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Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed...
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Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field

A special issue of Photochem (ISSN 2673-7256).

Deadline for manuscript submissions: closed (15 October 2022) | Viewed by 32568

Special Issue Editors

Prof. Dr. Gulce Ogruc Ildiz

E-Mail Website1 Website2
Guest Editor
1. Department of Chemistry, Coimbra Chemistry Center, University of Coimbra, Coimbra, Portugal
2. Faculty of Sciences & Letters, Department of Physics, Istanbul Kultur University, Istanbul, Turkey
Interests: photochemistry; molecular cryo- and biospectroscopy; quantum chemistry; molecular structure; photophysics; chemometrics
Special Issues, Collections and Topics in MDPI journals
Dr. Licinia L. G. Justino

E-Mail Website1 Website2
Guest Editor
CQC-IMS, Department of Chemistry, University of Coimbra, Rua Larga 3004-535, Coimbra, Portugal
Interests: spectroscopy; metal complexes; computational chemistry; matrix-isolation; photophysics

Special Issue Information

Dear Colleagues,

This Special Issue of Photochem is dedicated to Professor Rui Fausto for his contributions to our understanding of the photochemistry of organic molecules and of matrix-isolated reactive intermediates.

Professor Rui Fausto was born in Coimbra, Portugal, in 1961. He graduated from the University of Coimbra in 1984 and received his PhD in Chemistry (Molecular Spectroscopy) from the same university in 1988. He spent a post-doctoral period in the National Research Council of Canada, Ottawa, after which he returned to Coimbra where he initiated his research activities on molecular spectroscopy and computational chemistry. In 1994 he established the Laboratory for Molecular Cryospectroscopy and Biospectroscopy (LMCB), within the scope of the European Program PRAXIS XXI. This infrastructure has been growing since then and it now constitutes one of the leading research units at world scale in the field of low-temperature spectroscopy and photochemistry, centering its research on the topics of matrix-isolation infrared spectroscopy, the spectroscopy and photochemistry of biologically relevant systems, and the structure and reactivity of chemical intermediates.

Rui Fausto is Full Professor and Head of the Chemistry Research Centre of the Department of Chemistry of the University of Coimbra, Portugal. He is also the Coordinator of the Molecular Spectroscopy and Thermodynamics Research Group, co-Coordinator of the Research Group on Theoretical and Computational Chemistry of the Coimbra Chemistry Centre, and Director of the Laboratory for Molecular Cryospectroscopy and Biospectroscopy. He is a member of the European Academy of Arts, Sciences and Humanities, American Chemical Society, and European Photochemistry Association; President of the permanent Steering Committee of the EUCMOS (European Congress on Molecular Spectroscopy) series of meetings; and Editor-in-Chief of the Journal of Molecular Structure (Elsevier, Amsterdam). He belongs to the editorial boards of ten additional scientific journals. Along his career he has occupied many different positions in administration and scientific management in the University of Coimbra, including the presidency of the Academic Council and the vice-presidency of the Scientific and Directive boards of Faculty for Sciences and Technology as well as the presidency of the Institute for Interdisciplinary Research. He was also the President of the Physical-Chemistry Division of the Portuguese Chemical Society. In 2002 he received the RSC Journals Grant for International Authors Prize and in 2004 and 2005 he was one of the twelve Portuguese scientists awarded the Excellence Prize of the Portuguese Science Foundation. He is an author or editor of about 50 scientific books and author of ca. 500 scientific papers in prestigious peer-reviewed chemistry journals and book chapters. He has been the coordinator of many national and international research projects, which as a whole collected several millions of euros of funding, and has organized more than 50 international scientific congresses.

Rui Fausto has pioneered the use of vibrational excitation to induce chemical reactivity in organic molecules under matrix-isolation conditions (together with colleagues from Helsinki, and lately with the collaboration of other scientists all around the world), which appears as a powerful and elegant strategy for the manipulation of chemical structures. His work on both the infrared and ultraviolet induced photochemistry of reactive intermediates has opened new avenues for the understanding of the chemical reactivity of organic compounds, and his more recent work on quantum mechanical tunneling (including heavy atom tunneling) has been establishing new paradigms in chemical reactivity.

Rui Fausto has also been a notable tutor of many young scientists, which have permanently been receiving his dedication and stimulus. Besides his academic and scientific achievements, he has other interests that range from music to painting and the defense of human rights.

