Role of the conduction band in electroabsorption, two-photon absorption, and third-harmonic generation in polydiacetylenes

Ultrafast Transient Spectroscopy of Trans-Polyacetylene in the Midinfrared Spectral Range

Trans-polyacetylene [t-(CH)_{x}] possesses twofold ground state degeneracy. Using the Su-Schrieffer-Heeger Hamiltonian, scientists predicted charged solitons to be the primary photoexcitations in t-(CH)_{x}; this prediction, however, has led to sharp debate. To resolve this saga, we use subpicosecond transient photomodulation spectroscopy in the mid-IR spectral range (0.1-1.5 eV) in neat t-(CH)_{x} thin films. We show that odd-parity singlet excitons are the primary photoexcitations in t-(CH)_{x}, similar to many other nondegenerate π-conjugated polymers. The exciton transitions are characterized by two photoinduced absorption (PA) bands at 0.38 and 0.6 eV, and an associated photoluminescence band at ∼1.5  eV having similar polarization memory. The primary excitons undergo internal conversion within ∼100  fs to an even-parity (dark) singlet exciton with a PA band at ∼1.4  eV. We also find ultrafast photogeneration of charge polarons when pumping deep into the polymer continuum band, which are characterized by two other PA bands in the mid-IR and associated photoinduced IR vibrational modes.

Directional Self-Assembly and Photoinduced Polymerization of Diacetylene-Containing Platinum(II) Terpyridine Complexes

A series of newly designed and synthesized diacetylene-containing platinum(II) terpyridine complexes exhibited intriguing self-assembly properties. Facilitated by Pt⋅⋅⋅Pt, π-π stacking, hydrogen-bonding and hydrophobic-hydrophobic interactions, these complexes are preorganized to readily undergo topochemical polymerization reactions upon photoirradiation. The in situ polymerization of the diacetylene units to form polydiacetylene, indicated by the UV/Vis spectral changes, gel permeation chromatography and dynamic light scattering, was found to alter their assembly behaviours, as revealed by TEM images.

Hybrid-state emission in a polythienylenevinylene derivative with an electron deficient moiety

The photoluminescence (PL) of a novel imide-substituted poly(3-thienylenevinylene) derivative (imidePTV) was studied in film and solution. PL quantum efficiency was measured to be more than two orders of magnitude larger than its nonluminescent counterpart, namely, alkyl-substituted PTV and was interpreted as evidence for a near degeneracy of optically allowed 1(1)Bu and optically forbidden 2(1)Ag excitonic states. As a result, coexistence of 2(1)Ag and 1(1)Bu emissions was observed, and the predominance was found to be sensitive to temperature and morphological environment. PL of solutions in solvents of higher polarity and polarizability and from low-temperature films was dominated by the transition from the dipole allowed 1(1)Bu state. On the other hand, the PL spectra of films at high temperature and solutions in solvents of low polarity and polarizability were primarily from the 2(1)Ag state that obtains a finite transmission moment from an asymmetric perturbation mixing with the 1(1)Bu.

Ultrafast intersystem-crossing in platinum containing π-conjugated polymers with tunable spin-orbit coupling

The development of efficient organic light-emitting diodes (OLED) and organic photovoltaic cells requires control over the dynamics of spin sensitive excitations. Embedding heavy metal atoms in π-conjugated polymer chains enhances the spin-orbit coupling (SOC), and thus facilitates intersystem crossing (ISC) from the singlet to triplet manifolds. Here we use various nonlinear optical spectroscopies such as two-photon absorption and electroabsorption in conjunction with electronic structure calculations, for studying the energies, emission bands and ultrafast dynamics of spin photoexcitations in two newly synthesized π-conjugated polymers that contain intrachain platinum (Pt) atoms separated by one (Pt-1) or three (Pt-3) organic spacer units. The controllable SOC in these polymers leads to a record ISC time of <~1 ps in Pt-1 and ~6 ps in Pt-3. The tunable ultrafast ISC rate modulates the intensity ratio of the phosphorescence and fluorescence emission bands, with potential applications for white OLEDs.

