A study of the wavelength dependence of excited state lifetimes in a diarylethene
Introduction & Motivation
Jamie Somers1, Daniel R. Johnson1, and Christopher G. Elles1
1Department of Chemistry, University of Kansas, Lawrence, KS 66045
This work is proudly supported by the National Centre for Sensor Research (NCSR) and the NSF
1. Ward, C. L., & Elles, C. G. J. Phys. Chem. A, 2014 118 (43), 10011–10019
Photochromic switches are molecules which photoisomierize between two states with
distinct photophysical properties.Photoswitches have uses in optical data storage as
well as possible uses in molecular electronics.One such compound is diarylethene
(DAE), which can be reversibly switched between aclosed-ring form that has high
electrical conduction and absorbs UV light, and an open-ring form which has alow
electrical conduction and absorbs visible light.Although this compound is well studied
for its photoswitching capabilities, the underlying dynamics of the ring opening and
closing reactions are not fully understood.We studied the ring-opening reaction using
femtosecond transient absorption spectroscopy.Transient absorption (TA)
spectroscopy involves the excitation of aground state molecule using an ultrafast
pump laser pulse (~100 fs) to generate reactive excited states of the molecule.
Subsequently, awhite-light probe pulse measures the excited-state electronic
absorption spectrum as afunction of the time between pulses over the course of the
reaction.Previous ultrafast spectroscopy work has revealed two lifetimes on the
picosecond scale attributed to the ring-opening reaction;however, there is a
wavelength dependence of excited-state lifetimes, which may indicate that multiple
electronic states are accessed by the white-light probe.We explored this wavelength
dependence of the closed-ring excited state to better understand the dynamics of the
DAE ring-opening reaction.
Conclusions and Future Work
Transient Absorption Spectroscopy
•Use a femto-second laser at 800nm is split up into pump pulse and white light probe pulse.
•Pump laser pulse (550 nm) generated using an OPA with 100 femtosecond time resolution
excites molecule up into excited state.
•The sample is flowed through a 500-micron flow cell to avoid photoproduct.
•Solution is in a photo stationary state, the pump pulse excites the molecules from being in
ground state of the closed ring form into being in the excited state of the closed ring form,
some of these excited closed ring form molecules will react.
•Probe pulse scans a wavelength range of 345nm to 745 nm meanwhile automated stages
scan delays from -10 to 240 picoseconds.
•The pump pulse was rotated 54.7°(magic angle) from probe pulse using a λ/2 waveplate.
UV-Vis of DAE-Phenylthienyl1
Photoswitches are molecules which undergo structural
changes when irradiated with light.The photoswitches
investigated in this work are photochromic, so
illuminating with UV or visible light switches between a
colored or colorless solution respectively.This change in
color is due to the change in conjugation between the
open and closed forms resulting in anew absorption
band.Transient Absorption (TA) Spectroscopy provides
information about the electronic states of amolecule,
which in turn can be interpreted to determine which
initial conditions produce the best quantum yield of
open vs closed reactions.The goal of this project is to
analyze the TA spectra of the photoswitch in the closed
form undergoing the ring opening reaction.This will give
insight into the excited state electronic energy surfaces
that are present during the closed to open reaction.
•The Lifetimes associated with the electronic energy surfaces show a wavelength
dependence; 0.6 and 4 ps in the 345.3 –495 nm range, 20 ps in the 495 –595 nm
range and 7 ps in the 595 –745 nm range.
•By doing fit optimizations in small wavelength ranges before chi squared optimization
across the whole spectrum, we can get more specificity of the lifetimes.
•We can use this information to inform future three beam experiments (PReP)
Global analysis is a technique used to visualize data whereby the data is
broken up into the sum of exponential components based on lifetimes, each
specific lifetime has a variable amplitude across the spectrum, the amplitude
of the decay gives the Decay Associated Spectra (DAS).
Decay Associated Spectra (DAS) was used to determine the lifetimes present
at the excited state electronic energy surfaces for each wavelength region to
show a wavelength dependence.
Differences across the spectrum could indicate the white light accessing
multiple upper electronic states during the duration of the measurement
Range (nm) 345.3 -
(ps) 5.0 6.5 4.5 30 30 6.0 7.5 6.8
-9 ps 12 ps
1 ps 20 ps
2 ps 35 ps
3 ps 55 ps
4 ps 90 ps
7 ps 150 ps
Absorption (DAE) Closed
Figure 1: 2D Plot of Diarylethene (DAE)
Figure 2: Log Cut of Diarylethene (DAE)
Figure 1 depicts the full signal collected during the Transient Absorption Spectroscopy
measurement. The graph consists of two areas of high signal represented in dark blue. This
high signal present at 400 and 700nm is the result of new absorption after the pump generates
excited state closed ring molecules meanwhile, the low signal present at 550nm is the result of
ground state bleach. Ground state bleach is the depletion of ground state population to an
Figure 2 is a representation of the signal broken up by different delays and displayed as a function
of signal as wavelength is varied. In this representation we can once again see the peak of the
signal is present at around 400nm and 700nm meanwhile the negative signal caused by ground
state bleach is visible at 550nm.
The UV/VIS is overlayed on top to show that the extra population from the ground state has now
become excited and the signal has decayed where it previously absorbed the strongest (550nm)
and has grown in places where previously there was no absorption (400 and 700nm).
Time Constant (ps)
t1 = 0.6
Amplitude of t1
t2 = 4
Amplitude of t2
t3 = 7
Amplitude of t3
t4 = 20
Amplitude of t4