Optical studies of ordered and disordered α-sexithiophene films

Lane, Paul A.; Liess, Martin; Vardeny, Z. Valy; Partee, J.; Uhlhorn, Brian L.; Shinar, J.; Ibrahim, Mohamed F.; Frank, Arthur J.

doi:10.1117/12.279296

Cited by 4 publications

(4 citation statements)

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“…Thus, although the transition to the lower Davydov band is expected to carry no oscillator strength, such an absorption around 2.4 eV (labelled T 1 in our data in figure 4) has been observed in thin films correlating with our observations [47]. The energy separation of ca 0.2 eV between the peaks T 1 , T 2 and T 3 is close to the energy of the C=C stretching mode (∼ 0.18 eV) [51,[61][62][63] suggesting that T 2 and T 3 are then vibronic progressions of T 1 [50,64]. The pronounced absorption background in the spectrum of 6T below the fundamental transition can be explained by Mie scattering which comes into play when the crystallite size in a polycrystalline structure is in the range of the incident wavelength.…”

Section: Uv-vis-nir Spectroscopysupporting

confidence: 87%

Aggregate formation in crystalline blends of α-sexithiophene and para-sexiphenyl

et al. 2021

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Earlier reports on rod-like conjugated molecules of similar shape and size such as α-sexithiophene (6T) and para-sexiphenyl (6P) indicated mixed crystal growth in equimolar blends. The spectral overlap between the 6P fluorescence and 6T absorption might there give rise to resonant energy transfer between the two species. In marked contrast to H-type aggregation found for 6T bulk crystals, isolated monolayers of 6T as well as 6T monolayers sandwiched between 6P multilayers have been reported to show pronounced green (instead of red) fluorescence, which has been attributed to J-type aggregation. Here, we investigate whether these altered optical properties of 6T translate from the monolayer to a bulk equimolar blend with 6P. Insight into the mixed crystal structure for vacuum co-deposited films of 6T + 6P is provided by using synchrotron grazing-incidence x-ray diffraction on different substrates. By correlating the optical properties of the pure and the mixed systems using absorption and photoluminescence spectroscopy we identify the green emission known from 6T monolayers to prevail in the blend. Our analysis indicates the formation of aggregates which are promoted by the molecular arrangement in the mixed crystal structure highlighting that the remarkable optical properties of 6T/6P heterostacks translate into mixed crystalline films. This study underlines that tuning the opto-electronic properties of organic semiconductors by blending species of similar shape but distinct opto-electronic properties is a promising pathway to achieve altered material properties.

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Section: Uv-vis-nir Spectroscopysupporting

confidence: 87%

Aggregate formation in crystalline blends of α-sexithiophene and para-sexiphenyl

et al. 2021

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“…At the B3LYP/3-21G level, the oligothiophene substructure of the series 10 n is planar. However, a planar oligothiophene structure conflicts with experimental results on amino-group-free parent compounds . If the 3-21G basis set is replaced by the more extended 6-31G* basis set, then the thiophene rings of 5,5‘-bis(diphenylamino)-2,2-bithiophene 10 2 are twisted around the inter-ring CC-bond.…”

Section: Molecular and Electronic Structurementioning

confidence: 90%

“…However, a planar oligothiophene structure conflicts with experimental results on amino-group-free parent compounds. 30 If the 3-21G basis set is replaced by the more extended 6-31G* basis set, then the thiophene rings of 5,5′bis(diphenylamino)-2,2-bithiophene 10 2 are twisted around the inter-ring CC-bond. The CCCC dihedral angle of the two thiophene groups amounts to 158°.…”

Section: Molecular and Electronic Structurementioning

confidence: 99%

Radical Ions of α,α‘-Bis(diphenylamino)-capped Oligothiophenes: A Combined Spectroelectrochemical and Theoretical Study

