Determination of a Merocyanine J-Aggregate Structure and the Significant Contribution of the Electric Dipole Interaction to the Exciton Band Wavelength

The molecular alignment of a merocyanine (MC) J-aggregate monolayer at the air-water interface was determined by a grazing incidence x-ray diffraction method. The obtained molecular arrangement apparently shows that the conventional formula, which accounts only for the transition dipole interaction, is not sufficient to figure out the exciton band wavelength, suggesting the importance of the electric dipole (ED) interaction. We derived a simple formula for the ED interaction energy under an extended dipole approximation and clarified the ED contribution in the MC J aggregate.

Figure 1

Visible absorption spectra of the MC monolayer at the air-water interface in the J-aggregate and monomeric states. Inset indicates the molecular structure of the MC molecule. The left and right aromatic groups are benzothiazol and rhodanine rings, respectively. Both planar rings are on the same plane, which is termed “dye plane,” and the long axis of the dye group is termed “dye axis.”

Figure 2

(a) The 2D unit cell of the MC J aggregate. (b) Three parameters for simulating the diffraction intensities. (c) Determined arrangement of the MC molecules in the unit cell in (a). The dye group 1, the carboxylic acid group 2, and the alkyl chain 3 are highlighted for the molecule in the corner.

Figure 3

Extended dipole models for (a) TD and (b) ED interactions between molecules 00 and $mn$. The lengths of ${\mathit{\mu }}_{\mathrm{tr}}$, $\Delta {\mathit{\mu }}_{eg}$, and ${\mathit{\mu }}_g$ are ${\ell }_{\mathrm{tr}}$, ${\ell }_{eg}$, and ${\ell }_g$, respectively. In general, the vectors ${\mathit{\mu }}_{\mathrm{tr}}$, $\Delta {\mathit{\mu }}_{eg}$, and ${\mathit{\mu }}_g$ are not necessarily the same; neither are the lengths ${\ell }_{\mathrm{tr}}$, ${\ell }_{eg}$, and ${\ell }_g$.

Figure 4

(a) Contribution of the TD interaction. $|{\mathit{\mu }}_{\mathrm{tr}}|=8.256\mathrm{D}$ [8] and ${\epsilon }_{\mathrm{r}}=2.5$ [8, 10, 19]. The thin line indicates the observed $E_{\mathrm{shift}}=-0.36\mathrm{eV}$. Since the spread of the delocalized $\pi$ electrons around the methine chain (Fig. 1) is approximately $1\mathrm{nm}$, ${\ell }_{\mathrm{tr}}\lesssim 1\mathrm{nm}$ [19]. The maximum contribution in this region is given at ${\ell }_{\mathrm{tr}}=0.49\mathrm{nm}$ (indicated by a dot). (b) Contour plot of $E_{\mathrm{shift}}$. $|{\mathit{\mu }}_g|=7.68\mathrm{D}$ [21] and ${\ell }_{eg}={\ell }_{\mathrm{tr}}=0.49\mathrm{nm}$ [22]. The bold line indicates the observed $E_{\mathrm{shift}}=-0.36\mathrm{eV}$.