Aggregation control by vapour phase and heat treatments in Langmuir–Blodgett films of amphiphilic heteroarylazo dyes

Abstract

The effects of heat treatment (HT), and acidic- and basic-vapour-phase treatments (AT and BT, respectively) have been studied on Langmuir–Blodgett (LB) films of 2-[4-(octadecylamino)-1-naphthylazo]-N-methylbenzothiazolium perchlorate (azo V) and 2-[4-(dioctadecylamino)-1-naphthylazo]-N-methylbenzothiazolium perchlorate (azo VI). The as-deposited films for azo V and azo VI exhibit an H-like aggregate band at 419 and 427 nm, respectively. After HCl vapour treatment, the H-like band for azo V disappears to produce monomeric (M) band at 563 nm. The film, which dissociated under AT treatment, exhibited an H*-like band (at 480 nm) that differed from the original H-like band after HT, and also exhibited a band at ca. 429 nm after BT (exposure to NH₃ vapour). A combination of two different treatments, i.e. AT→HT gives a reversible change between M and H*-like states. Similar optical changes were also obtained for azo VI films. Comparison of infrared (IR) spectroscopic measurements with transmission and reflection–absorption spectra of azo V in the LB films after AT and HT gave characteristic information due to the H- and H*-like aggregations and the monomer state. Apart from evidence for gas–vapour penetration into the LB films under the employed conditions, the IR spectra of the films correlate closely with the corresponding visible absorption spectra.