–Blodgett phases of CnH2n+1–Q3CNQ

(Z)-α-cyano-β-(N-hexadecylquinolin-4-ylium)-4-styryldicyanomethanide, C16H33–Q3CNQ, has two Langmuir–Blodgett (LB) phases that differ in thickness, tilt angle and second-order susceptibility. For phase I, I=2.2 nm layer–1, ϕ= 8° and χzzz(2)= 18O pm V–1 at 1.064 µm, whereas for phase II, I= 1.6 nm layer–1, ϕ= 24° and χzzz(2)= 100 pm V–1. The susceptibility of phase I is the highest so far reported for an LB multilayer; the structure is non-centrosymmetric (Z-type) and, as predicted by theory, the second-harmonic intensity varies as I2ω(N)=I2ω(1)N2 as N increases from 1 to 200 layers. In contrast, C10H21–Q3CNQ has a low susceptibility χzzz(2)= 6 pm V–1 and neutron reflectivity studies have indicated an antiparallel alignment of chromophores at the air/water interface.

Rectifying characteristics of Mg|(C16H33-Q3CNQ LB film)|Pt structures

The Mg|(LB monolayer)|Pt structures of Z-β-(1-hexadecyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (C16H33–Q3CNQ) show asymmetric current–voltage characteristics; the behaviour is attributed to the organic monolayer although whether it is due to the presence of the permanent dipole moment or molecular rectification is unclear.