Spectral and Photophysical Studies on Cruciform Oligothiophenes in Solution and the Solid State

Abstract

The photophysical and spectroscopic properties of a new class of oligothiophene derivatives, designated as cruciform oligomers, have been investigated in solution (room and low temperature) and in the solid state (as thin films in Zeonex matrixes). The study comprises absorption, emission, and triplet-triplet absorption spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation) and lifetimes. The overall data allow the determination of the rate constants for all decay processes. From these, several conclusions are drawn. First, in solution, the main deactivation channels for the compounds are the radiationless processes: S 1 ' S 0 internal conversion and S 1 ' T 1 intersystem crossing. Second, in general, in the solid state, the fluorescence quantum yields decrease relative to solution. A comparison is made with the analogous linear R-oligothiophenes, revealing a lower fluorescence quantum efficiency and, in contrast to the normal oligothiophenes, that internal conversion is an important channel for the deactivation of the singlet excited state. Replacement of thiophene by 1,4-phenylene units in the longer-sized cruciform oligomer increases the fluorescence efficiency. The highly efficient generation of singlet oxygen through energy transfer from the triplet state (S ∆ ≈ 1) provides support for the measured intersystem crossing quantum yields and suggests that reaction with this may be an important pathway to consider for degradation of devices produced with these compounds.

Excited State Properties of Oligophenyl and Oligothienyl Swivel Cruciforms

“…8 Nanosecond fluorescence decays were measured using a home-built time-correlated single-photon counting (TCSPC) apparatus with an IBH NanoLED (339 or 373 nm) excitation source. 9 Fluorescence decay times with picosecond time resolution were obtained in an apparatus described elsewhere 10 except that a new Becker & Hickl SPC-630 board was used as a TCSPC device. Full details have previously been reported, and the time resolution of the equipment is approximately 3 ps.…”

“…13 The experimental setup used to obtain triplet state absorption spectra and quantum yields involves an Applied Photophysics laser flash photolysis apparatus pumped by a Nd:YAG laser (Spectra Physics), as described in detail elsewhere. 9 Transient spectra were obtained by monitoring the optical density change at intervals of 5-10 nm over the 300-850 nm range and averaging at least 10 decays at each wavelength. First-order kinetics were observed for the decay of the lowest triplet state.…”

Spectral and Photophysical Characterization of Donor-π-Acceptor Arylthienyl- and Bithienyl-Benzothiazole Derivatives in Solution and Solid State

“…Details on the experimental procedures and equations used can be found in ref 26,28 and in the Supporting Information section.…”

“…26,28 From these signals the singlet oxygen quantum yields were obtained. The procedures can be found in detail in the Supporting Information section.…”

Side‐Chain Degradation of Ultrapure π‐Conjugated Oligomers: Implications for Organic Electronics

“…Most likely, singlet oxygen is either formed through energy transfer from the thiophene core to triplet ground-state oxygen [39] or via a dioxygen-thiophene charge-transfer complex. [40] The presence of dimers could be due to noncovalent p-dimerization [41] or covalent cross-linking.…”