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제125회 대한화학회 학술발표회 및 총회 Systematical Investigation on Spatial Coherence in Size-Defined Merocyanine Dye Stacks

2020년 2월 3일 17시 34분 16초
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Physical Chemistry
저자 및
Seongsoo Kang, Taeyeon Kim1, Dongho Kim2,*
Yonsei University, Korea
1Department of Chemistry Graduate School, Yonsei University, Korea
2Department of Chemistry, Yonsei University, Korea

Molecular structures of merocyanine (MC) chromophores and schematic representations of size-defined MC stacks from dimer up to octamer are shown in Fig. 1a. Dipole-dipole interactions induced by the directionality of electrostatic dipole moments make MC dyes form highly aligned and favorable stacks with binding constants exhibiting the intermolecular distance of ~ 3.5Å between neighboring units. The absorption spectra of MC stacks are measured in tetrahydrofuran (THF), good solvating solvents, and methylcyclohexane (MCH), bad solvating solvents, mixtures (17.5:82.5) due to very low solubility in nonpolar MCH. When aggregated in THF/MCH mixtures, MC stacks from dimer up to octamer show diminished 0-0 and enhanced 0-1 peaks and blue-shifted absorption spectra compared to the monomer in toluene, which corresponds to the typical optical features of H-type aggregates according to the Kasha`s excitonic theory, implying the allowed upper and the forbidden lower Frenkel exciton states in H-type aggregates (Figure 1b). Particularly, larger MC stacks exhibit significantly blue-shifted absorption spectra more than those of smaller MC stacks, which qualitatively demonstrates fully-coupled electronic states in the ground state (at least in octamer). In fluorescence spectra, MC stacks from dimer up to octamer show broad and featureless emission spectra (QY < 1 %), which is typical excimer-like emission in H-type aggregates, but the totally quenched emission of MC monomer. In monomer, quenched emission originates from the intramolecular photoisomerization process within the time constant of ~ 3 ps. The formation of stacks by the dipole-dipole interactions in MC aggregates impede the photoisomerization of the MC subunits by restricting the structural rotation of MC subunits.