View source for Physics:Photosensitizer

{{Short description|Type of molecule reacting to light}}
[[File:Photodynamic therapy (red).jpg|thumb|right|300 px|A photosensitizer being used in photodynamic therapy]]
'''Photosensitizers''' are light absorbers that alter the course of a photochemical reaction.  They usually are [[Chemistry:Catalyst|catalyst]]s.<ref>{{cite web |title=Photosensitization |url=https://goldbook.iupac.org/terms/view/P04652 |website=IUPAC Gold Book |publisher=International Union of Pure and Applied Chemistry}}</ref>  They can function by many mechanisms, sometimes they donate an electron to the substrate, sometimes they abstract a hydrogen atom from the substrate. At the end of this process, the photosensitizer returns to its [[Physics:Ground state|ground state]], where it remains chemically intact, poised to absorb more light.<ref name=":02" /><ref name="Zhang 5934–5947">{{cite journal|vauthors=Zhang Y, Lee TS, Petersen JL, Milsmann C|date=May 2018|title=A Zirconium Photosensitizer with a Long-Lived Excited State: Mechanistic Insight into Photoinduced Single-Electron Transfer|journal=Journal of the American Chemical Society|volume=140|issue=18|pages=5934–5947|doi=10.1021/jacs.8b00742|pmid=29671586}}</ref><ref>{{cite book|year=2009|doi=10.1351/goldbook.P04652|title = IUPAC Compendium of Chemical Terminology|isbn = 978-0-9678550-9-7|chapter = Photosensitization}}</ref> One branch of chemistry which frequently utilizes photosensitizers is [[Chemistry:Polymer chemistry|polymer chemistry]], using photosensitizers in reactions such as photopolymerization, photocrosslinking, and [[Biology:Photodegradation|photodegradation]].<ref>{{cite book| vauthors = Alger M |title=Polymer science dictionary|date=1996|publisher=Chapman & Hall|location=London|isbn=978-0412608704|edition=2nd}}</ref> Photosensitizers are also used to generate prolonged excited electronic states in organic molecules with uses in [[Physics:Photocatalysis|photocatalysis]], [[Physics:Photon upconversion|photon upconversion]] and [[Physics:Photodynamic therapy|photodynamic therapy]]. Generally, photosensitizers absorb [[Physics:Electromagnetic radiation|electromagnetic radiation]] consisting of [[Physics:Infrared|infrared radiation]], [[Physics:Light|visible light radiation]], and [[Physics:Ultraviolet|ultraviolet radiation]] and transfer absorbed energy into neighboring molecules. This absorption of light is made possible by photosensitizers' large [[Physics:Delocalized electron|de-localized π-systems]], which lowers the energy of [[Chemistry:HOMO and LUMO|HOMO and LUMO]] orbitals to promote [[Physics:Photoexcitation|photoexcitation]]. While many photosensitizers are organic or organometallic compounds, there are also examples of using semiconductor [[Physics:Quantum dot|quantum dot]]s as photosensitizers.<ref>{{cite journal | vauthors = Liu Y, Ma Y, Zhao Y, Sun X, Gándara F, Furukawa H, Liu Z, Zhu H, Zhu C, Suenaga K, Oleynikov P, Alshammari AS, Zhang X, Terasaki O, Yaghi OM | display-authors = 6 | title = Weaving of organic threads into a crystalline covalent organic framework | journal = Science | volume = 351 | issue = 6271 | pages = 365–9 | date = January 2016 | pmid = 26798010 | doi = 10.1126/science.aad4011 | bibcode = 2016Sci...351..365L | doi-access = free }}</ref>

== Theory ==
[[File:Simplified_schematic_for_a_photosensitizer_affecting_a_substrate.svg|thumb|528x528px|Basic schematic for all photosensitizers (PS) wherein the photosensitizer absorbs light (hν) and transfers energy to create a physicochemical change]]

