Kronos is a portable kinetic spectrometer designed for measurements of light-induced absorption and emission changes that occur on the microsecond and longer time scales. It is targeted mainly as a chemical kinetics experiment in university or high school teaching laboratories. Kronos can measure solid and liquid samples in both transmission and emission modes. The Kronos (patent-pending) design utilizes a Xenon arc flash lamp as an excitation source, which allows for wavelength tunability. The photo-induced 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 is passed to the detector photodiode via an interference filter. In this way it is ensured that the detector sees only a 10 nm wide slice of the white light. The detector voltage output is digitized and transferred to a PC for generation of a voltage-time profile. The student can access the data array and input it into third-party software for manipulations by the student to establish the rate laws.
Features
- Time resolution: 15 us
- Spectral range: 425-700 nm (Transient Absorption), 200-1100 nm (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 is primarily targeted to chemical kinetics education, but it has potential uses in several research fields, as outlined below.
Type |
Field |
Kinetics of isomerization |
Development of light sensitive materials, glasses, optical limiters |
Kinetic studies of liquid crystals switching |
Development of displays, electrooptical window shutters, etc |
Excited state relaxation in liquids and solids |
Various basic and applied research |
Low temperature and solid state kinetic studies of excited states deactivation |
Various basic and applied research |
Kinetics studies of photodarkening |
Photonics and optical fiber telecommunication |
Charge separation and recombination in thin
films and solids |
Photovoltaics |
Photoinduced interlayer charge separation in molecular films |
Solar energy conversion |
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 hands-on experience that the students obtain with Kronos helps them understand how light is managed in the experiment and see how various transient species can have unique spectral and kinetics signatures. The experiments encourage students to examine the data with the different kinetic rate laws to discover the one that is appropriate to the experiment. The "black box" approach is discouraged.
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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