High pressure studies of the electronic behavior of bianthrones

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Abstract
High pressure optical studies have been made on a series of bianthrones dissolved in polymethyl methacrylate (PMMA). At sufficiently high pressure at room temperature, significant occupation of the photochromic (B) state occurs, but the yield is different for the different derivatives; it is especially small for the 4,4″ dimethyl bianthrone, where steric effects are important. From data obtained at 90°C at high pressure, it was established that equilibrium yields are obtained. From the pressure coefficient of the equilibrium constant, it was possible to calculate ΔΔ̄V̄A B, the change in partial molar volume upon conversion. The photochromic (B) state has a smaller volume but also a smaller compressibility than the A state. A comparison between bianthrone in PMMA and in polystyrene (PS) demonstrates that the medium affects the yield. The shifts and half‐width changes of the optical absorption peaks are used to characterize the configuration coordinates involved in optical excitation. High pressure data at low temperature (−80°C) demonstrate that the photochromic yield decreases rapidly with increasing pressure.

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