=== Mechanistic considerations ===
Photosensitizers absorb light (hν) and transfer the energy from the [[Physics:Ray (optics)|incident light]] into another nearby molecule either directly or by a chemical reaction.  Upon absorbing [[Physics:Photon|photon]]s of radiation from incident light, photosensitizers transform into an excited [[Physics:Singlet state|singlet state]]. The single electron in the excited [[Physics:Singlet state|singlet state]] then flips in its intrinsic spin state via [[Physics:Intersystem crossing|Intersystem crossing]] to become an excited [[Physics:Triplet state|triplet state]]. Triplet states typically have longer lifetimes than excited singlets. The prolonged lifetime increases the probability of interacting with other molecules nearby. Photosensitizers experience varying levels of efficiency for intersystem crossing at different wavelengths of light based on the internal electronic structure of the molecule.<ref name=":02">{{cite journal | vauthors = Gómez Alvarez E, Wortham H, Strekowski R, Zetzsch C, Gligorovski S | title = Atmospheric photosensitized heterogeneous and multiphase reactions: from outdoors to indoors | journal = Environmental Science & Technology | volume = 46 | issue = 4 | pages = 1955–63 | date = February 2012 | pmid = 22148293 | doi = 10.1021/es2019675 | bibcode = 2012EnST...46.1955G }}</ref><ref>{{Cite book |title=Spin crossover in transition metal compounds|date=2004 |publisher=Springer| vauthors = Gütlich P, Goodwin HA |isbn=978-3-540-40394-4|location=Berlin|oclc=56798940}}</ref>

=== Parameters ===
For a molecule to be considered a photosensitizer:

* The photosensitizer must impart a physicochemical change upon a substrate after absorbing incident light.
* Upon imparting a chemical change, the photosensitizer returns to its original chemical form.

It is important to differentiate photosensitizers from other [[Chemistry:Photochemistry|photochemical]] interactions including, but not limited to, [[Earth:Photoinitiator|photoinitiator]]s, [[Physics:Photocatalysis|photocatalysts]], [[Physics:Photoacid|photoacid]]s and photopolymerization. Photosensitizers utilize light to enact a chemical change in a substrate; after the chemical change, the photosensitizer returns to its initial state, remaining chemically unchanged from the process. [[Earth:Photoinitiator|Photoinitiator]]s absorb light to become a reactive species, commonly a [[Biology:Radical|radical]] or an [[Physics:Ion|ion]], where it then reacts with another chemical species. These photoinitiators are often completely chemically changed after their reaction. [[Physics:Photocatalysis|Photocatalysts]] accelerate chemical reactions which rely upon light. While some photosensitizers may act as photocatalysts, not all photocatalysts may act as photosensitizers. [[Physics:Photoacid|Photoacid]]s (or photobases) are molecules which become more acidic (or basic) upon the absorption of light. Photoacids increase in acidity upon absorbing light and thermally reassociate back into their original form upon relaxing. [[Physics:Photoacid|Photoacid generators]] undergo an irreversible change to become an acidic species upon light absorption. [[Chemistry:Polymerization|Photopolymerization]] can occur in two ways. Photopolymerization can occur directly wherein the monomers absorb the incident light and begin polymerizing, or it can occur through a photosensitizer-mediated process where the photosensitizer absorbs the light first before transferring energy into the monomer species.<ref name=":72">{{cite book|title=Modern molecular photochemistry|vauthors=Turro NJ|date=1978|publisher=Benjamin/Cummings Pub. Co|isbn=0-8053-9353-6|location=Menlo Park, Calif.|oclc=4417476}}</ref><ref>{{Cite book| vauthors = Allcock HR, Lampe FW, Mark JE |title=Contemporary polymer chemistry.|date=2003|publisher=Pearson/Prentice Hall |isbn=0-13-065056-0|edition=3rd|location=Upper Saddle River, N.J.|oclc=51096012}}</ref>

