The complex showed an initial change in the λmax at around 45 °C when measured spectroscopically but required heating to a minimum of 65 °C to elicit a visible colour change to orange, and complete conversion required a temperature of 68–70 °C ( Fig. 3). The bathochromic shift was demonstrable after heating to 85 °C. Time for complete reversion increased with increasing temperature, taking only about 3 min after heating to 65 °C, 2 to 3 h after 75 °C and up to 48 h after treatment at 85 °C. Reversion to the original state started to occur at around 45 °C.