posted on 2014-01-29, 00:00authored byKai Li, Yu Xiang, Xiaoyan Wang, Ji Li, Rongrong Hu, Aijun Tong, Ben Zhong Tang
Photochromic
molecules are widely applied in chemistry, physics,
biology, and materials science. Although a few photochromic systems
have been developed before, their applications are still limited by
complicated synthesis, low fatigue resistance, or incomplete light
conversion. Rhodamine is a class of dyes with excellent optical properties
including long-wavelength absorption, large absorption coefficient,
and high photostability in its ring-open form. It is an ideal chromophore
for the development of new photochromic systems. However, known photochromic
rhodamine derivatives, such as amides, exhibit only millisecond lifetimes
in their colored ring-open forms, making their application very limited
and difficult. In this work, rhodamine B salicylaldehyde hydrazone
metal complex was found to undergo intramolecular ring-open reactions
upon UV irradiation, which led to a distinct color and fluorescence
change both in solution and in solid matrix. The complex showed good
fatigue resistance for the reversible photochromism and long lifetime
for the ring-open state. Interestingly, the thermal bleaching rate
was tunable by using different metal ions, temperatures, solvents,
and chemical substitutions. It was proposed that UV light promoted
isomerization of the rhodamine B derivative from enol-form to keto-form,
which induced ring-opening of the rhodamine spirolactam in the complex
to generate color. The photochromic system was successfully applied
for photoprinting and UV strength measurement in the solid state.
As compared to other reported photochromic molecules, the system in
this study has its advantages of facile synthesis and tunable thermal
bleaching rate, and also provides new insights into the development
of photochromic materials based on metal complex and spirolactam-containing
dyes.