== History ==
Photosensitizers have existed within natural systems for as long as [[Biology:Chlorophyll|chlorophyll]] and other light sensitive molecules have been a part of plant life, but studies of photosensitizers began as early as the 1900s, where scientists observed photosensitization in biological substrates and in the treatment of cancer. Mechanistic studies related to photosensitizers began with scientists analyzing the results of chemical reactions where photosensitizers photo-oxidized molecular oxygen into peroxide species. The results were understood by calculating quantum efficiencies and fluorescent yields at varying wavelengths of light and comparing these results with the yield of [[Biology:Reactive oxygen species|reactive oxygen species]]. However, it was not until the 1960s that the electron donating mechanism was confirmed through various [[Physics:Spectroscopy|spectroscopic]] methods including reaction-intermediate studies and [[Physics:Luminescence|luminescence]] studies.<ref name=":72" /><ref>{{cite journal| vauthors = Kavarnos GJ, Turro NJ |date=1986-04-01|title=Photosensitization by reversible electron transfer: theories, experimental evidence, and examples |journal=Chemical Reviews|volume=86|issue=2|pages=401–449|doi=10.1021/cr00072a005|issn=0009-2665}}</ref><ref name=":82">{{cite journal | vauthors = Daniell MD, Hill JS | title = A history of photodynamic therapy | journal = The Australian and New Zealand Journal of Surgery | volume = 61 | issue = 5 | pages = 340–8 | date = May 1991 | pmid = 2025186 | doi = 10.1111/j.1445-2197.1991.tb00230.x }}</ref>

The term photosensitizer does not appear in scientific literature until the 1960s. Instead, scientists would refer to photosensitizers as sensitizers used in photo-oxidation or photo-oxygenation processes. Studies during this time period involving photosensitizers utilized organic photosensitizers,  consisting of [[Chemistry:Aromatic hydrocarbon|aromatic hydrocarbon]] molecules, which could facilitate synthetic chemistry reactions. However, by the 1970s and 1980s, photosensitizers gained attraction in the scientific community for their role within biologic processes and enzymatic processes.<ref name=":52">{{cite journal|vauthors=Gollnick K|date=1968|title=Type II Photooxygenation Reactions in Solution|journal=Advances in Photochemistry|language=en|publisher=John Wiley & Sons, Ltd|pages=1–122|doi=10.1002/9780470133361.ch1|isbn=978-0-470-13336-1}}</ref><ref>{{Cite journal| vauthors = Julliard M, Chanon M |date=1983-08-01|title=Photoelectron-transfer catalysis: its connections with thermal and electrochemical analogs |journal=Chemical Reviews|language=en|volume=83|issue=4|pages=425–506|doi=10.1021/cr00056a003|issn=0009-2665}}</ref> Currently, photosensitizers are studied for their contributions to fields such as energy harvesting, [[Physics:Photoredox catalysis|photoredox catalysis]] in synthetic chemistry, and cancer treatment.<ref name=":82" /><ref name=":62">{{Cite journal| vauthors = O'Regan B, Grätzel M |date=October 1991|title=A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO 2 films |journal=Nature|language=en|volume=353|issue=6346|pages=737–740|doi=10.1038/353737a0|bibcode=1991Natur.353..737O|s2cid=4340159|issn=1476-4687}}</ref>[[File:Type_1_Photosensitized_Reaction.svg|thumb|459x459px|Diagram of a Type I photosensitized reaction<ref name=":02" />|left]]

== Types of photosensitization processes ==

There are two main pathways for photosensitized reactions.<ref name=":02" />

=== Type I ===
In Type I photosensitized reactions, the photosensitizer is excited by a light source into a triplet state. The excited, triplet state photosensitizer then reacts with a substrate molecule which is not molecular oxygen to both form a product and reform the photosensitizer. Type I photosensitized reactions result in the photosensitizer being quenched by a different chemical substrate than molecular oxygen.<ref name=":02" /><ref name=":12">{{cite journal | vauthors = Sang X, Li J, Zhang L, Wang Z, Chen W, Zhu Z, Su Z, Wang E | display-authors = 6 | title = A novel carboxyethyltin functionalized sandwich-type germanotungstate: synthesis, crystal structure, photosensitivity, and application in dye-sensitized solar cells | journal = ACS Applied Materials & Interfaces | volume = 6 | issue = 10 | pages = 7876–84 | date = May 2014 | pmid = 24758570 | doi = 10.1021/am501192f }}</ref>
[[File:Diagram_of_a_Type_II_photosensitized_reaction.svg|thumb|459x459px|Diagram of a Type II photosensitized reaction<ref name=":02" />|left]]

