Kinetics of thermal dissociation-restoration processes of j-aggregate

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 ~590nm besides Bands I and II centered at 500-515 and 545-555 nm, respectively. In the case of dryheat 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(t, T) = exp[-k(T and ∞tH-, governed by an activation-type rate, k T and ∞ / k/1∞ exp/-E=/RT)] with k/1∞ / 4.50 x 108 s 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/1∞ and E for the forward reaction of 5.13 x 10 s and 68.5 kJ/mol, respectively, for the DHT-subjected films, are comparable to those of 4.31 x 10 s and 64.8 kJ/mol in the case of second-stage HTT reported previously. © 2009 The Japan Society of Applied Physics.