Interaction between the end groups and the main chain of conjugated polymers by time-resolved EPR and fluorescence spectroscopy

ABSTRACT

Interaction between a zinc porphyrin (ZnPor) as the end-group and poly(9,9-di-n-octylfluorene-2,7-vinylene) (PFV) as the main chain in a porphyrin end-modified fluorescent conjugated polymer, ZnPFV, was studied by time-resolved electron paramagnetic resonance (EPR) and fluorescence spectroscopy. While fluorescence from the PFV part of ZnPFV showed a spectral profile almost identical to that of a PFV oligomer without end-modification, the emission spectrum of the ZnPor part exhibited a much broader profile compared to that of the reference zinc porphyrin monomer. Based on the analysis of lifetimes and quantum yields, it was found that radiative rate constant of the ZnPor part was enhanced by nearly three times. The observed unusual enhancement in the radiative rate constant was rationalised in terms of a partial π-conjugation between the end group and the main chain, as a result of co-planarisation in fluid solution. On the other hand, the time-resolved EPR spectrum of ZnPFV at 100 K basically showed a similar spectral pattern to that of the reference zinc porphyrin, but with significant differences in zero-field spitting parameters and initial population ratios. The π-system of the excited triplet state is deduced to deviate from D_4h symmetry in the end zinc porphyrin groups. The obtained results show that interaction of the porphyrin end group with the main chain of the polymer significantly influences the excited singlet state properties of the porphyrin, while its triplet state properties were affected to a lesser extent.

GRAPHICAL ABSTRACT

KEYWORDS:

• Fluorescent conjugated polymer
• excited states
• zinc(II) porphyrin
• end group
• delocalisation
• precisely synthesised polymer

Acknowledgements

We would like to thank Professor M. Chikira (Chuo University) for providing a magnet for the TREPR system. We wish to thank Professor Y. Kobori (Kobe University) for helpful discussion. Mr. Y. Suka (Gunma University) was acknowledged for his assistance in analysing the time-resolved fluorescence data.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Motoko S. Asano http://orcid.org/0000-0003-2398-8747

Additional information

Funding

This work was partially supported by Advanced Catalytic Transformation for Carbon Utilization (ACT-C JPMJCR12YX), Japan Science and Technology Agency by MEXT/JSPS KAKENHI Grant Number JP17K05801 and by the Kurata Grants awarded by the Hitachi Global Foundation.