=== Type II ===
In Type II photosensitized reactions, the photosensitizer is excited by a light source into a triplet state. The excited photosensitizer then reacts with a ground state, [[Chemistry:Triplet oxygen|triplet oxygen]] molecule. This excites the oxygen molecule into the singlet state, making it a [[Biology:Reactive oxygen species|reactive oxygen species]]. Upon excitation, the [[Chemistry:Singlet oxygen|singlet oxygen]] molecule reacts with a substrate to form a product. Type II photosensitized reaction result in the photosensitizer being quenched by a ground state oxygen molecule which then goes on to react with a substrate to form a product.<ref name=":02" /><ref name="Karimi 4584–4610">{{cite journal | vauthors = Karimi M, Sahandi Zangabad P, Baghaee-Ravari S, Ghazadeh M, Mirshekari H, Hamblin MR | title = Smart Nanostructures for Cargo Delivery: Uncaging and Activating by Light | journal = Journal of the American Chemical Society | volume = 139 | issue = 13 | pages = 4584–4610 | date = April 2017 | pmid = 28192672 | pmc = 5475407 | doi = 10.1021/jacs.6b08313 }}</ref><ref name="Colloidal Quantum Dots as Photocata">{{cite journal | vauthors = Jiang Y, Weiss EA | title = Colloidal Quantum Dots as Photocatalysts for Triplet Excited State Reactions of Organic Molecules | journal = Journal of the American Chemical Society | volume = 142 | issue = 36 | pages = 15219–15229 | date = September 2020 | pmid = 32810396 | doi = 10.1021/jacs.0c07421 | s2cid = 221179722 }}</ref>

== Composition of photosensitizers ==
Photosensitizers can be placed into 3 generalized domains based on their molecular structure. These three domains are organometallic photosensitizers, organic photosensitizers, and nanomaterial photosensitizers.
[[File:Organometallic_photosensors_1.svg|left|thumb|290x290px|Pictured are Chlorophyll A (A) and Tris(2-phenylpyridine)iridium (B), two examples of organometallic photosensitizers.]]

=== Organometallic ===
[[File:Organic_photosensitizers_0.svg|thumb|351x351px|Pictured from top to bottom, (A) benzophenone, (B) methylene blue, and (C) rose Bengal are all organic photosensitizers. All metals involved are purely [[Chemistry:Counterion|counterion]]s to keep the material in the solid state as a salt.]]Organometallic photosensitizers contain a metal atom chelated to at least one organic ligand. The photosensitizing capacities of these molecules result from electronic interactions between the metal and ligand(s).  Popular electron-rich metal centers for these complexes include Iridium, Ruthenium, and Rhodium. These metals, as well as others, are common metal centers for photosensitizers due to their highly filled d-orbitals, or high d-electron counts, to promote metal to ligand charge transfer from pi-electron accepting ligands. This interaction between the metal center and the ligand leads to a large continuum of orbitals within both the highest occupied molecular orbital (HOMO) and the [[Chemistry:HOMO and LUMO|lowest unoccupied molecular orbital]] (LUMO) which allows for excited electrons to switch multiplicities via intersystem crossing.<ref name=":3">{{cite journal | vauthors = Zhang Y, Lee TS, Petersen JL, Milsmann C | title = A Zirconium Photosensitizer with a Long-Lived Excited State: Mechanistic Insight into Photoinduced Single-Electron Transfer | journal = Journal of the American Chemical Society | volume = 140 | issue = 18 | pages = 5934–5947 | date = May 2018 | pmid = 29671586 | doi = 10.1021/jacs.8b00742 }}</ref>  

