Transient Brewster angle reflectometry of spiropyran monolayers

Abstract

Brewster angle reflectometry has been developed as a tool for determining the absorbance and refractive index changes in molecular monolayers containing spiropyran. The method is sensitive to changes in both the real and imaginary parts of the refractive index in the monolayers. It was used to monitor the conversion of spiropyran to merocyanine and the reversal of this reaction when the molecules were immobilised on quartz using silane coupling. An analytical solution of Fresnel formula allowed the transient reflectometry data to be converted into transient absorption information. Absorbances of transients as low as~10 -6 were possible using the current apparatus with a single laser pulse transient measurement. It was found that spiropyran photoconverted to merocyanine with an efficiency of~0.1. The photochemical reversion of converted merocyanine to spiropyran occurred with efficiencies of 0.03-0.2 and this was probably site dependent. It was found that the thermal conversion from merocyanine to spiropyran was slow and even after 10 min there was no significant thermal reversion. This measurement was possible because the real part of the refractive index of the monolayer could be monitored with time using an off-resonance probe at a wavelength where the merocyanine did not absorb light meaning that the probe did not photobleach the sample. Thus our method also provides a non-intrusive method for probing changes in molecules in thin films. A. IntroductionSpiropyran (SP) conversion to merocyanine (MC) (Scheme 1) in monolayers on solid supports and in Langmuir and Langmuir Blodgett films has been extensively studied because of its potential for use in photoswitching applications. 1-10 For example the switchability has been harnessed to make the wetability of surfaces change, in order to alter capillary action, to lift heads of liquid, and to make water droplets move as UV and visible light excitation are alternated. 1-3 Light-induced phase transitions and pressure jumps have also been observed on timescales from minutes to hours, 4-6 as well as more recently by the current authors using nanosecond time-resolved Brewster angle microscopy. 7-9 Furthermore SP's ability to switch surface properties has been utilized in electrodes on gold for binding proteins, 10 in biopolymer films 11 and in hybrid surface architectures. 12 Scheme 1 Schematic representation of SP to MC and reverse photoconversions. Despite the plethora of works harnessing or studying the photo-switching of properties by SP, as well as other molecules, 13 on surfaces, there is little fundamental knowledge about the photochemical reactions themselves. This is primarily because making optical measurements in thin films and monolayers is difficult due to the short optical pathlength available. Ellipsometry has been reported on SP thin-films. 14,15 Reflectometry based on surface plasmon resonance (SPR) has been used to monitor reactions in self-assembled monolayers 16 and in polymer films 17 on metallic gold and silver surfaces, however the vast majority o...

Transient absorption spectroscopy on spiropyran monolayers using nanosecond pump—probe Brewster angle reflectometry

“…27 The second BAR is a modified NanoFilm EP 3 BAM that was used to verify monolayer conditions and measure Brewster's angle as a starting point for simulations. 26,28 Both systems and recent modifications are described below.…”

“…According to Fresnel's equations for p-polarized light, this results in a parabolic reflectivity increase around Brewster's angle. 26 The reflected light is collected with a CCD camera (DU420-BV, Andor Technology, cooled to −85°C for dark current suppression). The imaging area is a two dimensional array of pixels.…”

“…The precision of this value is important to get better estimations of monolayer thickness and initial refractive index from simulations. 26,27 Sample preparation A linear, Teflon Langmuir trough and a Hamilton microsyringe are cleaned using chloroform, ethanol, methanol, and deionised water. The spreading solution is prepared using ACS Spectro-photometric grade chloroform (stabilized with 0.5-1.0% ethanol) supplied by Sigma Aldrich without further purification.…”

“…Amphiphilic spiropyran, 11-(3′,3′-dimethyl-6,8-dinitrospiro [chromene-2,2′-indoline]-1′-yl) undecanoic acid, was synthesized from 2,3,4-trimethylindolenium 11-bromoundecanoic acid and 3,5-dinitrosalicylaldehyde via a two-step substitution and a condensation reaction as previously described. 26,28 Results and discussion…”

“…Previously we have shown that BAM is a useful tool for studying photodynamics in SP thin films and monolayers. 24,25 Brewster angle reflectometry (BAR), that is reflectometry in the vicinity of Brewster's angle, was used to study transient kinetics of SP monolayers on functionalised fused silica 26 and self-assembled monolayers at the air-water interface. 27 One drawback of the methods applied was the lack of spectral resolution in the kinetic studies due to the limitation of available probe wavelengths from the probe lasers used.…”