Photochromic systems quantum yields

Stokes-shifted tautomer emission and high quantum yield in solid state (10%), DHBO was successfully used as an energy transfer donor in the photochromic switching system [88]. 

Photochromism may be defined as the reversible light-induced interconversion of a system between two forms having distinguishably different absorption spectra.181 The generalized process is outlined in equation (55) where represents the quantum yield for photochemical reaction and... 

Fulgicldes such as (186) show well characterised photochromic behaviour and yield photocyclised species such as (187) which can be reconverted to (186) by Irradiation with light of appropriate wavelength. This photochromic system has been applied as an actinometer for lasers S while the effect of the size of the R group in (186) upon the quantum yield of the photocyclisation has also been... 

Depending on the molecule and the solvent, quantum yields l>coi ranging from 0.18 to 0.65 have been calculated for some representative spirooxazines.27 For other systems, 4>CO = 0.2-0.9 had already been reported.23,24 It has been observed that, in general, photochromes having high [Pg.75]

Using the example of a simple AB, 1 , Ik photochromic system, we can define the experiments required to obtain the quantum yield < >AB and the complete UV/ visible spectrum of the unstable photoisomer B (eB). The equation for the change in absorbance of such a system can be obtained by making BA = 0 in the kinetic... 

Dynamic analysis of photochromic systems under continuous irradiation represents a powerful method of investigation of the reaction mechanisms. The characteristic kinetic and spectral parameters such as the quantum yields of the photochemical steps and the molar extinction coefficients of the transient species can be derived using this method. The essence of the method is the inverse treatment based on numerical simulation and fitting of the plots (Abs versus t) obtained under continuous irradiation. This also exploits the information contained in the irradiation kinetics. In order to extract one or more of the relevant parameters of a given process, specially designed experiments need to be carried out in which the effect of the process under consideration is conspicuous. 

Photochromic materials based on different classes of spiropyrans (SPs) are widely used in various fields of science and technology, such as in the production of light filters regulating luminous fluxes, as photochromic organic media for processing optical information, for photochromic optics, and in the production of nonlinear optical materials. In recent years, the study of new SPs has been conducted mainly in two directions, namely, the search for new classes of SPs and structural modification of the known systems to improve their basic characteristics (quantum yield of photoconversion, the stability of the photoisomer produced, the number of cycles of operations). Only a profound comprehension of mechanisms of photoinitiated rupture of the Cspiro—O bond, structural isomerization, ways of stabilizing the photoisomer, routes of its breakdown, and influence of the structure of SPs on their properties can provide the basis for purposeful research in this area. Despite the vast number of investigations in this area, the mechanisms of the photochromic conversion of SPs and the influence of structural features on their photochemical properties are not well understood. This complicates the search for and synthesis of new SP classes. 

The photochromism of 1 is achieved by atom rearrangement into the intramolecular oxygen-atom insertion product 2, in contrast to the frequently reported organic photochromic systems based on photo-induced cyclization, cis/trans isomerization, or H atom transfer [1]. The quantum yield of the photoreaction from 1 to 2 at 509 nm in acetonitrile without 02 is 0.14 0.01. In solution, photoreaction of 1 causes the oxidation reaction by atmospheric oxygen, resulting in a mixture of 2 and further oxidation products such as [(RhCp )2((i-CH2)2( i-S03)] and [(RhCp )2 ( r-CH2)2( t-S04)]. In contrast, in the crystalline state, the photochromic system between 1 and 2 is stable and repeatable with essentially 100% interconversion ratio. 

Recently photochromism has also been detected for large systems such as XVIII lib. Further questions have to be solved such as, from which excited state these reactions occur, the quantum yield of these isomerizations and the reaction products in case of irreversibility. 

Interest in photochromic systems other than those based on the hexa-fluorocyclopentene moiety continues to grow. The photochemical reactivity of the two photoswitches (35) is similar, and irradiation is efficient with conversions of 85% and quantum yields of around 0.6. The novel photo-chromic systems (36) undergo reversible ring closure in a reaction analogous to that observed in the bisthienyl system. Qin et al. have studied the novel pyridyl substituted cyclopentene system (37). This undergoes photocyclization with an enhanced quantum yield when the reactions are carried out in the presence of a metal. The pyridine units are capable of co-ordinating with the metal. The photochromic dithienylethene unit tethered to 3-cyclodextrin (38) has been used as a photoswitch to control the uptake of porphyrin. A series of new photochromic molecules (39) have been synthesized and studied. These exhibit the usual cyclization on irradiation. " The terthiophene derivatives (40) exhibit reversible photochemical cyclization (at 313 nm) and reversion (at wavelengths >460 nm) reactions. The cycles can be carried out many times... 

The influence on the photochromism in dithienylethene systems of replacing the favoured perfluorocylopentene moiety by other cyclic ethene units has attracted the attention of a number of research groups. Dutch workers have shown that the use of the perhydrocyclopentene unit as in (91) has little effect on the photochromic behaviour of (92) but note that in situations where fatigue resistance and thermal stability are critical, the fluorinated moiety would be preferable. The cyclization quantum yield of (93) is reported to increase in the presence of metal ions, and more detailed accounts have been published of the interesting application of the dithienylcyclopentene-tethered jS-cyclodextrins (94) as photoswitchable hosts for meso tetrakis(4-sulfonatophenyl)porphyrin. ... 

As it is well known, photochromic materials have a wealth of applications, from sunglasses to information storage. The research on how these systems can be tuned (reversibility, rate and quantum yield in either direction, fatigue) by chemical elaboration is very active. In depth mechanistic studies with a variety of techniques have been carried out. 

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