Fig. 5.

UV–vis spectra of 4 in toluene at 20 mg l⁻¹ upon addition of acetic acid (AcOH). 1: without AcOH; 2: addition of 5 μl AcOH, reaction time 5 min; 3: as 2, but after 30 min; 4: as 3, but addition of 10 μl triethylamine (Et₃N); 5: control—addition of 10 μl Et₃N to 1