Photochromism
“Photochromism is the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra.[1][2] Trivially, this can be described as a reversible change of color upon exposure to light. The phenomenon was discovered in the late 1880s, including work by Markwald, who studied the reversible change of color of 2,3,4,4-tetrachloronaphthalen-1(4H)-one in the solid state. He labeled this phenomenon "phototropy", and this name was used until the 1950s when Yehuda Hirshberg, of the Weizmann Institute of Science in Israel proposed the term "photochromism".[3] Photochromism can take place in both organic and inorganic compounds, and also has its place in biological systems (for example retinal in the vision process).” (Wikipedia 8/17/2009) "Conventional photochromic compounds have at least two states, a first state having a first absorption spectrum and a second state having a second absorption spectrum that differs from the first absorption spectrum, and are capable of switching between the two states in response to at least actinic radiation. Further, conventional photochromic compounds can be thermally reversible. That is, conventional photochromic compounds are capable of switching between a first state and a second state in response to at least actinic radiation and reverting back to the first state in response to thermal energy. As used herein "actinic radiation" means electromagnetic radiation, such as but not limited to ultraviolet and visible radiation that is capable of causing a response. More specifically, conventional photochromic compounds can undergo a transformation in response to actinic radiation from one isomer to another, with each isomer having a characteristic absorption spectrum, and can further revert back to the first isomer in response to thermal energy (i.e., be thermally reversible). For example, conventional thermally reversible photochromic compounds are generally capable of switching from a first state, for example a "clear state," to a second state, for example a "colored state," in response to actinic radiation and reverting back to the "clear" state in response to thermal energy." (Kumar et al, Transitions Optical, Inc., US Patent 7,557,206, 7/7/2009)
Recent US Patents
To go to the text version of the patent, click on US Patent Number Search and enter the patent number in the search box. 7/7/2009 7,557,206 Photochromic compounds 6/9/2009 7,544,315 Photochromic h-annellated benzo[f]chromene compounds 5/5/2009 7,527,754 Photochromic indeno-fused naphthopyrans
Recent Journal Articles
To get to the abstract or article, copy the citation and paste it into favorite search engine (ex. Google). The date is the date the entry was found. 7/31/2009 Synthesis and Photochromic Behavior of Azo-Polydiphenylsilanes (348-354) Journal of Inorganic and Organometallic Polymers and Materials 19 #2 (2008)
Review Articles
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Editor's Notes
“Photochromism does not have a rigorous definition, but is usually used to describe compounds that undergo a reversible photochemical reaction where an absorption band in the visible part of the electromagnetic spectrum changes dramatically in strength or wavelength. In many cases, an absorbance band is present in only one form. The degree of change required for a photochemical reaction to be dubbed "photochromic" is that which appears dramatic by eye, but in essence there is no dividing line between photochromic reactions and other photochemistry. Therefore, while the trans-cis isomerization of azobenzene is considered a photochromic reaction, the analogous reaction of stilbene is not. Since photochromism is just a special case of a photochemical reaction, almost any photochemical reaction type may be used to produce photochromism with appropriate molecular design. Some of the most common processes involved in photochromism are pericyclic reactions, cis-trans isomerizations, intramolecular hydrogen transfer, intramolecular group transfers, dissociation processes and electron transfers (oxidation-reduction).” (Wikipedia 8/17/2009) ************************************************ As the literature is reviewed, and items of interest concerning this topic are found. These may be added in an abbreviated form with the reference. Readers and contributors are invited to add their own notes. Contributors may add them directly and other readers can simply send their notes to the editor, Roger Corneliussen at rcorneliussen@4spe.org. He may edit and add them to this page at his discretion.