Weikun Ge

Strong photoluminescence from monosubstituted polyacetylenes containing biphenylyl chromophores

Monosubstituted polyacetylenes are generally regarded as unlikely candidates for photoluminescent materials. We have, however, observed intense deep-blue emission in a series of photoexcited poly(1–alkynes) (PAs) containing biphenylyl pendants (–{HC=C[(CH2)m–OCO–biphenyl–OC7H15]}n–where m=2, 3, 4, and 9). The photoluminescence (PL) is readily observable by naked eyes under normal room illumination conditions, whose integrated intensity is threefold higher than that of poly(1–phenyl–1–butyne), a well-known highly luminescent disubstituted polyacetylene. A red PL band has also been detected in the PAs. Using the extended-Huckel-tight-binding method, we have calculated the density of states of the PAs, and it is found that the functional pendants have dramatically modified the band states of polyacetylene. The strong PL of the PAs is satisfactorily explained by the engineered electronic structures.

Influence of electric field on the photoluminescence of a liquid crystalline monosubstituted polyacetylene

It has been shown recently that strong photoluminescence (PL) can occur in monosubstituted polyacetylenes (PAs). In this letter, we investigate the PL of tetrahydrofuran solutions of a highly luminescent liquid crystalline PA, poly(11-{[4′-heptoxy-4-biphenylyl)carbonyl]oxy}-1-undecyne) (PBU), under the influence of direct current electric fields. The applied electric fields can dramatically tune the PL structures of a moderately concentrated PBU solution (∼11.3 mM) at and above an electric field threshold of about 3×105u200aV/m. Combined with the director simulations, our results indicate that the electrically induced molecular reorientation and intermolecular interaction are responsible for the observed spectral changes.

Behavior of nitrogen impurities in III–V semiconductors

Abstract A detailed review on a few important issues related to the isoelectronic impurity nitrogen in III-V semiconductors GaP and GaAs are given in this article. These issues include (1) the binding mechanism for the nitrogen bound exciton, (2) the electron binding energy calculation, (3) the exciton binding energy calculation, (4) the exciton–phonon coupling, and (5) the behavior of the nitrogen in dilute nitride alloys. We conclude that a key problem that remains not well answered is the interplay of the short-range impurity potential originated from the atomic energy difference, the strain field caused by the lattice relaxation and the accompanying electronic polarization. The solution to this problem is essential to solve the long-standing mystery: what exactly are those pairs configurations?

Poly(alkylacetylenes): a new class of highly luminescent polyacetylenes

Abstract It is generally believed that monosubstituted polyacetylenes are unlikely candidates for photoluminescence (PL) materials, but we demonstrate here that poly(alkylacetylenes) are a class of highly luminescent monosubstituted polyacetylenes. Poly(alkylacetylenes) –{HCue5fbC[(CH 2 ) m R]} n – with R=OCO-Biph-OC 7 H 15 [ m =2(2), 3(3), 4(4), 9(5)], CO 2 (CH 2 ) 6 OCO-Biph-OC 9 H 19 [ m =2(6), 8(7)], and OCO-Biph-OCOC 11 H 23 [ m =4(8)] emit strong deep-blue light, readily observable by naked eyes under normal room illumination conditions. The PL intensity of (5) is at least six times higher than that of poly(1-phenyl-1-butyne), a well-known photoluminescent disubstituted polyacetylene.

Photoluminescent properties and electronic structures of monosubstituted polyacetylenes: poly{n-[((4′-cyano-4-biphenylyl)oxy)carbonyl]-1-alkynes}

Abstract Poly{ n -[((4′-cyano-4-biphenylyl)oxy)carbonyl]-1-alkynes} (PA n CN, where n =2, 8), which is a class of monosubstituted polyacetylenes with biphenyl mesogens and cyano tails in their pendants, can emit strong deep-blue photoluminescence (PL). In this paper we have investigated their optical absorptions, PL evolutions with excitation power and excitation wavelength, electronic structures, and the electric-field effect on the PL of a moderately concentrated PA n CN solution that had been treated by electric fields. Having been treated by an electric-field pulse (6×10 5 xa0V/m) for about 3xa0s, the originally featureless PL spectrum of the PA n CN solution becomes structured and transient, which evolves with time and external electric field. Our results have demonstrated that both the absorption and the deep-blue PL originate from the biphenyl mesogens in the side chains rather than from the distorted backbones.

