Kinetics of Thermal Dissociation–Restoration Processes of J-Aggregate

Abstract

We investigated the kinetics of thermally induced dissociation–restoration processes of the J-band in merocyanine (MS)–Cd arachidate mixed Langmuir–Blodgett (LB) films. The UV–visible absorbance spectrum A of the as-deposited film is associated with a prominent J-band (Band III) with its peak located at ∼590 nm besides Bands I and II centered at 500–515 and 545–555 nm, respectively. In the case of dry-heat treatments (DHTs), the Band III fraction decreases and the Band I fraction increases as the process proceeds, while the Band II fraction remains almost constant. The remaining fraction of Band III is found to obey an exponential decay law, f(tH,TH) = exp [- k(TH)tH], governed by an activation-type rate, k(TH) = k(∞)exp [- E/(RTH)] with k(∞) = 4.50 ×108 s-1 and E = 79.7 kJ/mol, where tH and TH are the heating time and temperature, respectively. The above E is 12–16 kJ/mol higher than that for the J-band dissociation process in the first stage of the hydrothermal treatment (HTT) under 100% humidity. This difference is explained as a barrier-lowering effect due to the water present in the HTT case. In the DHT-treated LB films, further application of HTT restores the J-band in a similar manner to that observed in the second stage of HTT when HTT is directly applied to the as-deposited films. The J-band restoration is described in both cases as a first-order process where [Band I] → [Band III] predominates over the reverse reaction. The k(∞) and E for the forward reaction of 5.13 ×107 s-1 and 68.5 kJ/mol, respectively, for the DHT-subjected films, are comparable to those of 4.31 ×107 s-1 and 64.8 kJ/mol in the case of second-stage HTT reported previously.