Photocleavage of coumarin dimers studied by femtosecond UV transient absorption spectroscopy

Abstract

Coumarins are a class of UV absorbing compounds which exhibit fast, photoinduced cyclobutane ring formation and cleavage reactions. The photophysics behind such processes hold significant relevance for biomedical and photoresponsive materials research. In order to further understand the underlying dynamics of the cleavage reaction, and develop strategies for increasing the reaction efficiency, UV transient absorption spectroscopy was applied to three unsubstituted, isomeric coumarin dimers: anti-head-to-head (anti-hh), syn-head-to-head (syn-hh) and syn-head-to-tail (syn-ht). The experiments performed under 280 nm excitation and broadband (300-620 nm) probing revealed that the cleavage reaction of coumarin dimers occurs through non-radiative, short-lived (<200 fs) singlet states. From the data, two branched kinetic models were developed to describe the monomer formation and dimer relaxation dynamics, identify possible intermediate states, and determine the quantum yields of the dimer cleavage. The anti-hh dimer shows the highest cleavage efficiency with a value of about 20%. The differences in the cleavage efficiency for the three isomers are interpreted in terms of differing steric hindrances of the benzene groups attached to the cyclobutane ring and charge delocalisation of the intermediate state.

Nuclear Spin Singlet States in Photoactive Molecules: From Fluorescence/NMR Bimodality to a Bimolecular Switch for Spin Singlet States

“…Zuschriften it dimerizes the absorption above 300 nm disappears,and is in accordance to previous investigations. [49] When coumarin dimerizes,f our different constitutional isomers can form from anti-and syn-h ead-to-head or headto-tail isomerism, depending on the solvent. [51,52] Theresulting dimer in our case is the anti head-to-head isomer, which is formed almost exclusively during photoreactions in chloroform as used here.…”

“…Following this investigation, we studied coumarin. Coumarin and its derivatives are popular, not only because of their UV absorbing and fluorescence emission properties, [49] but also because of many other applications,s uch as controlled drug release. [50] If coumarin is directly irradiated with light at wavelengths above 300 nm it dimerizes to form ac yclobutane ring.…”

“…[50] If coumarin is directly irradiated with light at wavelengths above 300 nm it dimerizes to form ac yclobutane ring. [49] This dimerization occurs upon irradiation when a[ 2 + +2] cycloaddition reaction takes place between two monomers,l eading to the dimer (Scheme 2). [49] To perform singlet investigations of the dimerization process, we synthesized coumarin, substituting one proton for deuterium at the 3-position (3-2 H-coumarin), leaving an isolated proton in the 4-position (Figure 2a).…”

“…[49] This dimerization occurs upon irradiation when a[ 2 + +2] cycloaddition reaction takes place between two monomers,l eading to the dimer (Scheme 2). [49] To perform singlet investigations of the dimerization process, we synthesized coumarin, substituting one proton for deuterium at the 3-position (3-2 H-coumarin), leaving an isolated proton in the 4-position (Figure 2a). [47] In the monomeric form of 3-2 H-coumarin, asinglet state could not be populated because of the isolation of the 1 H-spin.…”

“…After irradiating the sample with light at aw avelength of 254 nm, the dimer can be cleaved into the monomer again, which we partly (16 %) achieved (Figure 2c). [49] We repeated the procedure to investigate its reversible nature and achieved 40 %d imerization starting from the previously cleaved compound. Further cleavage led to part conversion (15 %) of the total dimer content into the monomer.I na ddition to NMR experiments,w ep erformed UV/vis investigations to prove the formation of the dimer (Figure 3).…”