While many organometallic photosensitizer compounds are made synthetically, there also exists naturally occurring, light-harvesting organometallic photosensitizers as well. Some relevant naturally occurring examples of organometallic photosensitizers include [[Chemistry:Chlorophyll a|Chlorophyll A]] and [[Chemistry:Chlorophyll b|Chlorophyll B]].<ref name=":3" /><ref>{{cite journal | vauthors = Prier CK, Rankic DA, MacMillan DW | title = Visible light photoredox catalysis with transition metal complexes: applications in organic synthesis | journal = Chemical Reviews | volume = 113 | issue = 7 | pages = 5322–63 | date = July 2013 | pmid = 23509883 | pmc = 4028850 | doi = 10.1021/cr300503r }}</ref>

=== Organic ===
Organic photosensitizers are carbon-based molecules which are capable of photosensitizing. The earliest studied photosensitizers were aromatic hydrocarbons which absorbed light in the presence of oxygen to produce reactive oxygen species.<ref name=":4">{{ cite journal | vauthors = Bowen EJ |title=The Photochemistry of Aromatic Hydrocarbon Solutions|date=1963 |journal=Advances in Photochemistry|pages=23–42|publisher=John Wiley & Sons, Ltd |doi=10.1002/9780470133316.ch2|isbn=978-0-470-13331-6 }}</ref> These organic photosensitizers are made up of highly [[Chemistry:Conjugated system|conjugated]] systems which promote [[Physics:Delocalized electron|electron delocalization]]. Due to their high conjugation, these systems have a smaller gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as a continuum of orbitals within the HOMO and LUMO. The smaller [[Physics:Band gap|band gap]] and the continuum of orbitals in both the [[Physics:Valence and conduction bands|conduction band]] and the [[Physics:Valence and conduction bands|valence band]] allow for these materials to enter their triplet state more efficiently, making them better photosensitizers. Some notable organic photosensitizers which have been studied extensively include benzophenones, methylene blue, and rose Bengal.<ref name=":22">{{cite journal | vauthors = Romero NA, Nicewicz DA | title = Organic Photoredox Catalysis | journal = Chemical Reviews | volume = 116 | issue = 17 | pages = 10075–166 | date = September 2016 | pmid = 27285582 | doi = 10.1021/acs.chemrev.6b00057 }}</ref>

=== [[Physics:Nanomaterials|Nanomaterials]] ===

==== Quantum dots ====
Colloidal [[Physics:Quantum dot|quantum dot]]s are nanoscale [[Physics:Semiconductor|semiconductor]] materials with highly tunable optical and electronic properties. Quantum dots photosensitize via the same mechanism as organometallic photosensitizers and organic photosensitizers, but their nanoscale properties allow for greater control in distinctive aspects. Some key advantages to the use of quantum dots as photosensitizers includes their small, tunable [[Physics:Band gap|band gap]] which allows for efficient transitions to the triplet state, and their insolubility in many solvents which allows for easy retrieval from a synthetic reaction mixture.<ref name="Colloidal Quantum Dots as Photocata"/>

==== Nanorods ====
Nanorods, similar in size to quantum dots, have tunable optical and electronic properties. Based on their size and material composition, it is possible to tune the maximum absorption peak for nanorods during their synthesis. This control has led to the creation of photosensitizing nanorods.<ref name="Gold nanorod-photosensitizer comple">{{cite journal | vauthors = Jang B, Park JY, Tung CH, Kim IH, Choi Y | title = Gold nanorod-photosensitizer complex for near-infrared fluorescence imaging and photodynamic/photothermal therapy in vivo | journal = ACS Nano | volume = 5 | issue = 2 | pages = 1086–94 | date = February 2011 | pmid = 21244012 | doi = 10.1021/nn102722z }}</ref>

