Photochromic molecular switches that are also pH-sensitive are promising compounds for the development of advanced materials. Such multi-responsive compounds change optical and electronic properties reversibly under excitation in the UV or visible, whereas changing the pH shifts the absorption bands due to electronic effects. For example, the reversible protonation of an amine group affects the conjugation, and therefore the spectroscopy, of both the open- and closed-ring structures of phenylamine-substituted diarylethene (PA-DAE) photochromic switches. The influence of pH on the dynamics of the photoinduced ring-closing and ring-opening reactions of PA-DAE derivatives in solution were studied using steady-state and femtosecond time-resolved spectroscopy. The phenylamine-substituted DAEs were synthesized using an efficient microwave-assisted process. Steady-state spectroscopy indicates that the photocyclization and photocycloreversion reactions are reversible under different pH conditions, even though the addition of acid causes the absorption bands of the closed- and open-ring isomers to red-shift and blue-shift, respectively, compared with the neutral forms of the compound. Changing pH also affects the thermal stability and solubility of PA-DAEs. Specifically, the protonated closed-ring isomer is hydrophilic, whereas the protonated open-ring and both deprotonated forms are hydrophobic. Ultrafast pump-probe measurements monitor the solution-phase dynamics of protonated and neutral DAE following excitation to the first excited states. The decay of the S1 state as well as the recovery of the S0 state were monitored in the range of 350-750 nm using time-delayed broadband probe pulses. The cyclization reaction is faster and the cycloreversion reaction is slower for the protonated form of the PA-DAE compared with the neutral compound, consistent with different π-conjugation for the acid and base. The ultrafast time-resolved and steady-state absorption measurements reveal for the first time a complete picture of effect of pH as an external stimulus on the photoswitching behavior of PA-DAEs.

Influence of pH on the photochromic reaction dynamics of phenylamine-substituted diarylethene derivatives

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.