Indenopyrans – synthesis and photoluminescence properties

New indeno[1,2-c]pyran-3-ones bearing different substituents at the pyran moiety were synthesized and their photophysical properties were investigated. In solution all compounds were found to be blue emitters and the trans isomers exhibited significantly higher fluorescence quantum yields (relative to 9,10-diphenylanthracene) as compared to the corresponding cis isomers. The solid-state fluorescence spectra revealed an important red shift of λmax due to intermolecular interactions in the lattice, along with an emission-band broadening, as compared to the solution fluorescence spectra.


Introduction
Indenopyrans are functionalized oxygen-containing heterocycles consisting of fused indene and pyran units. Generally, oxygen-containing heteroaromatic compounds have been encountered as biologically active molecules and display exciting physico-chemical properties. Therefore, these compounds have found many applications in medicinal chemistry and material sciences. Besides, the annelated cyclic indenes containing N or O heteroatoms are the main structural frameworks of several natural alkaloids [1], while indenopyran derivatives were found to be valuable substrates for the synthesis of various compounds with biological activity [2,3]. Some indeno [1,2c]isochromen-5(11H)-ones have been reported as key intermediates for the development of several topoisomerase I (Top1) anticancer agents [4][5][6][7]. A natural product exhibiting very prom-ising antitumor activity is β-lapachone having a naphtho [1,2b]pyrandione skeleton [8,9]. Also worth mentioning in this context are benzo [b]indeno-[2,1-e]pyran-10,11-diones, which stimulate the biosynthesis of erythropoietin and are used to treat anemia [10]. Another application includes the use of some amino-spiroindenopyran derivatives as selective sensors for thallium(I) ions in human urine [11].
Based on their important biological functions in nature and the synthetic potential of α-pyrones for the construction of a variety of fluorescent heteroarenes, we decided to synthesize and study the optoelectronic properties of some indeno-fused pyran-3ones and the results are reported herein.

