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Potential advantages of photon-mode over heat-mode recording include higher resolution and speed of writing, and multiplex recording capability by use of wavelength, polarisation and phase changes. For a photochromic material to be of any use in optical memories it must meet the minimum requirements in the following physical properties:30 • Thermal stability – very low thermal re-conversion • Fatigue resistance – long-term stability during read and write cycles • Non-destructive readout capability – the read out process must not induce interconversion of the two forms • High efficiency of the photoreaction – large quantum yields and molar coefficients • High speed of the photoreactions • Diode laser susceptibility • Solubility in polymer matrix Of those properties on the list, the most difficult to attain is a non-destructive readout procedure.

21) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R1 and R2 can be part of a carbocyclic spiro ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups. 1. 6). Studies suggest that formation of the cisquinoidal species occurs in picoseconds, followed by generation of the trans-form in nanoseconds. 20) show quite different photochromic behaviour. 6 Photochromic behavior of chromenes. cis-quinoidal 16 Chapter 1 9 increase in coloration, but a very slow fade rate back to the colourless state.

By putting solubilising groups on the side chain of the polymer the colour change can be effected in a variety of solvents, including water when the side chain contains an ethanesulfonate group. The temperature range of the solid-state thermochromic effect can also be altered by changing the length and flexibility of the side chains. 2), they often require quite high temperatures to effect the change, and this has limited their use. Additionally, there is also a problem with synthesising compounds to cover the desired colour gamut across the visible spectrum.

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Chromic Phenomena by Bamfield P.

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