C. Cuniberti

Ultrafast exciton dynamics in highly oriented polydiacetylene films

The nonlinear optical response in uniaxially oriented films of poly[1.6‐di(N‐carbazolyl)‐2, 4‐hexadiyne], epitaxially grown on a potassium acid phthalate single crystal, has been measured by femtosecond pump and probe. A fast initial relaxation and the formation of long‐lived species at high excitation intensities have been observed. A high third‐order nonlinear optical susceptibility of 2×10−7 esu has been estimated at 627 nm. The effects of sample orientation on the relaxation dynamics have also been investigated.

Triplet exciton generation and decay in a red polydiacetylene studied by femtosecond spectroscopy

Abstract Results from femtosecond/picosecond transient absorption spectra measurements in the near-infrared/visible spectral region of the red form of a soluble polycarbazolyldiacetylene in a polyethylene matrix are reported. Intrachain singlet and triplet excitons are identified by the different dynamics of the photoinduced bands. The results are consistent with the ultrafast generation (

Photoinduced absorption spectra of poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (polyDCHD) by excitation on the carbazole group

We report photoinduced absorption measurements in the millisecond time domain of poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (poly-DCHD) obtained by pumping on the side group. We show that the photoexcited states are similar to those detected by pumping on the absorption band of the conjugated backbone. This fact can be explained in terms of fast migration of the excitation energy from the side group to the conjugated chain.

Photoinduced absorption of oriented poly[1,6-di( N -carbazolyl)-2,4-hexadiyne]

We report the photoinduced absorption spectra of an oriented form of poly[1,6- di(N-carbazolyl)-2,4-hexadiyne] (polyDCHD) grown by molecular epitaxy. The polarized absorption spectra of the polyDCHD point out the high quality of the sample and the very high degree of alignment of the polydiacetylene chains. The millisecond photoinduced absorption spectrum shows the same features already observed in polycrystalline samples assigned to triplet excitons in addition to bipolarons. The study of the dependence of the photoinduced absorptions on the polarization of the laser beam confirms this assignment. After correction for the same number of absorbed photons in both polarizations, the photoinduced signal assigned to bipolarons is stronger by pumping with the electric field of the radiation perpendicular to the chain axis direction. This fact is indicative of the interchain nature of the generation process of these photoexcited species. On the other hand, such an effect is not observed for the photoinduced triplet-triplet transition according to its intramolecular nature. textcopyright{} 1996 The American Physical Society.

Polarization properties of a novel oriented polydiacetylene

Polarized electronic absorption, Raman and luminescence spectra of gel-processed blends of a soluble polycarbazolyldiacetylene and ultrahigh molecular weight polyethylene are reported and compared with those observed in other physical states. The films are oriented by tensile drawing to a draw ratio of 62. The strong anisotropy of all the measured properties indicates a rather good orientation of the conjugated chains along the stretching direction. The data here presented are quite different from those described for other conjugated systems where the gel-processed stretch-oriented polyethylene chains play an important role in the improvement of the order and in the enhancement of delocalization for the conjugated guest. We attribute this peculiar behaviour to the stiffness of the conjugated skeleton.

Optical properties and photoinduced absorptions in unsymmetrical polycarbazolydiacetylenes

Abstract In this paper we report the optical absorption and photoinduced absorption spectra of poly(1- N -carbazolyl-penta-1,3 diyne-5-acetoxy) and of poly(1- N -carbazolyl-penta-1,3-diyne-5-ol) and their relation to temperature, excitation intensity, and frequency of the pump-probe cycles. Preliminary data on an oligomeric sample are also discussed. All experimental data point to the presence of long-lived photogenerated defects characterized by a bimolecular recombination process which appears typical of bipilaronic states.

Electronic structure of mixed quinoid/aromatic thiophene copolymers

Abstract We report results of geometry optimization and electronic structure calculations for a class of recently synthesized thiophene copolymers with mixed aromatic/quinoid character which exhibit unusually large third-order optical nonlinearities. The orbital charges on their aromatic/quinoid portions are calculated and a relation is suggested between them and the energy dispersion of the orbital. On the basis of these results, good localization properties are predicted for the HOMO and even for the LUMO, especially if somewhat larger quinoid sequences could be synthesized.

Photoexcitation studies in poly[1,6-di(N-carbazolyl)-2,4-hexadiyne]. Correlation of spectral features with the degree of order in polycrystalline samples

Abstract We have studied the electronic structure and the elementary excitations of the red phase of poly[1,6-di( N -carbazolyl)-2,4-hexadiyne] (polyDCHD) by electronic and Raman spectra and by photoinduced absorptions in the medium and near-infrared regions. The results are interpreted in terms of the coexistence in this polymer of both ordered and disordered regions. By comparing the present data with those previously obtained on a highly ordered blue form of the same polymer, the nature of the long-lived photoexcitations is found to be dependent on the degree of chain order. Indeed, in the blue polymer, charged bipolarons and triplet excitations were detected at 0.1, 0.8 and 1.25 eV, respectively. On the other hand, in the more disordered form, a signal at 1.5 eV represents the main component of the photoinduced spectra. A tentative assignment of this feature to triplet excitons trapped in a chain defect is proposed, although trapped polarons could similarly account for the large lifetime estimated from our experiments.

Photoexcitations in polycarbazolyldiacetylenes in different time regimes

Abstract We present the results of photoinduced absorption (PA) studies carried out on oligomers of 1(-N-carbazzolyl)penta-1,3-diyn-5-acetoxy (CPDA). From the temperature, laser intensity and chopper frequency dependence of the PA signals upon excitation at 488 nm, we have demonstrated that in the msec-sec range the dominant photocarries are charged bipolarons. The picosecond decay kinetics have been obtained on CPDA oligomer solutions by transient saturated absorption at 627 nm, measured by femtosecond pump and probe technique. The decay curves could be fitted to biexponential functions. These results are discussed in terms of the relaxation model proposed for other polydiacetylenes.

Theoretical calculations of the geometries and of the lowest optical transitions of singly and doubly charged oligodiacetylenes

Abstract We have performed theoretical calculations on the singly and doubly charged species of the lowest unsubstituted oligodiacetylenes, with the scope of achieving information on the charged photoexcitations of polydiacetylenes. On the basis of the AM1-optimized geometries of the radical ions and of the dications we have computed their low-energy vertical transitions, using the INDO-SCI method in order to take into account the electron correlation. Extrapolated polymer results are compared with the photoinduced absorption spectra.