Having being working closely with Professor Rui Fausto for several years, we are very pleased to give all his colleagues and friends the opportunity to celebrate his scientific career by contributing to this Special Issue dedicated to honoring his achievements in these fields of research.

Prof. Dr. Gulce Ogruc Ildiz
Dr. Licinia L. G. Justino
Guest Editors

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Keywords

  • photochemistry
  • IR-induced chemistry
  • organic molecules
  • reactive intermediates
  • conformationally dependent reactions
  • matrix-isolation
  • infrared and Raman spectroscopies
  • quantum mechanical tunneling
  • theoretical and computational organic chemistry
  • applied biospectroscopy

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Published Papers (12 papers)

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12 pages, 3366 KiB  
Article
Conformational-Dependent Photodissociation of Glycolic Acid in an Argon Matrix
by Jussi Ahokas, Timur Nikitin, Justyna Krupa, Iwona Kosendiak, Rui Fausto, Maria Wierzejewska and Jan Lundell
Photochem 2023, 3(2), 197-208; https://doi.org/10.3390/photochem3020013 - 31 Mar 2023
Cited by 1 | Viewed by 1892
Abstract
Ultraviolet-induced photodissociation and photo-isomerization of the three most stable conformers (SSC, GAC, and AAT) of glycolic acid are investigated in a low-temperature solid argon matrix using FTIR spectroscopy and employing laser radiation with wavelengths of 212 nm, 226 nm, and 230 nm. The [...] Read more.
Ultraviolet-induced photodissociation and photo-isomerization of the three most stable conformers (SSC, GAC, and AAT) of glycolic acid are investigated in a low-temperature solid argon matrix using FTIR spectroscopy and employing laser radiation with wavelengths of 212 nm, 226 nm, and 230 nm. The present work broadens the wavelength range of photochemical studies of glycolic acid, thus extending the understanding of the overall photochemistry of the compound. The proposed kinetic model for the photodissociation of glycolic acid proceeds from the lowest energy conformer (SSC). The model suggests that ultraviolet light induces isomerization only between the SSC and GAC conformers and between the SSC and AAT conformers. The relative reaction rate coefficients are reported for all proposed reactions. These results suggest that the direct photodissociation of GAC and AAT conformer does not occur in an argon matrix. The main photodissociation channel via the SSC conformer produces formaldehyde–water complexes. The proposed photodissociation mechanism emphasizes that the conformers’ relative abundancies can significantly affect the photodissociation rate of the molecule. For example, in the case of high relative GAC and AAT concentrations, the ultraviolet photodissociation of glycolic acid requires the proceeding photo-isomerization of GAC and AAT to SSC. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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21 pages, 4818 KiB  
Article
Spectroscopic and DFT Study of Alizarin Red S Complexes of Ga(III) in Semi-Aqueous Solution
by Licínia L. G. Justino, Sofia Braz and M. Luísa Ramos
Photochem 2023, 3(1), 61-81; https://doi.org/10.3390/photochem3010005 - 31 Jan 2023
Cited by 2 | Viewed by 2569
Abstract
A combined spectroscopic and computational approach has been used to study in detail the complexation between Ga(III) and ARS in solution. The NMR results revealed the formation of four Ga(III)/ARS complexes, at pH 4, differing in their metal:ligand stoichiometries or configuration, and point [...] Read more.
A combined spectroscopic and computational approach has been used to study in detail the complexation between Ga(III) and ARS in solution. The NMR results revealed the formation of four Ga(III)/ARS complexes, at pH 4, differing in their metal:ligand stoichiometries or configuration, and point to a coordination mode through the ligand positions C-1 and C-9. For equimolar metal:ligand solutions, a 1:1 [Ga(ARS)(H2O)4]+ complex was formed, while for 1:2 molar ratio solutions, a [Ga(ARS)2(H2O)2]− complex, in which the two ligands are magnetically equivalent, is proposed. Based on DFT calculations, it was determined that this is a centrosymmetric structure with the ligands in an anti configuration. For solutions with a 1:3 molar ratio, two isomeric [Ga(ARS)3]3− complexes were detected by NMR, in which the ligands have a mer and a fac configuration around the metal centre. The DFT calculations provided structural details on the complexes and support the proposal of a 1,9 coordination mode. The infrared spectroscopy results, together with the calculation of the infrared spectra for the theoretically proposed structures, give further support to the conclusions above. Changes in the UV/vis absorption and fluorescence spectra of the ligand upon complexation revealed that ARS is a highly sensitive fluorescent probe for the detection of Ga(III). Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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14 pages, 3364 KiB  