Polydiacetylenes: supramolecular smart materials with a structural hierarchy for sensing, imaging and display applications

While a large variety of conjugated polymers exist, polydiacetylenes (PDAs) remain a major research area among scientists due to their interesting optical, spectral, electronic, and structural properties. Heavily reviewed in regards to their stimuli responsive properties, much is known about the assortment of sensing and detection capabilities of PDAs. In this article, we look more upon the structural diversities of polydiacetylenes that have been achieved in recent years, particularly from a hierarchical perspective of 1, 2, and 3-dimensional configurations. In addition, we examine how these different dimensional arrangements of PDAs have heralded clear applications in several key areas. Successful integration of these stimuli-responsive "smart" materials into various geometries has required researchers to have a comprehensive understanding of both the fabrication and synthesis processes, as well as the signalling mechanism for the optical, fluorogenic or spectral transitions. The on-going discovery of new PDA formulations continues to provide interesting structural manifestations such as liposomes, tubes, fibres, organic/inorganic incorporated hybrids and composite structures. By highlighting some of the recent conceptual and technological developments, we hope to provide a measure of the current pace in new PDA derivative development as core components in efficient sensor, imaging and display systems.

COUPLED-CLUSTER EQUATION OF MOTION STUDY FOR THE ELECTRONIC AND OPTICAL PROPERTIES OF CONJUGATED SYSTEMS

We have implemented the Coupled-Cluster Equation of Motion (CC-EOM) with single and double excitations coupled with semiempirical parameterizations, Pariser–Parr–Pople model and INDO Hamiltonians, to investigate the optical and nonlinear optical properties, electronic structures and the excited states properties for the conjugated polymers. The semiempirical parameters allow us to study the conjugated systems with extensive sizes. Firstly, by comparing with the quasi-exact Density Matrix Renormalization Group theory for the 1D conjugated chain, we find that the CC-EOM approach can give satisfactory results for both the ground state and the excited states energies. We demonstrate that our approach can be adopted to evaluate linear and nonlinear response. We find that both the real and imaginary parts of the third-order polarizability can be solved either for static and dynamic responses. Then we apply the CC-EOM approach to study the optical signatures for the polarons in conjugated polymers. We have established a solid relationship between the rigidity of a polymer and its optical signature of the polarons.

Electron-electron interaction effects on the photophysics of metallic single-walled carbon nanotubes

Single-walled carbon nanotubes are strongly correlated systems with large Coulomb repulsion between two electrons occupying the same p(z) orbital. Within a molecular Hamiltonian appropriate for correlated π-electron systems, we show that optical excitations polarized parallel to the nanotube axes in the so-called metallic single-walled carbon nanotubes are excitons. Our calculated absolute exciton energies in twelve different metallic single-walled carbon nanotubes, with diameters in the range 0.8-1.4 nm, are in nearly quantitative agreement with experimental results. We have also calculated the absorption spectrum for the (21, 21) single-walled carbon nanotube in the E(22) region. Our calculated spectrum gives an excellent fit to the experimental absorption spectrum. In all cases our calculated exciton binding energies are only slightly smaller than those of semiconducting nanotubes with comparable diameters, in contradiction to results obtained within the ab initio approach, which predicts much smaller binding energies. We ascribe this difference to the difficulty of determining the behavior of systems with strong on-site Coulomb interactions within theories based on the density functional approach. As in the semiconducting nanotubes we predict in the metallic nanotubes a two-photon exciton above the lowest longitudinally polarized exciton that can be detected by ultrafast pump-probe spectroscopy. We also predict a subgap absorption polarized perpendicular to the nanotube axes below the lowest longitudinal exciton, blueshifted from the exact midgap by electron-electron interactions.

