Single Crystals Of Polydiacetylene Research Articles

New approach to achieving the potentially high conductivity of polydiacetylene

It is postulated that only the surface layers of the structural formations are doped in single crystals of various polydiacetylenes because of the high packing density of their macromolecules and that this is the reason for the low level of conductivity attained in these conjugated polymers. For the purpose of raising the efficacy of the doping process by increasing the surface-to-volume ratio of the polymer, single crystals of the polydiacetylene poly-1,1,6,6-tetraphenylhexadiinediamine (poly-THD) are ground by various methods. A correlation between the particle size of the ground single crystals and their dc conductivity in the doped state is discovered. A record value of the conductivity (3×10−2 S/cm) for polydiacetylenes, which is more than two orders of magnitude higher than published values (10−3–10−5 S/cm), is achieved.

Photo-induced bi-directional phase transitions in polydiacetylene single crystals

We report the occurrence of a unique photo-induced phenomenon in conjugated polymers: reversible photo-induced phase transitions in single crystals of polydiacetylene (PDA). The dependence of phase conversion efficiency on the excitation photon energy shows that the photo-generation of charge-carriers (polarons), rather than excitons, is responsible for this macroscopic phase change. The possible dynamics of this unconventional phenomenon in the nanosecond time-domain is discussed.

Photoinduced bidirectional phase transitions in single crystals of polydiacetylenes

Photoinduced phase transitions have been observed between the two phases in single crystals of polydiacetylenes with side-groups of alkylurethanes. The threshold of the excitation intensity for the photochromism is extremely low, one absorbed photon per 140 repeated backbone units, indicating a collective nature of the photoconversion. The excitation spectra for the converted fraction clearly show that the photoinduced phase transition is mediated by photogeneration of polaronic species and not of singlet excitons.

Reversibly Photo-Induced Phase Transition in Single Crystals of Polydiacititlenes

Abstract We have observed novel photo-induced phenomena in conjugated polymers; the reversibly photo-induced phase transitions in single crystals of poly-diacetylene(PDA). Dependence of the phase conversion efficiency on excitation photon energy shows that the photo-generation of charge-carriers (polarons), not excitons, is responsible for the macroscopic phase change.

Reversible photoinduced phase transitions in single crystals of polydiacetylenes.

It was observed that bi-directional phase changes can be triggered by genuine photoexcitation between two phases with different bond structures in polydiacetylenes. The observed photoinduced phase transition was found to be mediated by photogeneration of charge carriers (and not of excitons). The extremely high efficiency is perhaps due to cooperative interaction between the locally photoconverted domains.

Dynamics of thermally- and photo-induced phase changes in single crystals of polydiacetylene

Thermally- and photo-induced transition between the two distinct phases (A- and B- phases) have been investigated for a new family of polydiacetylenes (PDAs) substituted with the side-groups of alkyl-urethanes ( R 1= R 2= ( CH 2) 4 OCONH ( CH 2) n−1 CH 3: n=1–10). Measurements of optical spectra as well as calorimetric studies have revealed the thermally-induced reversible transition accompanying the hysteresis of 0–70K in width. A large photo-induced change of reflectance spectra and anomalously long decay time have been observed in the hysteresis region, indicating the occurrence of photo-induced phase transition.

Photoconductivity action spectra in single crystals of polydiacetylenes

Photoconductivity action spectra in single crystals of polydiacetylenes

Anisotropy of the third-order nonlinear-optical susceptibility in a degenerate-ground-state conjugated polymer: trans-(CH)x.

We present the results of a series of third-harmonic-generation (THG) measurements on the conjugated polymer polyacetylene which probe the magnitude and the origin of the third-order nonlinear-optical susceptibility of this material. By performing reflection THG measurements relative to a silicon standard, we have unambiguously determined the magnitude of the third-order susceptibility associated with tripling the fundamental of the Nd:YAG laser to be ${\ensuremath{\chi}}_{?}^{(3)}$(3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega}/B) =(4\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ esu, where ${\ensuremath{\chi}}_{?}^{(3)}$ refers to that component of the third-order susceptibility tensor with all indices parallel to the chain direction. By measuring anisotropic THG in oriented films, we have shown that this component dominates. The magntiude and anisotropy are directly compared with results obtained from single crystals of polydiacetylene-(toluene-sulfonate) measured in the same apparatus. Finally, we have measured THG in both cis-rich and trans isomers of the same sample. We found that the measured response of the cis-rich samples scales with the residual trans content of the sample; ${\ensuremath{\chi}}_{?}^{(3)}$ of the trans isomer is 15--20 times larger than that of the cis isomer. This symmetry-specific aspect of ${\ensuremath{\chi}}^{(3)}$ implies a mechanism which is sensitive to the existence of a degenerate ground state, as in trans-(CH${)}_{\mathrm{x}}$; it is consistent with the virtual generation of nonlinear solitons as a mechanism for the large measured third-order nonlinear-optical susceptibility of polyacetylene.

Carrier photogeneration and mobility in polydiacetylene: Fast transient photoconductivity.

Transient photoconductivity experiments have been carried out on single crystals of polydiacetylene-(bis p-toluene sulfonate), PDA-TS. The low--electric-field photocurrent decay consists of a temperature-independent fast (picosecond) initial component and a longer-time (nanosecond) component with magnitude that is strongly temperature dependent. Using very small spacings between electrodes on the sample, we have succeeded in achieving sweepout for the longer-lived carriers; the data yield a mobility of \ensuremath{\approxeq}5 ${\mathrm{cm}}^{2}$/V\ensuremath{\cdot}s at room temperature in the nanosecond regime. These results demonstrate that the Onsager geminate recombination model, previously used extensively for the polydiacetylenes, is not applicable to PDA-TS.

Toward Highly Oriented Conjugated Polymers for Non-Linear Optics

The promise of conjugated polymers as fast response nonlinear optical materials has been emphasized, since the nonresonant third order nonlinear susceptibilities of these π-electron polymers are among the largest measured in any material. For example, third harmonic generation (THG) experiments have shown that for trans-polyacetylene, χ║(3)(3ω:ωω,ω) = (4± 2) × 10-10 esu, where χ║(3) refers to that component of the third order susceptibility tensor with all indices parallel to the chain direction [1]. The large nonlinear response is anisotropic and arises from the π-electrons of the conjugated polymer. The magnitude and anisotropy are comparable to results obtained from single crystals of polydiacetylene-(toluenesulfonate). Given these large χ║(3) values, it is important to develop methods for fabricating chain-aligned (oriented) conjugated polymer films which have optical quality.