Article
Ar-Matrix Studies of the Photochemical Reaction between CS2 and ClF: Prereactive Complexes and Bond Isomerism of the Photoproducts
by Michelle T. Custodio Castro, Carlos O. Della Védova, Helge Willner and Rosana M. Romano
Photochem 2022, 2(3), 765-778; https://doi.org/10.3390/photochem2030049 - 2 Sep 2022
Cited by 1 | Viewed by 1901
Abstract
In this work, prereactive complexes, reaction products, and conformational preferences derived from the photochemical reaction between CS2 and ClF were analyzed following the codeposition of the reactants trapped in argon matrices at cryogenic temperatures. After codeposition of CS2 and ClF diluted [...] Read more.
In this work, prereactive complexes, reaction products, and conformational preferences derived from the photochemical reaction between CS2 and ClF were analyzed following the codeposition of the reactants trapped in argon matrices at cryogenic temperatures. After codeposition of CS2 and ClF diluted in Ar, the formation of van der Waals complexes is observed. When the mixture is subsequently irradiated by means of broad-band UV-visible light (225 ≤ λ ≤ 800 nm), fluorothiocarbonylsulfenyl chloride (FC(S)SCl) and chlorothiocarbonylsulfenyl fluoride (ClC(S)SF) are produced. These species exist as two stable planar anti- and syn-conformers (anti- and syn- of the C=S double bond with respect to the S–Cl or S–F single bond, respectively). For both novel molecules, anti-FC(S)SCl and anti-ClC(S)SF are the lowest-energy computed rotamers. As expected due to the photochemical activity of these species, additional reaction products due to alternative or subsequent photochannels are formed during this process. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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13 pages, 1564 KiB  
Article
Decoding Breast Cancer Metabolism: Hunting BRCA Mutations by Raman Spectroscopy
by Monika Kopec, Beata Romanowska-Pietrasiak and Halina Abramczyk
Photochem 2022, 2(3), 752-764; https://doi.org/10.3390/photochem2030048 - 25 Aug 2022
Cited by 2 | Viewed by 2290
Abstract
Presented study included human blood from healthy people and patients with BReast CAncer gene (BRCA) mutation. We used Raman spectroscopy for BRCA mutation detection and the bioanalytical characterization of pathologically changed samples. The aim of this study is to evaluate the Raman biomarkers [...] Read more.
Presented study included human blood from healthy people and patients with BReast CAncer gene (BRCA) mutation. We used Raman spectroscopy for BRCA mutation detection and the bioanalytical characterization of pathologically changed samples. The aim of this study is to evaluate the Raman biomarkers to distinguish blood samples from healthy people and patients with BRCA mutation. We demonstrated that Raman spectroscopy is a powerful technique to distinguish between healthy blood and blood with BRCA mutation and to characterize the biochemical composition of samples. We applied partial least squares discriminant analysis (PLS-DA) to discriminate BRCA1/2 mutations and control samples without the mutations based on vibrational features. The sensitivity and specificity for calibration obtained directly from PLS-DA are equal to 94.2% and 97.6% and for cross-validation are equal to 93.3% and 97%. Our combination (Raman spectroscopy and PLS-DA) provides quick methods to reliably visualize the biochemical differences in human blood plasma. We proved that Raman spectroscopy combined with the chemometric method is a promising tool for hunting BRCA mutation in breast cancer. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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16 pages, 7566 KiB  
Article
UPS, XPS, NEXAFS and Computational Investigation of Acrylamide Monomer
by Luca Evangelisti, Sonia Melandri, Fabrizia Negri, Marcello Coreno, Kevin C. Prince and Assimo Maris
Photochem 2022, 2(3), 463-478; https://doi.org/10.3390/photochem2030032 - 22 Jun 2022
Cited by 3 | Viewed by 2333
Abstract
Acrylamide is a small conjugated organic compound widely used in industrial processes and agriculture, generally in the form of a polymer. It can also be formed from food and tobacco as a result of Maillard reaction from reducing sugars and asparagine during heat [...] Read more.