Poly(diiododiacetylene): preparation, isolation, and full characterization of a very simple poly(diacetylene)

Poly(diiodiacetylene), or PIDA, is a conjugated polymer containing the poly(diacetylene) (PDA) backbone but with only iodine atom substituents. The monomer diiodobutadiyne (1) can be aligned in the solid state with bis(nitrile) oxalamide hosts by hydrogen bonds between oxalamide groups and weak Lewis acid-base interactions (halogen bonds) between nitriles and iodoalkynes. The resulting cocrystals start out pale blue but turn shiny and copper-colored as the polymerization progresses. The development of a crystallization methodology that greatly improves the yield of PIDA to about 50% now allows the full characterization of the polymer by X-ray diffraction, solid-state (13)C MAS NMR, Raman, and electron absorption spectroscopy. Comparison of a series of hosts reveals an odd-even effect in the topochemical polymerization, based on the alkyl chain length of the host. In the cocrystals formed with bis(pentanenitrile) oxalamide (4) and bis(heptanenitrile) oxalamide (6), the host/guest ratio is 1:2 and the monomer polymerizes spontaneously at room temperature, while in the case of bis(butanenitrile) oxalamide (3) and bis(hexanenitrile) oxalamide (5), where the host and guest form cocrystals in a 1:1 ratio, the polymerization is disfavored and does not go to completion. The topochemical polymerization can also be observed in water suspensions of micrometer-sized 6.1 cocrystals; the size distribution of these microcrystals, and the resulting polymer chains, can be controlled by sonication. Completely polymerized PIDA cocrystals show a highly resolved vibronic progression in their UV/vis absorption spectra. Extensive rinsing of the crystals in organic solvents such as methanol, THF, and chloroform separates the polymer from the soluble host. Once isolated, PIDA forms blue suspensions in a variety of solvents. The UV/vis absorption spectra of these suspensions match the cocrystal spectrum, without the vibronic resolution. However, they also include a new longer-wavelength absorption peak, associated with aggregation of the polymer chains.

Elucidation of the electronic structure of semiconducting single-walled carbon nanotubes by electroabsorption spectroscopy

We report benchmark calculations of electroabsorption in semiconducting single-walled carbon nanotubes that provide motivation to perform electroabsorption measurement on these systems. We show that electroabsorption can detect the continuum bands in different energy manifolds. Direct determination of the binding energies of excitons in higher manifolds thereby becomes possible. We also find that electroabsorption can provide evidence for Fano-type coupling between the second exciton and the lowest continuum band states.

All-optical tunable photonic bandgap microcavities with a femtosecond time response

An all-optical tunable photonic bandgap microcavity made from a two-dimensional polystyrene photonic crystal is fabricated by focused ion beam etching. The pump and probe scheme is adopted to measure tunability based on the femtosecond optical Kerr effect. An ultrafast response time of less than 120 fs is achieved for the tunable photonic bandgap microcavity. The microcavity resonant wavelength shifts 3.1 nm under excitation of 9.4 GW/cm2 pump intensity, which is in agreement with the theoretical prediction.

Long-lived charged states in single-walled carbon nanotubes

We present continuous wave photoinduced absorption spectroscopy of single-walled carbon nanotubes dispersed in a polymer matrix. The spectrum is dominated by a modulation of the absorption line shape, predominantly of large diameter tubes, that we assigned to electroabsorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation points to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.

Direct observation of excitons and a continuum of one-dimensional Mott insulators: a reflection-type third-harmonic-generation study of Ni-halogen chain compounds

The third-harmonic-generation (THG) spectrum was measured for a NiBr-chain compound, which is a one-dimensional Mott insulator, in a reflection configuration. A sharp peak and a shoulder structure in the THG spectrum are attributed to three-photon resonance to an exciton and a continuum, respectively. The band-edge energy, the exciton binding energy, and the spectral weights for the exciton and the continuum were determined from comparative studies of linear absorption, THG, and electroreflectance spectra. The excitonic effect is more pronounced in the NiCl chain than in the NiBr chain.

Electron-electron interaction effects on the optical excitations of semiconducting single-walled carbon nanotubes

We report correlated-electron calculations of optically excited states in ten semiconducting single-walled carbon nanotubes with a wide range of diameters. Optical excitation occurs to excitons whose binding energies decrease with increasing nanotube diameter, and are smaller than the binding energy of an isolated strand of poly-(paraphenylene vinylene). The ratio of the energy of the second optical exciton polarized along the nanotube axis to that of the lowest exciton is smaller than the value predicted within single-particle theory. The experimentally observed weak photoluminescence is an intrinsic feature of semiconducting nanotubes.