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“…To get a deeper insight into the photophysical properties of D7, femtosecond transient absorption measurements (λ exc = 280 nm) were performed using KP as a reference. As expected, the triplet excited state of KP, with λ max = 530 nm, 40,41 was formed within ca. 9.5 ps.…”
Benzophenone (BP) is present in a variety of bioactive molecules. This chromophore is able to photosensitize DNA damage, where one of the most relevant BP/DNA interactions occurs with thymine (Thy). In view of the complex photoreactivity previously observed for dyads containing BP covalently linked to thymidine, the aim of this work is to investigate whether appropriate changes in the nature of the spacer could modulate the intramolecular BP/Thy photoreactivity, resulting in an enhanced selectivity. Accordingly, the photobehavior of a series of dyads derived from BP and Thy, separated by linear linkers of different length, has been investigated by steady-state photolysis, as well as femtosecond and nanosecond transient absorption spectroscopy. Irradiation of the dyads led to photoproducts arising from formal hydrogen abstraction or Paternò-Büchi (PB) photoreaction, with a chemoselectivity that was clearly dependent on the nature of the linking bridge; moreover, the PB process occurred with complete regio-and stereoselectivity. The overall photoreactivity increased with the length of the spacer and correlated well with the rate constants estimated from the BP triplet lifetimes. A reaction mechanism explaining these results is proposed, where key features are the strain associated with the reactive conformations and the participation of triplet exciplexes.
ASSOCIATED CONTENTSupporting Information. 1 H and 13 C NMR spectra of the synthesized dyads and intermediates. UV, femtosecond transient absorption and laser flash photolysis spectra and kinetics. X-Ray crystallography data for Ox-D10.
“…To get a deeper insight into the photophysical properties of D7, femtosecond transient absorption measurements (λ exc = 280 nm) were performed using KP as a reference. As expected, the triplet excited state of KP, with λ max = 530 nm, 40,41 was formed within ca. 9.5 ps.…”
Benzophenone (BP) is present in a variety of bioactive molecules. This chromophore is able to photosensitize DNA damage, where one of the most relevant BP/DNA interactions occurs with thymine (Thy). In view of the complex photoreactivity previously observed for dyads containing BP covalently linked to thymidine, the aim of this work is to investigate whether appropriate changes in the nature of the spacer could modulate the intramolecular BP/Thy photoreactivity, resulting in an enhanced selectivity. Accordingly, the photobehavior of a series of dyads derived from BP and Thy, separated by linear linkers of different length, has been investigated by steady-state photolysis, as well as femtosecond and nanosecond transient absorption spectroscopy. Irradiation of the dyads led to photoproducts arising from formal hydrogen abstraction or Paternò-Büchi (PB) photoreaction, with a chemoselectivity that was clearly dependent on the nature of the linking bridge; moreover, the PB process occurred with complete regio-and stereoselectivity. The overall photoreactivity increased with the length of the spacer and correlated well with the rate constants estimated from the BP triplet lifetimes. A reaction mechanism explaining these results is proposed, where key features are the strain associated with the reactive conformations and the participation of triplet exciplexes.
ASSOCIATED CONTENTSupporting Information. 1 H and 13 C NMR spectra of the synthesized dyads and intermediates. UV, femtosecond transient absorption and laser flash photolysis spectra and kinetics. X-Ray crystallography data for Ox-D10.
“…This asymmetric recognition can be clearly assessed by the difference in lifetime of the intermediate depending on the configuration of the 2-butanol asymmetric center (Figure ). Miranda and co-workers have recently reported precedents in which chiral n,π* triplet excited states of benzophenones abstract hydrogen atoms at different rate constants depending on the configuration of chiral centers. − Upon purging under O 2 , quenching of the transient absorption band assigned to the radical salen metal was observed and the temporal profile of the decay was dramatically affected (Figure ). 7 Decays of the signal monitored at 480 nm under N 2 (○) and O 2 (▪) of the salen Al(II) radical in ( R )-2-butanol (left) and ( S )-2-butanol (right).…”
The photochemistry of a chiral (salen)aluminum(III) chloride complex has been studied in nonconventional solvents, namely, two imidazolium ionic liquids differing on the hydrophobicity (hydrophilic BF(4)(-) or hydrophobic PF(6)(-) counter anions) and in chiral 2-butanols (R and S). Upon 355 nm laser excitation, the same transient absorption spectrum (with some solvatochromic shift in lambda(max)) was recorded in all cases and assigned to the (salen)Al(II) complex with radicaloid character at the metal atom. This intermediate arises from the photoinduced homolytic cleavage of the apical Al-Cl bond. The half-life of this radicaloid Al(II) species varies depending on the solvent, indicating that its reactivity is governed by the nature of the ionic liquid and also on the R or S configuration of the chiral alcohol.
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