Co-oligomer Single Crystals Research Articles

Optoelectronic characteristics of furan substituted thiophene/phenylene co-oligomer single crystals for organic lasing

A new lasing molecule BPTFT was synthesized by substituting furan to the central thiophene ring of BP3T. It exhibits better rigidity and optoelectronic properties, and is suitable to develop an electrically pumped organic laser.

Crystal packing control of a trifluoromethyl-substituted furan/phenylene co-oligomer.

Furan/phenylene co-oligomer single crystals are considered as future materials for organic optoelectronics. Here, the effects of trifluoromethyl substituents on the crystallization, structure and optical properties of furan/phenylene co-oligomer 1,4-bis{5-[4-(trifluoromethyl)phenyl]furan-2-yl}benzene are studied systematically. The solution growth methods and physical vapor transport result in the formation of three polymorphs depending on the growth method and the solvent. Single-crystal X-ray analysis reveals the crystal structures to correspond to H-, J- or mixed aggregates. All obtained crystals exhibit high photoluminescence efficiency and have optical properties which strongly depend on the crystal packing. Variable-temperature X-ray powder diffraction analysis shows the thermal transition of two forms (H- and J-aggregates) into a third one (mixed aggregate). Terminal trifluoromethyl groups induce weak intermolecular interactions which control the crystal packing and optical properties of co-oligomer single crystals.

Comparative Study of Single and Dual Gain-Narrowed Emission in Thiophene/Furan/Phenylene Co-Oligomer Single Crystals

Three organic semiconductors consisting of thiophene, furan, and phenylene groups showed either one or two gain-narrowed emission peaks by excitation with a laser pulse. The two gain-narrowed emission peaks are optically studied and assigned to 0 → 1 and 0 → 2 vibronic transitions. The number of gain-narrowed emission peaks is determined by a degree of overlap of the vibronic emissions with ground-state and excited-state absorption of the material. Dependence of the gained emission intensity both on the total input energy and the density of the excited states shows that the two gain-narrowed peaks are attributed to two different decay processes: amplified spontaneous emission (ASE) and superfluorescence (SF). The input energy to be conserved in total is distributed between ASE and SF processes strongly dependent on the transition probability and the self-absorption of organic semiconductors as a gain media.

Methyl substituent effect on structure, luminescence and semiconducting properties of furan/phenylene co-oligomer single crystals

Single crystals of furan/phenylene co-oligomers are among the most promising highly-emissive materials for applications in various optoelectronic devices. In this work, we synthesized and studied furan/phenylene co-oligomers with the same conjugated core 1,4-bis(5-phenylfuran-2-yl)benzene and methyl substituents at p- and m-positions of the terminal phenyls. The effect of substituents on the crystal packing, charge transport and luminescence of the single crystals was studied. Compared to the unsubstituted compound, the methyl-substituted co-oligomers demonstrated improved thermostability and enhanced photoluminescence, which we assign to J-aggregation resulting from the strong inclination of the molecules against the main crystal facet. The charge mobility in single crystal organic field-effect transistors decreased upon the inclination of the molecules. We conclude that the molecular tilt angle, intermolecular distances and interactions in crystals of heteroaryl-containing linear conjugated oligomers can be controlled by the introduction of end methyl groups in the appropriate positions.

Highly-emissive solution-grown furan/phenylene co-oligomer single crystals

Solution-grown single crystals of furan/phenylene co-oligomer combine efficient charge transport properties and high fluorescence efficiency.

Vertical cavity surface emitting lasing from cyano-substituted thiophene/phenylene co-oligomer single crystals

We have achieved to observe vertical cavity surface emitting lasing (VCSEL) from an optically pumped organic single crystal. A vapor-grown thiophene/phenylene co-oligomer (TPCO) single crystal with in-plane polarization component was used as the gain medium in an optical microcavity. Since the top and bottom surfaces of the TPCO crystal serve as high-quality resonators, well-defined Fabry-Pérot resonance is achieved. Furthermore, by using high reflectivity distributed Bragg reflector as the cavity resonator, we have observed lasing emissions with a line width of 0.9 nm and a threshold of ∼3 mJ/cm2. The VCSEL-type optical feedback configuration demonstrated in this study is expected to be one of the available means to realize the electrically pumped organic laser device.

Dispersion of the refractive indices of thiophene/phenylene co-oligomer single crystals

We have determined the wavelength dependence of group refractive indices for crystals of thiophene/phenylene co-oligomers (TPCOs). We calculated the group indices from the broadband interference modulation of the emission spectra. The interference is caused by a pair of parallel crystal edges that function as an optical resonator. These indices steeply increased with decrease in wavelengths and were in good agreement with those estimated from the mode intervals of the longitudinal multimode at the laser oscillation. Furthermore, we derived the dispersion of phase refractive indices on the basis of the Sellmeier dispersion equation. These refractive indices gradually increased with decrease in wavelengths and ranged 2.5–3.2 for the materials. The implications of the experimental results are discussed.

Amplified Pulse Emissions with Variable Delay Times in Vibronic Transition Bands of Thiophene/Phenylene Co-Oligomer Single Crystals

Time-resolved photoluminescence of thiophene/phenylene co-oligomer (TPCO) single crystals has been measured at 7 K. The amplified pulse emissions with time delay are demonstrated in the 0–2 vibronic transition band of the TPCO single crystal under high-intensity excitation. We identify the delayed pulse emission to the 2A1 vibration line. The pulse peak of the 2A1 line exhibits a maximum delay time of 70 ps at 7 K. We consider that the amplified pulse emissions with time delay in single crystals occur at a vibronic transition band with the largest spontaneous emission intensity in the PL spectrum under weak excitation.

Optical selection rule for the lower Davydov excitons in co-oligomer single crystals

Intrinsic optical transitions of lower Davydov excitons in thiophene/phenylene co-oligomer (TPCO) crystals were investigated. Lower Davydov excitons are either optically allowed or forbidden depending on the number of thiophene rings constituting the TPCOs. The TPCO molecules are orientated nearly parallel to each other like $H$ aggregates and are aligned in herringbone fashion within a crystalline layer. The transition selection rules are dominated by the components of the molecular transition dipole moments lying in the herringbone planes of the respective crystals. The selection rules at the exciton band bottoms of the TPCOs differ from those of oligothiophenes in spite of these two compounds having similar molecular structures. The selection rules vary drastically with the molecular alignment because of the $H$ aggregation. TPCO and oligothiophene crystals are treated as $H$ aggregates as a first approximation. This treatment is very useful for understanding the optical transitions of conjugated materials.

Origin of the amplified spontaneous emission from thiophene/phenylene co-oligomer single crystals: Towards co-oligomer lasers

Photoluminescence (PL) and optical gain measurements have been performed for single crystals of thiophene/phenylene co-oligomer at room and low temperatures. Broad PL bands are transformed to be an ensemble of several spectrally narrower vibronic peaks with decreasing temperature. Very sharp lines as narrow as ∼3meV are observed at 10K under weak excitation. Intensities of sharp emission lines superlinearly increased at 10K under intense excitation, showing the amplified spontaneous emission (ASE). The ASE bands were clearly identified to the sidebands of B1 and A1+B1 vibronic modes. The ASE is also observed at room temperature under the intense excitation. Spectroscopic investigation at varied temperatures enabled us to identify the origin of the ASE band to the vibronic sidebands of the electronic transition of the thiophene/phenylene co-oligomer crystals. In addition, highly polarized optical gains ∼50cm−1 were obtained for the two ASE bands at room temperature.