== Applications ==

=== Medical ===

==== Photodynamic therapy ====

In February 2019, medical scientists announced that [[Chemistry:Iridium|iridium]] attached to [[Biology:Albumin|albumin]], creating a photosensitized molecule, can penetrate cancer cells and, after being irradiated with light (a process called [[Physics:Photodynamic therapy|photodynamic therapy]]), destroy the cancer cells.<ref name="EA-20190203">{{cite news |author=University of Warwick |title=Simply shining light on dinosaur metal compound kills cancer cells |url=https://www.eurekalert.org/pub_releases/2019-02/uow-ssl013119.php |date=3 February 2019 |work=EurekAlert! |access-date=3 February 2019 }}</ref><ref name="ANG-20181215">{{cite journal | vauthors = Zhang P, Huang H, Banerjee S, Clarkson GJ, Ge C, Imberti C, Sadler PJ | title = Nucleus-Targeted Organoiridium-Albumin Conjugate for Photodynamic Cancer Therapy | journal = Angewandte Chemie | volume = 58 | issue = 8 | pages = 2350–2354 | date = February 2019 | pmid = 30552796 | pmc = 6468315 | doi = 10.1002/anie.201813002 }}</ref>
[[File:Dye_sensitized_solar_cell.svg|thumb|Dye sensitized solar cells are photosensitizers which transfer energy to semiconductors to generate energy from solar light<ref name="Zhang 5934–5947"/>]]

=== Energy sources ===

==== Dye sensitized solar cells ====
In 1972, scientists discovered that chlorophyll could absorb sunlight and transfer energy into electrochemical cells.<ref>{{Cite journal| vauthors = Tributsch H |date=1972|title=Reaction of Excited Chlorophyll Molecules at Electrodes and in Photosynthesis |journal=Photochemistry and Photobiology|language=en|volume=16|issue=4|pages=261–269|doi=10.1111/j.1751-1097.1972.tb06297.x|s2cid=94054808|issn=1751-1097}}</ref> This discovery eventually led to the use of photosensitizers as sunlight-harvesting materials in solar cells, mainly through the use of photosensitizer dyes. [[Physics:Dye-sensitized solar cell|Dye Sensitized Solar cells]] utilize these photosensitizer dyes to absorb photons from solar light and transfer energy rich electrons to the neighboring [[Physics:Semiconductor|semiconductor]] material to generate electric energy output. These dyes act as [[Chemistry:Dopant|dopant]]s to semiconductor surfaces which allows for the transfer of light energy from the photosensitizer to electronic energy within the semiconductor. These photosensitizers are not limited to dyes. They may take the form of any photosensitizing structure, dependent on the semiconductor material to which they are attached.<ref name=":12"/><ref name=":62"/><ref>{{cite journal | vauthors = Teodor AH, Bruce BD | title = Putting Photosystem I to Work: Truly Green Energy | journal = Trends in Biotechnology | volume = 38 | issue = 12 | pages = 1329–1342 | date = December 2020 | pmid = 32448469 | doi = 10.1016/j.tibtech.2020.04.004 | doi-access = free }}</ref><ref>{{Cite journal| vauthors = Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, Wang F, Pan C, Wang P | display-authors = 6 |date=2010-03-09|title=Efficient Dye-Sensitized Solar Cells with an Organic Photosensitizer Featuring Orderly Conjugated Ethylenedioxythiophene and Dithienosilole Blocks |journal=Chemistry of Materials|volume=22|issue=5|pages=1915–1925|doi=10.1021/cm9036988|issn=0897-4756}}</ref>

==== Hydrogen generating catalysts ====
Via the absorption of light, photosensitizers can utilize triplet state transfer to reduce small molecules, such as water, to generate Hydrogen gas. As of right now, photosensitizers have generated hydrogen gas by splitting water molecules at a small, laboratory scale.<ref>{{cite journal | vauthors = McCullough BJ, Neyhouse BJ, Schrage BR, Reed DT, Osinski AJ, Ziegler CJ, White TA | title = Visible-Light-Driven Photosystems Using Heteroleptic Cu(I) Photosensitizers and Rh(III) Catalysts To Produce H<sub>2</sub> | journal = Inorganic Chemistry | volume = 57 | issue = 5 | pages = 2865–2875 | date = March 2018 | pmid = 29446925 | doi = 10.1021/acs.inorgchem.7b03273 }}</ref><ref>{{cite journal | vauthors = Zhou Q, Shi G | title = Conducting Polymer-Based Catalysts | journal = Journal of the American Chemical Society | volume = 138 | issue = 9 | pages = 2868–76 | date = March 2016 | pmid = 26863332 | doi = 10.1021/jacs.5b12474 }}</ref>

