Kronos is a portable flash photolysis spectrometer designed for transient absorption and emission measurements on the microsecond and longer time scale. Kronos can measure solid and liquid samples in the transmission as well as the reflection mode. 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 the output of a white light emitting diode (LED) through the 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 the sample, the probe light passes through an interference filter. After that 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 for further manipulations
Features
- Time resolution: 30 us
- Spectral range: 425-700 ns (Transient Absorption), 200-1100 (Emission)
- Spectral resolution: 10 nm
- Digitizer: 16 bit, 250 kHz
- Comes with an optional PC, comprehensive software, laboratory manuals and sets of chemicals
- Sensitive in the visible spectral region
- Dimensions: 12”x17”
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Kronos can also serve 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 experiments are:
- To introduce students to the methods of a kinetic investigation
- To provide data acquisition and computational opportunities
- To introduce the principles of first and second order reactions
- To demonstrate how bimolecular processes can show first order properties
- To teach competition kinetic analysis
- To generate a familiarity with absorption of light, the Jablonski diagram and the dynamics of electronically excited species
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.
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.
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