Electrically tunable photoluminescence of liquid crystalline polyacetylene solutions

Abstract Light emission from tetrahydrofuran solutions of a liquid crystalline polyacetylene, poly(11-{[(4′-heptoxy-4-biphenylyl)carbonyl]oxy}-1-undecyne), is investigated in the electrical field. The field exerts little effect on the photoluminescence of the polymer solution with a low concentration (1.03×10 −4 M). The photoluminescence of a concentrated solution (113×10 −4 M) is, however, noticeably quenched under an electrical field with a field strength of >3×10 5 V m −1 . When the field strength is increased to ⩾3.67×10 5 V m −1 , the bimodal emission spectrum of the solution changes to a monomodal one. Thus, both the emission intensity and spectral profile of the photoluminescence of the concentrated solution can be tuned by the electrical field, which is probably caused by the aggregate dissociation and mesogen realignment induced by the external stimulus.

Monomer and dimer emissions in the solutions of a monosubstituted polyacetylene

Abstract Photoluminescence (PL) of tetrahydrofuran solutions of a monosubstituted polyacetylene with liquid crystalline pendants, poly(11-{[4′-heptoxy-4-biphenylyl)carbonyl]oxy}-1-undecyne) ( 1 ), has been investigated. Intense ultraviolet emission (∼370 nm) has been detected in its dilute solution (2.5 μM). As the solution becomes concentrated (up to 39 mM), the emission color changes to deep blue and finally to blue (∼430 nm). In addition to the large-scale red shift in PL peak, its luminescent intensity changes about two orders of magnitude. Based on the extended Huckel tight-binding calculations, the 370 nm peak is assigned to monomer emission, which originates from the biphenyl mesogen in the pendants, and the peak at 430 nm corresponds to dimer emission, which is related to the aggregates formed in concentrated solutions. Lyotropic mesophase was observed as 1 dissolves in tetrahydrofuran.

The role of the phenyl and biphenyl chromophores in the blue luminescent liquid crystalline polyacetylenes

Abstract We have studied the photoluminescence (PL) of liquid crystalline polyacetylenes {ue5f8[HCue605C(CH 2 ) 9 ue5f8OCOue5f8Biphue5f8Oue5f8(CH 2 ) 6 CH 3 ] n ue5f8 ( 1 ), ue5f8[HCue605C(CH 2 ) 3 ue5f8Oue5f8C 6 H 4 ue5f8CO 2 ue5f8C 6 H 4 ue5f8O ue5f8(CH 2 ) 5 CH 3 ] n ue5f8 ( 2 ), and ue5f8[HCue605Cue5f8C 6 H 4 ue5f8CO 2 ue5f8(CH 2 ) 6 ue5f8OCOue5f8Biphue5f8O(CH 2 ) 6 CH 3 ] n ue5f8 ( 3 )}. The textures of the polymers are characterized by polarized optical microscope. In dilute tetrahydrofuran (THF) solution, 1 emits strong deep-blue PL with a single PL peak at ∼380 nm, while 2 emits faint blue PL with a single peak locating at ∼450 nm. Interestingly, the PL of 3 is composed of two peaks, one of which locates at 380 nm and the other at 450 nm. Using extended Huckel tight-binding method, we have calculated their electronic structures and the results show that the electronic states of the polymers are essentially an ensemble of the extended states characteristic of the backbone and the localized states characteristic of the pendant. Our experimental and calculated results prove that both the absorption and blue emissions take place in the phenyl or biphenyl mesogens in the pendants and the types of chromophores determine the emission colors of the polymers.

Liquid crystalline polyacetylenes: a new class of mesomorphic materials with novel optical and electronic properties

A large number (more than 20 different kinds) of new polyacetylenes with general molecular structures of -[HC equals C(C6H4-mesogen)]p- [poly(arylacetylene) type] and -{HC equals C[(CH2)n-mesogen]}p- [poly(alkylacetylene) type] are designed and synthesized. Pendant interaction and backbone rigidity in the polymers are tuned through systematic molecular engineering endeavor, and liquid crystalline polyacetylenes (LCPAs) with novel mesomorphic, optical, and electronic properties are successfully developed. The rigid polyacetylene backbones enable ready alignments of the LCPA molecules by simple mechanical perturbations. Upon photoexcitation, the LCPAs with the poly(alkylacetylene) skeleton structure emit strong blue light clearly observable by naked eyes under normal room illumination conditions. The shape and position of the emission peaks and the color of the emitted light can be manipulated by the application of external electrical fields. The LCPAs exhibit excellent intrinsic photoconductivity in the visible spectral region in the undoped (pure) states, and doping with electron acceptor/donor further increases the photoconduction efficiency of the LCPAs.