Excited State Proton Transfers in Hybrid Compound Based on Indoline Spiropyran of the Coumarin Type and Azomethinocoumarin in the Presence of Metal Ions

Abstract

Spectral-luminescence properties of a hybrid compound containing a coumarin-type spiropyran and an azomethinocoumarin fragment in toluene-acetonitrile solution in the presence of Li+, Ca2+, Zn2+ and Mg2+ ions are reported. Two excited state proton transfers can occur in the hybrid compound—the transfer of a proton from the OH group of the 7-hydroxy coumarin tautomer to the N atom of the C=N bond of the azomethine fragment leading to green ESIPT fluorescence with a maximum at 540 nm and from the OH group of the 7-hydroxy coumarin tautomer to the carbonyl group of the pyrone chromophore, which leads to the formation of the 2-hydroxyl-tautomer T of coumarin with blue fluorescence with a maximum at 475 nm. Dependence of these excited state proton transfers on the metal nature and irradiation with an external UV source is discussed.

Ground-State Tautomerism and Excited-State Proton Transfer in 7-Hydroxy-4-methyl-8-((phenylimino)methyl)-2H-chromen-2-one as a Potential Proton Crane

“…The absorption spectra of the possible tautomers of 6, as a model of 5, are shown in Figure 1 and Figure S1 in toluene and acetonitrile, respectively. In line with the experiment, the E tautomer absorbs around 330-340 nm, which corresponds very well to the experimental data in these two solvents [82,83]. The existence of the keto tautomers (KE, KK and K) should be manifested by the appearance of red-shifted absorbance, each of them with their own signature.…”

“…Physchem 2024, 4, FOR PEER REVIEW 5 energies of KK, and especially of K, are too high to be observed in solution. According to the spectrum of 5 in toluene [82], a very low intensive tail is observed in the range 400-500 nm, while in acetonitrile [83], the red-shifted band is well defined at 450 nm, which corresponds to the suggested spectra of KE and KK. As discussed above, due to the high relative energy (>6 kcal/mol in respect of E, Table S1), the KK tautomer should be excluded.…”

“…In spite of its structural suitability as a PTF, 7-hydroxycoumarine and its derivative 4 have never been considered in the design of proton cranes. Very recently, the synthesis and spectral properties of 5 have been reported, along with some photochemical information [82,83]. This creates a solid background for detailed theoretical studies that can reveal the ground-state tautomeric properties and excited-state PT in 6 as simplified models of 5.…”