Acrylamide is a small conjugated organic compound widely used in industrial processes and agriculture, generally in the form of a polymer. It can also be formed from food and tobacco as a result of Maillard reaction from reducing sugars and asparagine during heat treatment. Due to its toxicity and possible carcinogenicity, there is a risk in its release into the environment or human intake. In order to provide molecular and energetic information, we use synchrotron radiation to record the UV and X-ray photoelectron and photoabsorption spectra of acrylamide. The data are rationalized with the support of density functional theory and ab initio calculations, providing precise assignment of the observed features. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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18 pages, 2594 KiB  
Article
Conformational Structure, Infrared Spectra and Light-Induced Transformations of Thymol Isolated in Noble Gas Cryomatrices
by Antόnio Jorge Lopes Jesus, Cláudio M. Nunes and Igor Reva
Photochem 2022, 2(2), 405-422; https://doi.org/10.3390/photochem2020028 - 7 Jun 2022
Cited by 3 | Viewed by 2412
Abstract
The conformational space of the natural product thymol (2-isopropyl-5-methylphenol) was investigated using quantum chemical calculations at the B3LYP and MP2 levels, which revealed the existence of four types of conformers differing in the orientation of the isopropyl and hydroxyl groups. Thymol monomers were [...] Read more.
The conformational space of the natural product thymol (2-isopropyl-5-methylphenol) was investigated using quantum chemical calculations at the B3LYP and MP2 levels, which revealed the existence of four types of conformers differing in the orientation of the isopropyl and hydroxyl groups. Thymol monomers were isolated in noble gas (Ar and Xe) matrices (at 15 K) and characterized by IR spectroscopy. With the support of B3LYP harmonic vibrational calculations, the two most stable trans-OH-conformers, differing in the isopropyl orientation, were identified in the cryomatrices. The two less stable cis-OH conformers were not detected as they shall undergo fast tunneling to the most stable ones. Annealing experiments in a Xe matrix up to 75 K did not lead to any conversion between the two isolated conformers, which is in accordance with the significative energy barrier computed for rotamerization of the bulky isopropyl group (~24 kJ mol−1). Vibrational excitation promoted by broadband or by narrowband irradiation, at the 2ν(OH) frequencies of the isolated conformers, did not lead to any conversion either, which was interpreted in terms of a more efficient energy transfer to the hydroxyl rotamerization (associated with a lower energy barrier and a light H-atom) than to the isopropyl rotamerization coordinate. Broadband UV irradiation experiments (λ > 200 nm) led to a prompt transformation of matrix isolated thymol, with spectroscopic evidence suggesting the formation of isomeric alkyl-substituted cyclohexadienones, Dewar isomers and open-chain conjugated ketenes. The photochemical mechanism interpretation concords with that reported for analogous phenol derivatives. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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18 pages, 8157 KiB  
Article
Photooxidation of 2,2′-(Ethyne-1,2-diyl)dianilines: An Enhanced Photocatalytic Properties of New Salophen-Based Zn(II) Complexes
by Mahesh Subburu, Ramesh Gade, Prabhakar Chetti and Someshwar Pola
Photochem 2022, 2(2), 358-375; https://doi.org/10.3390/photochem2020025 - 23 May 2022
Cited by 4 | Viewed by 2061
Abstract
Under solvothermal conditions, the Zn(II) complexes formed from salophen-based ligands with N and O donor atoms are reported. These Zn(II) complexes were initially confirmed through elemental analysis and supported by mass spectral data. The purity of the ligands and Zn(II) complexes was confirmed [...] Read more.
Under solvothermal conditions, the Zn(II) complexes formed from salophen-based ligands with N and O donor atoms are reported. These Zn(II) complexes were initially confirmed through elemental analysis and supported by mass spectral data. The purity of the ligands and Zn(II) complexes was confirmed by using NMR spectral studies. The functional group complexation was established by FT-IR analysis. Additional supportive information about the complexes is also reported through molar conductance and thermal studies. The bandgap energies of the ligands and Zn(II) complexes are estimated with UV-visible DRS studies. The rate of recombination of hole–electron pairs is directly related to photocatalytic activity, which is confirmed by using emission spectral analysis. The surface metaphors for ligands and complexes are obtained from FESEM analysis. These new sequences of Zn(II) complexes were used for the photooxidation of 2,2′-(ethyne-1,2-diyl)dianiline and its derivatives. Mechanistic studies on the fast degradation of dyes were supported in the presence of several scavengers. The rapid photooxidation process in the presence of [Zn(CPAMN)] has been demonstrated, and a highly efficient photocatalyst for the photooxidation of 2,2′-(ethyne-1,2-diyl) dianiline has been proposed. Furthermore, the experimental findings are supported by the DFT studies. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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8 pages, 1123 KiB  
