Abstract
Oligothiophenes and polythiophenes are building blocks of organic-based energy conversion materials. Therefore the lifetime of the excited states plays a central role. As a first step to understand the factors influencing the performance, we investigated the deactivation processes from the first excited state S1 of thiophene and small oligothiophenes containing up to four rings using quantum chemical calculations. For thiophene a low-lying S1/S0 conical intersection seam is easily accessible and drives the fast internal conversion. In oligothiophenes barriers inhibit this passage while deactivation pathways via intersystem crossing channels open. The first one is responsible for the high triplet quantum yields and takes place shortly after the Franck-Condon region. The second one occurs in the vicinity of a local S1 minimum. The calculated spin-orbit coupling strength together with the singlet-triplet energy gaps can explain the decreasing triplet and increasing fluorescence quantum yields for growing chain length. From the triplets the ground state is reachable by inter-ring torsions and T1/S0 intersections. The present results allow a deeper understanding of the deactivation pathways of thiophene and small oligothiophenes and are of potential interest for the photophysics of longer oligothiophenes and polythiophenes used in optical devices.
Highlights
Based on the presented results we suggest that in 4T the fast intersystem crossing (ISC) to the T3 state accounts for the high triplet quantum yield of 0.73.25 Reasons for the decreasing triplet quantum yield from 2T to 4T are the diminishing spin–orbit coupling (SOC) values along the initial relaxation (ISC1) and the increasing S1–T2 energy gap at S1-Min-a (ISC2), shown by photodetachment photoelectron spectroscopy.[47]
We demonstrated that 1T decays primarily via its singlet states
The S1/S0 conical intersections (CoIns) are inactive their energetic position with respect to the ground state minimum is similar for all systems
Summary
In the last few decades polythiophenes and oligothiophenes have been shown to be some of the most promising candidates of organic materials for technological applications.[1,2] In particular, they have been used in solar cells,[3,4] light emitting diodes,[5,6] photoswitches[7] and as biological labels.[8,9,10] Gaining detailed knowledge of the radiative and nonradiative mechanisms and the factors tuning these processes in the isolated oligomers should be the first step in improving the performance of oligothiophene-based devices. Effective triplet generation.[28,33,34] In addition oligothiophenes are considered to be quite flexible molecules with respect to the rotation around the inter-ring bonds. The T4 and T3 states were regarded until now to play only a minor role in the efficient ISC pathways of the oligothiophenes. We will demonstrate the crucial role of the T3 state for the effective triplet formation and elucidate the details of the ISC processes and relaxation pathways of bithiophene (2T), terthiophene (3T) and quaterthiophene (4T). The current results in conjunction with previous work offer quite a complete picture of the photophysical properties of the molecules and may be linked to the application of the oligothiophenes in optical devices
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
