Photochemical Sciences Ph.D. Dissertations

Title

Photochemical, Photophysical, and Electronic Properties of Fused Ring Systems with Alternating Benzene and Thiophene Units

Date of Award

2005

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Douglas Neckers

Abstract

We have synthesized a series of polycyclic benzo-fused thiophene compounds. Specifically, we have achieved the isomer-pure synthesis of two new pentacyclic representatives of TBBT that is thieno[2,3-f:5,4-f']bis[1]benzothiophene (anti) and thieno[3,2-f:4,5-f']bis[1]benzothiophene (syn).

Using benzo[1,2-b:4,5-b']dithiophene, we have shown a successful photoaddition to dimethyl acetylenedicarboxylate and have discovered a new photoaddition between benzo[1,2-b:4,5-b']dithiophene and a series of 1,4-diarylbutadiynes. This photoreaction led to acetylene-substituted cyclobutene derivatives in regioselective manner.

When embedded into a field-effect transistor, the new TBBT materials exhibited p-type semiconducting properties. X-ray diffraction studies revealed a favorable edge-on packing of both compounds on the substrate. We observed competitive field-effect mobilities and on/off current ratios, yet a distinct difference among the isomers.

We have investigated a wide series of benzo-fused thiophene compounds for both, hole-vibrational and electron-vibrational couplings. Specifically, we compared acenes, acenedithiophenes, and phenacenes to both TBBT isomers. Photoelectron measurements and density functional theory calculations showed an electronic structure of the radical-cation state of the TBBT isomers very different from that in anthradithiophene and pentacene and a close resemblance to the corresponding electronic structure of pentaphene. The calculations further showed that the nature of the hole-vibrational interactions in TBBT was very different from those found in acenedithiophenes and oligoacenes, as a significant coupling in TBBT between holes and low-energy vibrational modes was found.

A complete photophysical characterization was carried out on the series of benzo-fused thiophenes. We found that the fusion of one (two) thiophene(s) to benzothiophene (dibenzothiophene) caused a redshift in absorption and emission. The phosphorescence lifetimes and quantum yields were larger for the syn series of compounds compared to the anti series, yet no comparable trend could be found for fluorescence lifetimes. In fact, thieno[2,3-f:5,4-f']bis[1]benzothiophene showed the longest fluorescence and triplet lifetime in the entire series with 4.6 nanoseconds and 128 microseconds, respectively. Using time-resolved absorption spectroscopy, we characterized the lowest energy singlet and triplet excited state in the series.

Finally, low temperature UV-vis absorption spectroscopy was combined with quantum chemical computations to demonstrate a reorganization of the molecular orbitals in two isomers of benzodithiophene.

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