Article
Phosphorescence and Photophysical Parameters of Porphycene in Cryogenic Matrices
by Barbara Golec, Aleksander Gorski and Jacek Waluk
Photochem 2022, 2(1), 217-224; https://doi.org/10.3390/photochem2010016 - 9 Mar 2022
Viewed by 2128
Abstract
Matrix isolation studies were carried out for porphycene, an isomer of porphyrin, embedded in solid nitrogen and xenon. The external heavy atom effect resulted in nearly a 100% population of the triplet state and in the appearance of phosphorescence, with the origin located [...] Read more.
Matrix isolation studies were carried out for porphycene, an isomer of porphyrin, embedded in solid nitrogen and xenon. The external heavy atom effect resulted in nearly a 100% population of the triplet state and in the appearance of phosphorescence, with the origin located at 10163 cm−1. This energy is much lower than that corresponding to the T1 position in porphyrin. This difference could be explained by postulating that the orbital origin corresponds in both isomers to the second excited singlet state, which lies much closer to S1 in porphycene. Most of the vibrational frequencies observed in the phosphorescence spectrum correspond to totally symmetric modes, but several ones were assigned to the out-of-plane Bg vibrations. These bands are not observed in fluorescence, which suggests their possible role in vibronic-spin-orbit coupling. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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15 pages, 4835 KiB  
Article
Theoretical Modeling of Absorption and Fluorescent Characteristics of Cyanine Dyes
by Sonia Ilieva, Meglena Kandinska, Aleksey Vasilev and Diana Cheshmedzhieva
Photochem 2022, 2(1), 202-216; https://doi.org/10.3390/photochem2010015 - 4 Mar 2022
Cited by 5 | Viewed by 2761
Abstract
The rational design of cyanine dyes for the fine-tuning of their photophysical properties undoubtedly requires theoretical considerations for understanding and predicting their absorption and fluorescence characteristics. The present study aims to assess the applicability and accuracy of several DFT functionals for calculating the [...] Read more.
The rational design of cyanine dyes for the fine-tuning of their photophysical properties undoubtedly requires theoretical considerations for understanding and predicting their absorption and fluorescence characteristics. The present study aims to assess the applicability and accuracy of several DFT functionals for calculating the absorption and fluorescence maxima of monomethine cyanine dyes. Ten DFT functionals and different basis sets were examined to select the proper theoretical model for calculating the electronic transitions of eight representative molecules from this class of compounds. The self-aggregation of the dyes was also considered. The pure exchange functionals (M06L, HFS, HFB, B97D) combined with the triple-zeta basis set 6-311+G(2d,p) showed the best performance during the theoretical estimation of the absorption and fluorescent characteristics of cyanine dyes. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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20 pages, 2553 KiB  
Article
Local and Remote Conformational Switching in 2-Fluoro-4-Hydroxy Benzoic Acid
by Sándor Góbi, Mirjam Balbisi and György Tarczay
Photochem 2022, 2(1), 102-121; https://doi.org/10.3390/photochem2010009 - 28 Jan 2022
Cited by 2 | Viewed by 2521
Abstract
In this work, 2-F-4-OH benzoic acid was isolated in Ar matrices and conformational changes were induced by near-IR irradiating the sample. Upon deposition, three conformers could be observed in the matrix, denoted as A1, A2, and D1, respectively. A1 and [...] Read more.
In this work, 2-F-4-OH benzoic acid was isolated in Ar matrices and conformational changes were induced by near-IR irradiating the sample. Upon deposition, three conformers could be observed in the matrix, denoted as A1, A2, and D1, respectively. A1 and A2 are trans carboxylic acids, i.e., there is an intramolecular H bond between the H and the carbonyl O atoms in the COOH group, whereas D1 is a cis carboxylic acid with an intramolecular H bond between the F atom and the H atom in the COOH group, which otherwise has the same structure as A1. The difference between A1 and A2 is in the orientation of the carbonyl O atom with regard to the F atom, i.e., whether they are on the opposite or on the same side of the molecule, respectively. All three conformers have their H atom in their 4-OH group, facing the opposite direction with regard to the F atom. The stretching overtones of the 4-OH and the carboxylic OH groups were selectively excited in the case of each conformer. Unlike A2, which did