Mechanism of photochromism

It was mentioned earlier that there are two independent chromophores in fulgide molecules. Kurita et al. studied the excited state characteristics of phenyl-substituted fulgide, and found that when fulgide molecules were excited by ultraviolet light, only excited singlets were generated, and some fluorescence of fulgide could be observed at low temperature. The existence of (n, r “) excited singlet was further confirmed in nanosecond laser flash photolysis experiment, and no evidence of excited triplet was observed, and oxygen had no effect on transient spectrum and photochromic reaction-211 kinetics. This excited singlet feature was also confirmed by 11ge et al. They also found that the internal or external heavy atom effect had no effect on the intersystem crossover. Kurita et al. studied furosemide and its chromosomes by picosecond laser flash photolysis technique, and found that the excited states of furosemide and its chromosomes have the characteristics of (r, r*) excited singlets. Ku-rita and others believe that the non-radiative tunneling process is the main way of excited state decay to ground state.

Heller et al. deeply studied the photochromic mechanism of phenyl-substituted fulgide. Under the irradiation of ultraviolet light, the compound (1) or (5) undergoes a cis-rotating ring closure to generate a chromophore, and the 1,8a- dihydronaphthalene derivative (1,8a-DHN)(2) or (6), while the 1,8a-DHN can undergo the opposite change under the irradiation of white light. The main side reactions are thermal pair unfastening to form trans isomer (5) or (1) of substrate, and [1,5] hydrogen migration reaction to form 1,2- dihydronaphthalene derivative (3) or (7), and the activation energy of [1,5] hydrogen migration reaction is low. Cyclization products (2) and (6) are dehydrogenated in the presence of oxidant or oxygen to generate naphthalene derivatives (4), thus losing photochromic properties.

The side reactions of chromophores are mainly hydrogen migration and thermodynamic ring-opening reaction. These side reactions are all related to H in the 8a- position of 1,8a-DHN. If it is replaced by methyl, it can eliminate hydrogen migration reaction and improve thermal stability. However, the photochromism reaction and photochromism reaction have little influence, which is due to their cis-rotation reaction mode, which makes the steric interaction between 1,8a groups smaller. Thermodynamics of 1,8a- dihydronaphthalene derivatives 13h,13i,13j and 13o are listed in Table 11.13 and Table 11.14 respectively. Regarding the mechanism of cyclization reaction, Fan Meigong and others studied it with nanosecond laser flash photolysis technology. According to the results of nanosecond laser flash photolysis on the photochromic process of pyrrole fulgide anhydride, a new photochromic mechanism is proposed. Different from fulgide such as furan and thiophene, in the time-resolved transient spectrum, the absorption of a new intermediate except the closed-loop can be detected, which can be quenched by oxygen, so it is presumed to be an excited triplet. With the increase of time delay, the optical density of the excited triplet decreases while the optical density of the closed loop increases. The attenuation of the triplet accords with the first-order reaction kinetics formula, and its lifetime in cyclohexane is 0.8us. Therefore, the photochromic reaction of pyrrole fulgide includes both excited singlets and excited triplets, but excited singlets are the main ones. The photo-reaction of fulgide includes not only photo-ring reaction, but also cis-trans isomerization reaction.

The photoisomerization reaction of phenylfulgide is listed in Figure 11.21. Ilge et al.’ s research shows that direct light isomerization has nothing to do with oxygen and triplet quencher, so it is a singlet reaction There is no potential barrier in the photoisomerization reactions of EE→EZ,EE-ZE and EZ→ZZ; However, in the reaction of ZZ→EZ or ZE, there is a small potential barrier, and the fluorescence of ZZ isomer can be observed at 77K, but it disappears above 130K K. Triplet sensitization reaction only exists in the reaction betwEEn ee-ZZ and EZ-ZE.