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AbstractThis Glossary contains definitions, explanatory notes, and sources for terms used in physical organic chemistry. Its aim is to provide guidance on the terminology of physical organic chemistry, with a view to achieving a consensus on the meaning and applicability of useful terms and the abandonment of unsatisfactory ones. Owing to the substantial progress in the field, this 2021 revision of the Glossary is much expanded relative to the previous edition, and it includes terms from cognate fields.

The history of the IUPAC Photochemistry Committee since its creation in 1976 and its transition in 2001 to the IUPAC Photochemistry Sub-Committee are reviewed as well as the connections of Committee and Sub-Committee to the various photochemical associations (European Photochemical Association, EPA, Inter-American Photochemical Society, I-APS and Asian and Oceanian Photochemistry Association, APA). The participants in both the Commission and the Sub-Committee over the years are listed as well as the Recommendations and Technical Reports produced since the creation of the Committee until the present days. [ABSTRACT FROM AUTHOR]

Optical and optoacoustic characterization of absolute two-photon absorption cross-section for use as standard two-photon sensitizers in different solvents is presented. Some key aspects of the techniques used to quantify values of [delta] are discussed.

A nanosecond laser-induced optoacoustic spectroscopy was applied to several electron-donor-acceptor pairs in aqueous solution and the data obtained were analyzed with the respective Marcus reorganization energy determined experimentally. Both [gamma] and [DELTA.sub.FR]V react to the same phenomena which help in the understanding of the origin of gamma values and their evaluation at a molecular level.

The structural volume changes derived from laser-induced optoacoustic spectroscopy (LIOAS) are determined for trans to cis and cis to trans photoisomerization of 4.4'-dinitro-2,2'-disulfonylstilbene (DS) in aereated neat water and in aereated aqueous solutions of various monovalent cations.

Researchers trapped the intermediate states in the photoconversion of phytochrome and show that the chromophore of the intermediate is similar to the final active form. This indicates that the ultimate structural change occurs during this step.

Research was conducted to examine the photoisomerization of water-soluble carboxy-azobenzenes that could be dissolved as acids in the series of cycloalkanes as well as carboxylates in potassium phosphate buffer at pH8 using laser-induced optoacoustic spectroscopy. A comparison of the different factors determining the structural volume change in water and in nonpolar media was performed. Results suggest that the E -> Z photoisomerization of the azobenzenes in organic solvents after excitation in the n-pi* bands takes place through the singlet state and that the fluorescence and the triplet yields are very low.

The electron transfer-reaction between the metal-to-ligand charge-transfer triplet (3MCLT) state of Ru(bpy)3(super 2+) and the methyl viologen cation MV2+ has been investigated. The reactions were studied by laser-induced optoacoustic spectroscopy in the 8-35 degree C temperature range in aqueous solutions in the absence and in the presence of 0.1 M sodium salts. The results showed that the enthalpy and structural changes for the formation of the 3MCLT state were independent of the presence of quencher or salt.

The charge separation process occurring in the excited state of a donor-bridge-acceptor (D-br-A) compound in alkane solvents was investigated by time-resolved laser-induced optoacoustic spectroscopy. Findings showed that the intramolecular charge separation process is accompanied by a very large contraction. Since the molecule itself can be regarded to be rigid, the contraction can only be attributed to electrostriction of the solvent around the highly dipolar D+-br-A- species.

A study was conducted to characterize the total structure volume and enthalpy changes related to the formation of the triplet metal-to-ligand charge transfer state in ruthenium(II) bipyridine cyano complexes. Laser-induced optoacoustic spectroscopy was carried out to analyze the complexes. Experimental results indicated that photoinduced structural volume changes correlate with changes in the structural entropy influenced by the addition of salt.

Experiments were performed to study the expression of N-terminal apolipoprotein fragments of oat phytochrome A (phyA) of 65kDa and potato phyb of 66 kDa in Escherichia coli and in the yeasts Saccharomyces cerevisiae and Pichia pastoris. Both oat phyA65 and potato phyB66 expressed larger amounts of N-terminal halves than full-length lipoproteins. Moreover, it was also observed that the difference absorption maxima and the kinetics of chromoproteins derived from E. coli and the two yeasts were similar.

Raman spectroscopy and nanosecond flash photolysis were used to study a recombinant phytochrome of the cyanobacterium Synechocystis. The findings indicate a resemblance between recombinant Synechocystis phytochrome and plant phytochrome with regard to Raman and visible spectral properties of the Pr and Pfr states. However, although the Synechocystis phytochrome undergoes a photoreversible conversion, red-shifted intermediates with kinetics different from those determined for the phytochrome intermediates were observed.

The cDNAs encoding full-length type A and B phytochromes (phyA and phyB, respectively) from potato were expressed in inducible yeast systems (Saccharomyces cerevisiae and Pichia pastoris). Also, a deletion mutant of phyB was expressed. The apoproteins were reconstituted into chromoproteins by incorporation of the native chromophore, phytochromobilin and of phycocyanobilin. There were significant differences between the full-length phyA and phyB regarding the decay kinetics of the I700 intermediates, irrespective of which chromophore the two recombinant chromoproteins had been reconstituted with.

The low-temperature Fourier-transform resonance Raman spectrum of the intermediary forms Pr and Pfr of the phytochrome is given and a mechanism is proposed for the changes in the phytochrome during the photocycle. The signal-to-noise ratio and the resolution of the spectrum obtained by this method is better than that obtained by other methods. The interconversion of these forms is by the absorption of light and involves the a Z to E isomerization in the methine C-D bridge and torsion around two single bonds. In Pr the configuration of the exocyclic bonds is ZZZ and in Pfr it is ZZE.

A study is made on the photophysical and photochemical properties of new tetrapyrrolic macrocycle 8 and its lower 22pi homologs 6 and 7. These properties are compared to those of 18pi porphycene 5. The results indicate the absence of phosphorescence in any of compounds 6-8, thereby, necessitating other means of triplet energy determination. The results also exhibit favorable comparison with the corresponding performance of the parent porphycene 5.

Fourier transform resonance Raman spectroscopy with near infrared excitation was used to study the PR and PFR forms of phytochrome in H2O and D2O. This technique is used to analyze the chromophore structures of photosensitive pigments. Vibrational assignments based on an empirical approach using previously published data based on model compounds, can be discussed.

The laser-induced optoacoustic spectroscopy technique is employed to study the impact of the entropic changes on the charge separation process in triads, which is shown to mimic the photosynthetic charge separation. The results show that the entropy changes completely control the discussed charge separation.

Time-resolved laser-induced optoacoustic spectroscopy was used for the determination of the enthalpy, [DELTA.sub.T]H, and structural volume changes [DELTA.sub.T]V concomitant with triplet state formation upon excitation of tetrasulfonated porphyrin [3.sup.T]SP[P.sup.4-] as well as with benzoquinone. The values of [DELTA.sub.T]H and [DELTA.sub.T]V for [3.sup.T]SP[P.sup.4-] formation in the presence of different cations correlated with each other and afforded a value of [DELTA.sub.T]G.