not show any response to irradiation, A1 could be converted to the higher energy form D1. The D1 conformer spontaneously converts back to A1 via tunneling; however, the conversion rate could be significantly increased by selectively exciting the OH vibrational overtones of D1. Quantum efficiencies have been determined for the ‘local’ or ‘remote’ excitations, i.e., when the carboxylic OH or the 4-OH group is excited in order to induce the rotamerization of the carboxylic OH group. Both ‘local’ and ‘remote’ conformational switching are induced by the same type of vibration, which allows for a direct comparison of how much energy is lost by energy dissipation during the two processes. The experimental findings indicate that the ‘local’ excitation is only marginally more efficient than the ‘remote’ one. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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14 pages, 1203 KiB  
Article
UV Photolysis Study of Para-Aminobenzoic Acid Using Parahydrogen Matrix Isolated Spectroscopy
by Alexandra McKinnon, Brendan Moore, Pavle Djuricanin and Takamasa Momose
Photochem 2022, 2(1), 88-101; https://doi.org/10.3390/photochem2010008 - 26 Jan 2022
Viewed by 3821
Abstract
Many sunscreen chemical agents are designed to absorb UVB radiation (and in some cases UVA) to protect the skin from sunlight, but UV absorption is often accompanied by photodissociation of the chemical agent, which may reduce its UV absorption capacity. Therefore, it is [...] Read more.
Many sunscreen chemical agents are designed to absorb UVB radiation (and in some cases UVA) to protect the skin from sunlight, but UV absorption is often accompanied by photodissociation of the chemical agent, which may reduce its UV absorption capacity. Therefore, it is important to understand the photochemical processes of sunscreen agents. In this study, the photolysis of para-aminobenzoic acid (PABA), one of the original sunscreen chemical agents, at three different UV ranges (UVA: 355 nm, UVB: >280 nm, and UVC: 266 nm and 213 nm) was investigated using parahydrogen (pH2) matrix isolation Fourier-Transform Infrared (FTIR) Spectroscopy. PABA was found to be stable under UVA (355 nm) irradiation, while it dissociated into 4-aminylbenzoic acid (the PABA radical) through the loss of an amino hydrogen atom under UVB (>280 nm) and UVC (266 nm and 213 nm) irradiation. The radical production supports a proposed mechanism of carcinogenic PABA-thymine adduct formation. The infrared spectrum of the PABA radical was analyzed by referring to quantum chemical calculations, and two conformers were found in solid pH2. The PABA radicals were stable in solid pH2 for hours after irradiation. The trans-hydrocarboxyl (HOCO) radical was also observed as a minor secondary photoproduct of PABA following 213 nm irradiation. This work shows that pH2 matrix isolation spectroscopy is effective for photochemical studies of sunscreen agents. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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8 pages, 1427 KiB  
Communication
Demonstration of a Stereospecific Photochemical Meta Effect
by Hoai Pham, Madelyn Hunsley, Chou-Hsun Yang, Haobin Wang and Scott M. Reed
Photochem 2022, 2(1), 69-76; https://doi.org/10.3390/photochem2010006 - 12 Jan 2022
Viewed by 3765
Abstract
A fundamental goal of photochemistry is to understand how structural features of a chromophore can make specific bonds within a molecule prone to cleavage by light, or photolabile. The meta effect is an example of a regiochemical explanation for photolability, in which electron [...] Read more.
A fundamental goal of photochemistry is to understand how structural features of a chromophore can make specific bonds within a molecule prone to cleavage by light, or photolabile. The meta effect is an example of a regiochemical explanation for photolability, in which electron donating groups on an aromatic ring cause photolability selectively at the meta position. Here, we show, using a chromophore containing one ring with a meta-methoxy group and one ring with a para-methoxy group, that two stereoisomers of the same compounds can react with light differently, based simply on the three-dimensional positioning of a meta anisyl ring. The result is that the stereoisomers of the compound with the same configuration at both stereogenic centers are photolabile while the stereoisomers with opposite configuration do not react with light. Furthermore, time-dependent density functional theory (TD-DFT) calculations show distinct excitation pathways for each stereoisomer. Full article
(This article belongs to the Special Issue Photochemistry of Organic Molecules and of Matrix-Isolated Reactive Intermediates: Themed Issue Honoring Professor Rui Fausto for His Contributions to the Field)
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