Abstract
Photodeprotection is an important reaction that has been attracting broad interest for use in a variety of applications. Recent advances in ultrafast and vibrational time-resolved spectroscopies can facilitate obtaining data to help unravel the reaction mechanisms involving in the photochemical reactions of interest. The kinetics and reaction mechanisms for the photodeprotection reactions of ketoprofen derivatives containing three different substituents (ibuprofen, Br and I) were investigated by femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy methods in phosphate buffered solutions (PBS). Fs-TA allows us to detect the decay kinetics of the triplet species as the key precursor for formation of a carbanion species for three different substituents attached to ketoprofen. To characterize the structural and electronic properties of the corresponding carbanion and triplet intermediates, TR3 spectroscopic experiments were conducted. The transient spectroscopy work reveals that the different substituents affect the photodecarboxylation reaction to produce carbon dioxide which in turn influences the generation of the carbanion species which determines the rate of the photorelease of the functional groups attached on the ketoprofen parent molecule. The fingerprint TR3 spectroscopy results suggest that ketoprofen derivatives may be deactivated to produce a triplet carbanion when increasing the atom mass of the halogen atoms.
Highlights
In this work, we investigated the mechanisms of selected ketoprofenate phototrigger systems to release species like ibuprofen, bromide and Iodine by time-resolved spectroscopy in phosphate buffered solutions (PBS) with a pH value of 7.4
An absorption band at 340 nm gradually decays to 328 nm while another absorption band at 575 nm grows in and shifts to 530 nm. This process is associated with intersystem crossing (ISC) from S1 to T1 according to its similarity to results from previous femtosecond transient absorption (fs-TA) studies of ketoprofen derivatives[21]
The isosbestic point for this reaction clearly indicates that the KP-ibuprofen triplet anion is the precursor of the 610 nm species which is readily assigned to a KP carbanion species[21]
Summary
We investigated the mechanisms of selected ketoprofenate phototrigger systems to release species like ibuprofen, bromide and Iodine (see compound structures in Fig. 1) by time-resolved spectroscopy in PBS with a pH value of 7.4. The rate constants and reaction mechanisms of these ketoprofenate phototrigger compounds were found to be greatly dependent on the different substituent in the PBS. The KP-Br, I and ibuprofen photocages underwent intersystem crossing (ISC) with different time constants. A clear and convincing mechanism can be concluded which is helpful for a better understanding of how these ketoprofenate phototrigger compounds work and may be useful in the design of new phototrigger compounds of NSAID in the future and for related functional group delivery
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