=== Synthetic chemistry ===

==== Photoredox chemistry ====
In the early 20th century, chemists observed that various aromatic hydrocarbons in the presence of oxygen could absorb wavelength specific light to generate a peroxide species.<ref name=":52"/> This discovery of oxygen's reduction by a photosensitizer led to chemists studying photosensitizers as [[Physics:Photoredox catalysis|photoredox catalysts]] for their roles in the catalysis of [[Chemistry:Pericyclic reaction|pericyclic reaction]]s and other [[Chemistry:Redox|reduction]] and [[Chemistry:Redox|oxidation]] reactions. Photosensitizers in synthetic chemistry allow for the manipulation of electronic transitions within molecules through an externally applied light source. These photosensitizers used in redox chemistry may be organic, organometallic, or nanomaterials depending on the physical and spectral properties required for the reaction.<ref name=":12" /><ref name=":22"/>

== Biological effects of photosensitizers ==
Photosensitizers that are readily incorporated into the external tissues can increase the rate at which reactive oxygen species are generated upon exposure to UV light (such as UV-containing sunlight). Some photosensitizing agents, such as St. John's Wort, appear to increase the incidence of inflammatory skin conditions in animals and have been observed to slightly reduce the minimum tanning dose in humans.<ref>Kumper H. [Hypericum poisoning in sheep]. ''Tierarztl Prax'' 1989;17:257-261.</ref><ref name=":0">Brockmoller J, et al. Hypericin and pseudohypericin: Pharmacokinetics and effects on photosensitivity in humans. ''Pharmacopsychiatry'' 1997;30(Suppl 2):

94-101.</ref> 

Some examples of photosensitizing medications (both investigatory and approved for human use) are:

* [[Biology:Hypericum perforatum|St. John's Wort]]<ref name=":0" />
* [[Chemistry:9-Methyl-β-carboline|9-me-bc]]<ref>{{Cite journal |last1=Vignoni |first1=Mariana |last2=Rasse-Suriani |first2=Federico A. O. |last3=Butzbach |first3=Kathrin |last4=Erra-Balsells |first4=Rosa |last5=Epe |first5=Bernd |last6=Cabrerizo |first6=Franco M. |date=2013-07-24 |title=Mechanisms of DNA damage by photoexcited 9-methyl-β-carbolines |url=https://pubs.rsc.org/en/content/articlelanding/2013/ob/c3ob40344k |journal=Organic & Biomolecular Chemistry |language=en |volume=11 |issue=32 |pages=5300–5309 |doi=10.1039/C3OB40344K |pmid=23842892 |issn=1477-0539}}</ref>
* [[Chemistry:Doxepin|Doxepin]]<ref name=":1">{{Cite web |date=2013-07-11 |title=Medications and other Agents that Increase Sensitivity to Light |url=https://www.dhs.wisconsin.gov/radiation/medications.htm |access-date=2022-11-01 |website=Wisconsin Department of Health Services |language=en}}</ref>
* [[Chemistry:Amoxapine|Amoxapine]]<ref name=":1" />
* [[Chemistry:Ethinylestradiol|Ethinyl estradiol]]<ref name=":1" />

== See also ==
* [[Physics:Artificial photosynthesis|Artificial photosynthesis]]
* [[Medicine:Photosensitivity|Photosensitivity]]
* [[Physics:Photodynamic therapy|Photodynamic therapy]]
*[[Physics:Photocatalysis|Photocatalysis]]
*[[Physics:Dye-sensitized solar cell|Dye-sensitized solar cell]]
*[[Physics:Photoredox catalysis|Photoredox catalysis]]
*[[Physics:Light harvesting materials|Light harvesting materials]]
*[[Engineering:Photoswitch|Photoswitch]]

== References ==
{{Reflist}}

== External links ==

[[Category:Drug delivery devices]]
[[Category:Photochemistry]]

{{Sourceattribution|Photosensitizer}}