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01:26Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
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01:26Thermal and Photochemical Electrocyclic Reactions: Overview
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
01:29The Photochemical Reaction Center
Reaction centers are pigment-protein complexes that initiate energy conversion from photons to chemical entities. Therefore, photochemical reaction center is a more appropriate term that describes these complexes. The Nobel laureates Robert Emerson and William Arnold provided the first experimental evidence of photochemical reaction centers by demonstrating the participation of nearly 2,500 chlorophyll molecules for the release of just one molecule of oxygen. Despite thousands of photosynthetic...
01:22Photoreceptors and Visual Pathways
At the molecular level, visual signals trigger transformations in photopigment molecules, resulting in changes in the photoreceptor cell's membrane potential. The photon's energy level is denoted by its wavelength, with each specific wavelength of visible light associated with a distinct color. The spectral range of visible light, classified as electromagnetic radiation, spans from 380 to 720 nm. Electromagnetic radiation wavelengths exceeding 720 nm fall under the infrared category, whereas...
01:12Cycloaddition Reactions: MO Requirements for Photochemical Activation
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
01:22Photosystem II
The multi-protein complex photosystem II (PS II) harvests photons and transfers their energy through its bound pigments to its reaction center, and ultimately to photosystem I (PSI) through the electron transport chain. The pigments responsible for caputirng the light energy in photosystems include chlorophyll a, chlorophyll b, and carotenoids.
The pigment molecules are arranged across two photosystem domains — the antenna complex and the reaction center. The main aim of the pigment molecules...
The pigment molecules are arranged across two photosystem domains — the antenna complex and the reaction center. The main aim of the pigment molecules...
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11:26
Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light
Published on: September 12, 2014
Entropically driven photochemical upconversion.
Yuen Yap Cheng1, Burkhard Fückel, Tony Khoury
1School of Chemistry, The University of Sydney, Sydney, NSW, Australia.
The Journal of Physical Chemistry. A
|January 27, 2011
Summary
Researchers achieved a record energy margin in photochemical upconversion (UC) by reversing the typical energy transfer direction. This novel approach utilizes entropy to drive triplet excitation, enhancing the upconversion process.
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Area of Science:
- Photochemistry
- Materials Science
- Physical Chemistry
Background:
- Conventional photochemical upconversion (UC) via triplet-triplet annihilation (TTA) suffers from enthalpic losses.
- Triplet energy transfer (TET) from sensitizer to emitter is a key loss mechanism, typically requiring the emitter's triplet energy level to be below the sensitizer's.
Purpose of the Study:
- To overcome enthalpic losses in TTA-UC by inverting the conventional energy transfer pathway.
- To investigate the use of entropy as a driving force for triplet excitation migration.
Main Methods:
- Designed a system where the emitter's triplet energy level exceeds the sensitizer's by approximately 600 cm(-1).
- Optimized sensitizer and emitter concentrations to leverage entropy for triplet migration.
- Employed a modified Stern-Volmer analysis to determine the TET rate constant.
Main Results:
- Achieved a new record peak-to-peak TTA-UC energy margin of 0.94 eV.
- Determined a triplet energy transfer (TET) rate constant of 2.0 × 10(7) M(-1) s(-1).
- Observed visible upconverted fluorescence at low irradiation intensities (2 W cm(-2)), despite relative inefficiency.
Conclusions:
- Exploiting entropy can drive triplet excitation migration against conventional energy gradients, improving TTA-UC energy margins.
- This inverted energy transfer strategy offers a novel pathway for efficient photochemical upconversion.
- The system demonstrates practical potential with visible fluorescence under low irradiation conditions.