Kronos is an inexpensive
miniature flash photolysis spectrometer designed
to introduce chemistry students to the principles
of chemical kinetics using time-resolved spectrometry,
which is an analytical tool widely used in chemistry,
biology, materials science and nanoscience.
With Kronos, students can quickly and easily
generate reaction time profiles and thence perform
kinetic analysis of a variety of transient species
such as free radicals, excited states, metal-coordination
complexes in unusual valence states, and the
like. Additionally Kronos serves to introduce
the students to the principles of photochemistry.
Kronos comes with a comprehensive manual complete
with a suggested set of well-tried experiments.
The common goals of all the experiments are
to:
- Introduce students to the methods of a kinetic
investigation
- Provide data acquisition and computational
opportunities
- Introduce the principles of first and second
order reactions
- Demonstrate how bimolecular processes can
show first order properties
- Teach competition kinetic analysis
- Generate a familiarity with absorption
of light, the Jablonski diagram and the dynamics
of electronically excited species
For more details and pricing information
please contact
us.
Features
- Compact footprint: 12”x17”
- Sensitive in the visible spectral region
- No laser required
- Extremely user friendly
- Safe to use without laser goggles
- Can be easily relocated without need for
realignment
- Comes with an optionalPC, comprehensive
software, laboratory manuals and sets of chemicals
The spectrometer’s optical bench can be disassembled
and put back together in just a couple of minutes.
Additionally students themselves need to change
optical filters in order to tune the monitored
wavelength for different experiments. This hands-on
experience helps the students to understand
how light is managed in the experiment and see
how various transient species can have unique
spectral and kinetics signatures.
Technical information
The Kronos patent pending design utilizes a
Xe arc flash lamp as an excitation source, which
allows for wavelength tunability. The photoinduced
transient species are investigated by passing
light generated by a white light emitting diode
(LED) through a sample. Using an LED as a probe
light source results in superior stability and
low noise. The probe wavelength range of Kronos
is 425-700 nm. After traveling through a sample,
the probe light passes through an interference
filter. After which a ~10 nm wide section of
the spectrum is delivered to the detector photodiode.
The detector voltage output is digitized and
transferred to a PC for generation of a kinetic
trace and further manipulations.
Light induced isomerisation
In this experiment photoexcitation of the solution
of the chromophore induces cis-to-trans isomerisation
and the compound changes color from orange to
blue (Lambda max = 605 nm). The color reverts
thermally but the rate is catalyzed by acid.
The decay of the transient absorption at 605
nm is monitored as a function of time after
the excitation flash. The decay is exponential
in time with a rate that is first order in the
concentration of an acid. A plot of the observed
rate constant as a function of acid concentration
is linear with a slope that provides the bimolecular
rate constant for the catalysis process.
Energy transfer
In this experiment the chromophore aqueous solution
is luminescent when excited by visible light
and emits green light as the excited state decays.
The decay is exponential with a lifetime of
1.6 ms. In the presence of energy acceptor,
the rate of luminescence decay increases as
energy is transferred during collisions between
the photoexcited donor and the energy acceptor.
The decay rates as a function of the acceptor
concentration are linear and a plot of these
variables leads to the bimolecular rate constant
for the energy transfer process.
Kinetics of the triplet
state
The chromophore in this experiment is promoted
to its excited triplet state by visible radiation.
In an aqueous solution this state shows an exponential
decay with a lifetime of ca 260 microseconds
and it can be detected by its optical absorption
at 450 nm or its phosphorescence emission at
700 nm. Carrying out the experiment under both
absorption and luminescence conditions shows
that the phosphorescence signal and the absorption
signal decay with the same lifetime, allowing
the 450 nm signal to be identified as belonging
to the T1-Tn absorption transition. Moreover,
by measuring the initial absorption at 450 nm
post excitation flash at different flash intensities
shows that the triplet absorption can easily
be saturated. This occurs when all ground state
molecules are excited to the triplet state.
Under these conditions it is possible to compute
the molar extinction coefficient of the chromophore
triplet absorption at 450 nm.
Kinetics of the radical
anion
The excited triplet state of the chromophore
in this experiment is generated by the photoflash.
In alcohol solution the electronically excited
molecule readily abstracts an H atom from the
solvent generating the corresponding free radicals.
In the presence of NaOH (alkaline solution)
these radicals deprotonate to the conjugate
bases and undergo dimerization/dismutation processes
that are kinetically second order. It can be
shown that the first half life for the second
order reaction is dependent on the flash intensity,
but the extracted rate parameter is not. Furthermore,
when observations are made at different wavelengths
inside the radical absorption band (500-650
nm) the rate parameter depends on the wavelength
since it is a composite of the bimolecular rate
constant and the extinction coefficient of the
absorbing species.
The experiments for Kronos have been
developed in collaboration with the research group of
Michael A.J. Rodgers, Ohio Eminent Scholar and Professor
at the Center for Photochemical Sciences,
Bowling Green State University, USA