In this study, the optical properties and excited-state dynamics of the unique self-assembled donor-acceptor (DA) merocyanine dye stacks from dimer up to octamer, prepared via dipole-dipole interactions, are reported in terms of coherent exciton dynamics and formation of an excimer-like state. Our findings are based on the steady-state absorption/emission, time-resolved fluorescence, and transient absorption (anisotropy) measurements, including wavepacket analysis and quantum mechanical calculations. Coherent exciton of torsional motions-restricted dye stacks rapidly localizes into the weakly emissive excimer-like state, by shortening the inter-moiety distance and changing the bond-length alternation pattern. The inner merocyanine moiety, having two neighboring units, has a reversed resonance character (non-polar (N) < zwitterionic (Z)) compared with the outer moiety (N > Z) in the ground state. This difference has led to two conclusions: (1) tetramers and octamers exhibit different features of excimer-like state than the dimer, and (2) octamers exhibit slower localization dynamics due to the enhanced homogeneity (six inner-moieties) compared with tetramers (two inner moieties).
Bibliographical noteFunding Information:
The work was supported by the National Research Foundation of Korea (NRF) through a grant funded by the South Korean Government (MEST) (no. 2016R1E1A1A01943379) and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (no. 2020R1A5A1019141). The quantum mechanical calculations were supported by the National Institute of Supercomputing and Network (NISN)/Korea Institute of Science and Technology Information (KISTI) with needed supercomputing resources, including technical support (KSC-2019-CRE-0201). This work was also partially supported by the Graduate School of Yonsei University Research Scholarship Grants in 2020. The authors thank the Bavarian State Ministry of Science and the Arts for the generous support to establish the Key Laboratory for Supramolecular Polymers of the Bavarian Polymer Institute (BPI) at the Center for Nanosystems Chemistry in Würzburg. D.K. directed the research. T.K. and S.K. led the experimental and computational activities, analyzed the data, and wrote the manuscript. T.K. and W.K. made an ultrafast TA setup. F.W. designed the dyes for the desired assembly into defined stack sizes. E.K. synthesized the dyes. E.K and D.B. confirmed the self-assembly structures by experimental and theoretical work. All the authors discussed the results and implications and commented on the manuscript. The authors declare no competing interests.
© 2020 Elsevier Inc.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry
- Chemical Engineering(all)
- Biochemistry, medical
- Materials Chemistry