Elucidating the mechanisms of acidochromic spiropyran-merocyanine interconversion.
The thermal and photochemical processes associated with the acid-induced conversions of 6-nitroBIPS, SP-1, to form the protonated merocyanine (MC-OH+) were investigated via UV/vis spectrophotometric studies in acetone. It was found that the mechanism of trifluoroacetic acid (TFA)-induced ring-opening of the SP and the rate of MC-OH+ formation follows a general acid catalysis mechanism. In accord with this mechanism, the thermal growth of the acid-induced ring-opened form (MC-OH+) was retarded as the concentration of TFA in the medium was increased. The N-protonated SP, i.e., SP-NH+, is formed in a competing side-equilibrium process as an unreactive "sink", with the nitrogen lone-pair no longer available to drive the ring-opening process and resulting in the inverse rate dependence as a linear 1/kobs vs [HA] plot. Addition of a tertiary amine to MC-OH+ regenerated MC which underwent thermal ring closure to the SP, thus restoring its function as a molecular switch. NMR titration of SP samples showed a downfield shift of the N-substituent peak upon increasing the TFA concentration. However, a saturation behavior could not be observed with SP-1 up to 1 M acid, unlike the model compound, N,N-dimethylaniline (N,N-DMA), which indicates a base strength order of N,N-DMA > SP-1. Further, we have demonstrated that in solvent acetone, on acidification, the normal photo- and thermochromic behavior is reversed; now MC-OH+ is photochemically transformed into SP-H+, which undergoes thermal ring-opening to MC-OH+.
Wojtyk
James T C
JT
Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Canada. James.Wojtyk@rmc.ca
Wasey
Adnaan
A
Xiao
Ning-Ning
NN
Kazmaier
Peter M
PM
Hoz
Shmaryahu
S
Yu
Chen
C
Lemieux
Robert P
RP
Buncel
Erwin
E
D000143:Acids / Q000737:chemistry*; D001578:Benzopyrans / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D006900:Hydroxylation; D007211:Indoles / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009574:Nitro Compounds / Q000737:chemistry*; D011744:Pyrimidinones / Q000737:chemistry*
D000143:Acids; D001578:Benzopyrans; D005609:Free Radicals; D007211:Indoles; D009574:Nitro Compounds; D011744:Pyrimidinones; C088184:spiropyran; C003954:merocyanine dye