Rohde¹,

Dunsch²,

Tabet³

et al. 2006

J. Phys. Chem. B

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A new homologous series of alpha,alpha'-bis(diphenylamino)-capped oligothiophenes, prepared by a palladium-catalyzed coupling reaction of stannylated 2-diphenylaminothiophenes with 2-mono- or 2,5-dibromothiophenes and their homologues, was studied by in situ ESR/UV-vis/NIR spectroelectrochemistry. In general, the oxidation potentials of these oligothiophenes were found to be proportional to the inverse number of thiophene units. However, the potential slope of the first oxidation is completely different from that of higher oxidation steps. Trication radicals were identified by electron spin resonance (ESR) for higher thiophene homologues in addition to monocation radicals (polarons). According to the ESR hyperfine structures, the unpaired electron is delocalized in the conjugated system. In contrast to the parent alpha,alpha'-bis(diphenylamino)-capped oligothiophenes, the UV-vis/NIR absorption maxima of the oxidized species strongly depend on the number of thiophene units. Spin-restricted and spin-unrestricted Kohn-Sham density functional calculations were used to explain and to understand these properties. Absorption wavelength and intensities were calculated by the time-dependent density functional response theory. Unrestricted density functional calculations of oligothiophene dications (bipolarons) with five or more thiophene rings result in spin-broken structures which may be considered as two-polaron biradicals (polaron pairs).

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“…[32][33][34][35] The first EA spectroscopy in conducting polymer films was applied to the structurally simplest polymer, namely, the trans and cis isomers of polyacetylene (CH) x ; [36][37][38][39] later studies have focused on its oligomers such as ␤-carotene. 40,41 Other conducting polymers have since attracted much attention and numerous EA studies of various conducting polymer films [42][43][44][45][46][47] and crystals 48,49 were made.…”

Section: Introductionmentioning

confidence: 99%

Electroabsorption spectroscopy of luminescent and nonluminescent π-conjugated polymers

et al. 1997

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We have measured the quadratic electroabsorption ͑EA͒ spectrum of a variety of soluble luminescent and nonluminescent-conjugated polymer films in the spectral range of 1.5-4.5 eV. The luminescent polymers include MEH and DOO derivatives of poly͑phenylene-vinylene͒, poly͑phenylene ethylene͒, and polythiophene; the nonluminescent polymers include poly͑diethynyl silane͒ and monosubstituted polyacetylene. All EA spectra show a Stark shift of the low-lying odd-parity exciton (1B u) and imply the presence of phonon sidebands. There are also higher-energy bands due to transfer of oscillator strength to even-parity exciton states (A g), the strongest of which (mA g) is located at an energy about 1.3 times that of the 1B u exciton in both luminescent and nonluminescent polymers; in the luminescent polymers the EA spectra also show a second prominent A g state (kA g) at an energy of about 1.6 times that of the 1B u. We have successfully fitted the EA spectra by calculating the imaginary part of the third order optical susceptibility, Im͓ 3 (Ϫ;,0,0)͔, using a summation over states model dominated by the ground state, the 1B u exciton, two strongly coupled A g states ͑mA g and kA g ͒, and their most strongly coupled vibrations, using Frank-Condon overlap integrals. A distribution of conjugation lengths, which results in a distribution of excited state energies, was also incorporated into the model. The decomposition of the EA spectra due to the conjugation length distribution was then used to calculate the 1B u exciton polarizability (⌬p) using first derivative analysis. For the longest conjugation lengths in our films, we found ⌬p to be of order 10 4 (Å) 3 in luminescent polymers and 10 3 Å 3 in nonluminescent polymers, respectively, in good agreement with recent subnanosecond transient photoconductivity measurements. We also found that the Huang-Rhys parameter of the 1B u exciton varies between 0.25 and 0.9, being in general smaller for the luminescent polymers. The consequent exciton relaxation energies were calculated to be of order 100 meV. ͓S0163-1829͑97͒03948-9͔

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Optical studies of ordered and disordered α-sexithiophene films

Cited by 4 publications

References 0 publications

Aggregate formation in crystalline blends of α-sexithiophene and para-sexiphenyl

Aggregate formation in crystalline blends of α-sexithiophene and para-sexiphenyl

Radical Ions of α,α‘-Bis(diphenylamino)-capped Oligothiophenes: A Combined Spectroelectrochemical and Theoretical Study

Electroabsorption spectroscopy of luminescent and nonluminescent π-conjugated polymers

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