Thermally induced J-band narrowing in merocyanine LB films

Abstract

We have examined the effect of heat treatments (HT) at temperatures below 90 °C by comparing UV–vis spectra of meorcyanine (MS)-cadmium arachidate (Cd-C 20) mixed Langmuir–Blodgett (LB) films before and after the application of HT. It has been found that narrowing of the red-shifted J-band is thermally induced in the mixed LB films by HT at temperatures ranging from 60 °C to 70 °C. The J-band is centered at 590 nm in the as-deposited state, and it is associated with a prominent in-plane anisotropy. After HT, the band becomes sharper in spectral shape with its peak further red-shifted to 596 nm, and the in-pane anisotropy is drastically decreased at the same time. We have not found the original and the thermally induced bands coexisting in one and the same spectrum. It is suggested that the observed thermal annealing effect is due to a total reorganization of the MS chromophore arrangements rather than an additional growth of the original MS J-aggregates. The reorganization of MS chromophores seems to be closely related with the microbrownian motion of alkyl chains that is plausibly assumed to be activated in the present temperature range. The mild heat treatments as shown here is characterized as a purely physical process, and will present another method of modifying the properties of LB films with a lower risk of degradations compared with those chemical processes such as acid or basic treatments which are so far known to be effective for controlling the aggregation state.