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

Area of Science:

• Surface chemistry
• Photochemistry
• Spectroscopy

Background:

• Self-assembled monolayers (SAMs) are crucial for surface functionalization.
• Spiropyran compounds exhibit photochromism, reversibly changing color upon light exposure.
• Understanding photoinduced transformations in SAMs is key for molecular electronics and sensors.

Purpose of the Study:

• To investigate the photoinduced conversion kinetics of spiropyran SAMs at the air-water interface.
• To characterize intermediate states during the spiropyran to merocyanine transformation.
• To elucidate the structural changes associated with the observed kinetic and spectral dynamics.

Main Methods:

• Brewster angle reflectometry for SAM analysis.
• UV-pump, VIS-probe spectroscopy for transient kinetics.
• Optical parametric oscillator for wavelength-tunable probing.
• Nanosecond time-resolved absorption spectroscopy.

Main Results:

• UV irradiation induced a two-stage conversion of spiropyran to merocyanine.
• The first conversion stage occurred within tens of nanoseconds, overlapping with system response.
• The second stage proceeded over hundreds of nanoseconds, accompanied by a ~20 nm red shift in absorption spectra.
• Slower kinetics and spectral shifts were attributed to cis-trans isomerization of the merocyanine intermediate.

Conclusions:

• The study reveals the complex, multi-stage photoisomerization pathway of spiropyran SAMs.
• Nanosecond time-resolved spectroscopy provides insights into intermediate states and isomerization dynamics.
• The findings contribute to the understanding of photoresponsive materials at interfaces.