Colloids and Surfaces A: Physicochemical and Engineering Aspects
Photo-induced fluorescence emission enhancement of azobenzene thin films
Introduction
There have been many studies on the fabrication of optical memory devices that can store highly dense information within a limited area [1]. It is well known that azobenzene is a model compound for photo-switched memory devices by using their characteristic property of reversible trans-to-cis photoisomerization [2], as well as for representative dye molecules in industries [3].
In these areas, the characteristic features of fluorescence emission also play an important role in functional materials like light-emitting semiconductors [4]. With regard to the emission properties of azobenzene derivatives, a large number of studies have been reported [5], suggesting that an azobenzene unit in itself hardly fluoresces. However, it has been reported that J-aggregated azobenzene in the bilayer membrane led to two efficient fluorescence emissions at around 400 nm and 600 nm [6]. Hence, the aggregation of azobenzene unit assumed a key role for the intense fluorescence. Recently, it have been also reported that the self-assembled micelle-like aggregates of cis-azobenzene derivatives by UV illumination in solution showed aggregate size- and structure-dependent fluorescence emissions [7].
Herein we report UV irradiation-induced fluorescence enhancement in azobenzene thin films. Langmuir-Blodgett (LB) monolayers and cast films of azobenzene amphiphiles (AzoN+) were prepared and irradiated by UV light. The UV irradiation-induced fluorescence emission properties of the azobenzene thin films will be discussed.
Section snippets
Surface pressure–area (π–A) isotherm measurement
The preparation of the azobenzene amphiphile, C12AzoC10N+ (AzoN+), employed in this experiment has been reported previously [8]. An adequate amount of a 1 mM chloroform solution of AzoN+ was spread onto water purified to 18.2 MΩ·cm by a Milli-Q SP reagent system (Millipore Co.). The surface pressure–area (π–A) isotherm was measured by using a computer controlled film balance system (FSD-300, USI). The trough size was 190 mm × 90 mm, and the temperature of the subphase was maintained at 20.0 ± 0.1 °C
Results and discussion
The result of π–A isotherm measurement of AzoN+ on pure water is shown in Fig. 1(b). The molecular area extrapolated to zero pressure in the condensed phase (solid phase) was calculated to be about 0.41 nm2/molecule, which was consistent with the value reported in a previous article [9].
In the inset of Fig. 1(b), UV–Vis absorption spectra of AzoN+ monolayers transferred onto the tapered quartz substrates at surface pressures of 5 mN/m (A ≈ 0.6 nm2) and 20 mN/m (A < 0.4 nm2) are shown. Two strong
Conclusion
Monolayers of AzoN+ transferred onto glass substrates by the LB technique at surface pressures of 5 mN/m, 20 mN/m and 25 mN/m, were irradiated by UV light (330–380 nm, >600 mW/cm2), resulting in the enhancement of the fluorescence emissions with a wide range of excitation wavelengths. The UV irradiation-induced fluorescence enhancement was independent of the aggregation structures of the AzoN+ monolayers. Moreover, UV light irradiation to the monolayers covered with a square mesh grid enabled the
Acknowledgement
This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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