Unveiling the Triplet State of a 4‐Amino‐7‐Nitrobenzofurazan Derivative in Cyclohexane

Abstract

The nitrobenzofurazan (NBD) moiety has gained tremendous popularity over the last decades due to its fluorogenic nature. Indeed, upon interaction with aliphatic amines, it generates a stable fluorescent adduct, which has been used for protein and lipid labeling. In fact the 4-amino substituted NBD belongs to the broad family of intramolecular charge transfer molecules, with the amino group acting as an electron donor upon photoexcitation, and the nitro group as an electron acceptor. Although the singlet excited state of 4-amino NBD derivatives has been abundantly studied, investigation of its triplet manifold is scarce and even the absence of intersystem crossing for this type of molecules has been suggested. However, intramolecular charge transfer molecules are known to undergo intersystem crossing and high phosphorescence quantum yields have been reported in a nonpolar solvent. In the present paper, we have investigated the photophysical and photochemical properties of N-hexyl-7-nitrobenzo[c][1,2,5]xadiazole-4-amine. We have shown the existence of a triplet state for this molecule in cyclohexane via nanosecond laser flash photolysis. Interestingly, deactivation of the triplet state leads to photoproducts formation, which are only present in the absence of oxygen.

A High‐Performing Metal‐Free Photoactivatable Nitric Oxide Donor with a Green Fluorescent Reporter

“…This hypothesis is also in good agreement with the inefficient intersystem crossing to the excited triplet state reported for ABF derivatives, unless than in very non polar solvents ( i. e . cyclohexane) . The identical nature of the photoproduct observed under both aerobic and anaerobic conditions, is in good agreement with the formation of the anilinyl radical as logical key intermediate after the loss of NO via homolytic N‐NO cleavage.…”

“…Finally, unless in very nonpolar solvents ( i. e . cyclohexane), ABF derivatives do not effectively populate the lowest triplet state after excitation with light, precluding the formation of singlet oxygen and consequently avoiding undesired intra‐ and intermolecular oxidation processes. These spectroscopic and photochemical properties combined with the derivatization through straightforward synthetic procedures, have made ABF derivatives very suited for biological labelling as well as a fluorescent sensors for the detection of reactive oxygen species…”

Imaging of intermittent lipid-receptor interactions reflects changes in live cell membranes upon agonist-receptor binding

“…2A,B (see Methods and Materials). The excitation rate was calculated by k 01 = σ ⋅Φ exc , with an excitation cross section of σ = 8.6 ⋅ 10 −17 cm 2 36,39 and where Φ exc is the excitation photon flux. The triplet relaxation rate, k T , was allowed to vary freely for each curve, the intersystem crossing rate, k isc , was determined as a linear function of [O 2 ], while k ox and k red were set as global variables.…”

Using photochemistry to understand and control the production of reactive oxygen species in biological environments

“…[110] This result is substantiated by our recent study where we showed another ICT complex to have a triplet state solely in non-polar environments. [37] Furthermore, we were the first to demonstrate the DNA binding mode of a molecule affects its ability to react with 1 O 2 . [111] Similarly, the extent of 1 O 2 generation by berberine and palmatine is dependent on the DNA sequence.…”

“…1), where E ox (receptor) and E red (reporter) are the oxidation potential of the receptor (electron donor) and the reduction potential of the reporter (electron acceptor), respectively, E 00 (reporter) is the energy of the S 0 to S 1 transition of the fluorophore, and C corresponds to the coulombic attraction energy experienced by the ion pair following the electron transfer reaction. In our case, the oxidation potential of phenol (+0.5V vs. SHE), [36] the reduction potential of NBF (-1.49V vs. SHE, acetonitrile) [37] and the energy of its singlet excited state (E 00 = 57.60 kcal mol -1 , acetonitrile) [37] have to be known. For NBFhd, a free energy G PeT of -11.52 kcal mol -1 is obtained, suggesting a spontaneous transfer of electron between the phenolic moiety and the NBF fluorophore.…”