Results and Discussion
For a long time, the Diels-Alder reaction of simple heterosubstituted 1,3-dienes has been considered a standard procedure to prepare highly functionalized ring systems [28]. Taking advantage of diazalactones to act as the dienophile component in inverse-electron demand Diels-Alder reactions, a series of new dihydroindeno[1,2-c]pyran derivatives was synthesized. Thus, the addition of freshly distilled indene as a diene component to oxadiazinones 1 as the dienophiles [29] under acidic conditions (TFA, TFAA) led to the formation of dihydroindeno [1,2c]pyran-3-ones 2 and 3 as a mixture of isomers (Scheme 1). Additionally, the corresponding phenyl-substituted compounds 2a and 3a (previously reported [30]) were synthesized to investigate the effect of an aryl substituent in position 1 on the photophysical properties of the compounds. The selective formation of type 3 isomers has been reported earlier [31,32]. They are accessible through a cycloaddition reaction of indene and methyl 6-oxo-5-phenyl-6H-1,3,4-oxadiazine-2-carboxylate followed by the treatment with bromine (or hydrogen chloride), thus yielding the corresponding halogenated lactones having the 4a,9a-fused skeleton and an ester group at position 1.
The cycloaddition reaction proceeded with moderate regio-and stereoselectivity. As reported in the literature, the inverse-electron demand Diels-Alder cycloaddition of unsymmetrical olefins and oxadiazinones [33][34][35] may lead to two possible regioisomers: isomer 2 with the benzene ring fused at C5a-C9a and its regioisomer 3 with the benzene ring fused at C4b-C8a ( Figure 1).
Additionally, due to the new stereocenters created at positions 4 and 4a, each regioisomer could be expressed as a set of diastereoisomers. Thus, the isomers denoted 2' and 3' exhibit the phenyl group at position 4 and the indeno fragment at position 4a in a trans configuration, while the isomers 2'' and 3'' bear the two aforementioned groups in a cis configuration ( Figure 1).
In most cases, regioisomer 2 predominates and occasionally, formation of derivative 3 has not even been observed (e.g., d derivatives). Moreover, the nature of the substituent influences the regio-and diastereoisomeric ratio ( Table 1).
The diastereoisomers exhibit very close R f values which makes their separation by conventional chromatography quite difficult. Nevertheless, successful separation of diastereoisomers 2' and 2'' as individual isomers has been achieved for all compounds.
The configuration of the diastereoisomers was assigned based on 1 H NMR spectroscopic measurements ( Figure 2). Thus, for  3 1.    cis configuration. The cis configuration was also assigned for diastereoisomer 3''b which displays one doublet at 4.87 ppm for the hydrogen atom at position 4 with a coupling constant of 6.7 Hz.
Generally, the expected structures for all compounds were fully confirmed by the characteristic signals in the 1 H NMR spectra ( Figure 2). The protons at positions 5 and 9 of compounds 2 or 3 exhibit large coupling constants, typically observed for geminal protons. Noteworthy, in case of isomer 3''b, the specific methylene protons connected to the enol-lactone moiety display more deshielded signals than those of isomer 2''b ( Figure 2).
Once the 1,4-disubstituted indenopyrone derivatives were synthesized and their structures confirmed by spectroscopical methods, we further investigated their photophysical properties. No significant differences could be observed in the shape of the electronic bands in the absorption spectra of 2a-d recorded in acetonitrile ( Figure 3) and methylene chloride ( Figure S1, Supporting Information File 1). The spectra exhibit intense broad absorption bands between 275-282 nm and 307-317 nm, re-spectively, corresponding to π-π * and n-π * transitions. As a general remark, when compared to the reference compound 2a, the maxima of absorption bands for 2b-d are slightly redshifted due to the substituents in position 1: C 6 H 4 -m-OCH 3 , C 6 H 4 -p-t-Bu and C 6 H 4 -p-Br, respectively ( Table 2). The use of methylene chloride or acetonitrile as solvents led to a similar trend of the electronic spectra depending on the nature of the substituents (Table 2, Figure 3, Figure S1 in Supporting Information File 1). However, a slight bathochromic shift could be observed when methylene chloride was used in contrast to a hypsochromic shift in the more polar solvent acetonitrile.
The analysis of the absorption spectra revealed some differences between the regioisomers 2''b and 3''b and diastereoisomers 2' and 2'', as well ( Figure 3, Figure S1, Supporting Information File 1). Thus, a red shift of 35 nm was observed when comparing the longest wavelength for 2''b (λ max = 317 nm) with that of its regioisomer 3''b (λ max = 282 nm). This behavior could be explained by an extended electron delocalization in the conjugated system of isomer 2''b. From the spectra obtained from the diastereoisomers 2' and 2'', one can observe a small shift to higher wavelengths in case of the cis isomers 2''.  Moreover, the spectrum of the trans isomer 2'b displays the most intense absorption band at a lower wavelength (280 nm) in contrast to the cis isomers 2''b (313 nm, in acetonitrile).
All derivatives 2a-d showed an intense emission in acetonitrile in the range 380-393 nm (Figure 4, Table 3), while for the cis isomer 3''b a weaker emission was observed. This may be explained by a less extended conjugation and an intersystemcrossing process that occurs when an electron is transferred from a singlet energy level into a triplet energy level [36].
The derivatives 2a-d are blue emitters, displaying maximumemission bands at around 370-393 nm. Remarkably, the fluorescence quantum yields calculated relative to 9,10-diphenylanthracene are significantly higher in the case of the trans isomers 2' as compared to the corresponding cis isomers 2'' ( Table 3).
The solid-state photoluminescence properties of the dihydroindenopyrones 2'a-c and 2''a-d were also investigated. These de-rivatives exhibit emission maxima in the range 423-491 nm when excited at 310 nm ( Figure 5, Table 3).
Comparing the solution (acetonitrile) and solid-state fluorescence spectra of both diastereoisomers ( Figure 5), one can observe a bathochromic shift of λ max in the solid state and a broadening of the emission band because of intermolecular interactions.
Investigations carried out for the dehydrogenation reaction of the isomeric mixture 2'a/3''a (Scheme 2) revealed the forma- tion of the α-pyrone 6a (15% yield), which was the oxidation product of isomer 3''a. In the same time, most of the isomer 2'a was recovered and traces of derivative 4a could only be detected in the NMR spectrum.
Using the same procedure, the enol-lactone isomeric mixture 2'c/3'c was subjected to the dehydrogenation reaction yielding the regioisomers 4c and 5c (Scheme 2). The 1 H NMR spectrum revealed the formation of the fully oxidized derivative 6c in small amounts that could not be isolated. However, silica gel column chromatography of the reaction mixture allowed the recovery of the starting enol-lactone derivative 2'c in 50% yield.
The 1 H NMR spectra of α-pyrones 4c and 5c ( Figure S4, Supporting Information File 1) exhibit characteristic signals for the aliphatic protons: one singlet at 4.19 ppm for derivative 5c and one signal at 3.97 ppm for compound 4c, respectively.
Next, the photoluminescence properties of the isolated indenopyrones 4c, 5c and 6a were investigated. The UV-vis spectra ( Figure S5, Supporting Information File 1) recorded in methylene chloride displayed two absorption maxima assigned to π-π* and n-π* transitions, respectively. All derivatives showed very weak emission in the range 380-550 nm ( Figure  S5, Supporting Information File 1) when excited at 370 nm, while the calculated quantum yields relative to 9,10-diphenylanthracene were quite low (<1%).
In the crystals of α-pyrone 6a, the asymmetric unit consists of a single molecule. The two phenyl rings attached to the pyran-3-one moiety are disposed in a staggered conformation. The angle between the planes defined by the two phenyl rings is 85.8(1)°a nd the angle between the non-intersecting C1-C13 and the C3-C19 bond vectors is 12.9(3)°. Both phenyl rings are rotated with respect to the best plane defined through the pyran-3-one moiety (C1-O1-C(2-4)-C12), with angles of 35.8(2)° for the (C13-18) ring and 51.1(1)° for the (C19-24) ring, respectively (Figure 6a).

Conclusion
Several indenopyrone derivatives were synthesized and characterized by NMR, UV-vis and fluorescence spectroscopy as well as mass spectrometry. Column chromatography of the crude product furnished the diastereoisomers as pure samples. The electronic spectra display two intense absorption bands ranging between 275-282 nm and 307-317 nm, while their photoluminescence spectra revealed that all compounds are blue emitters.
The calculated fluorescence quantum yield relative to 9,10diphenylanthracene is significantly higher for the trans isomers when compared to the corresponding cis isomers. The solidstate fluorescence investigations revealed in the case of the regioisomers 2 a bathochromic shift together with an emissionband broadening in contrast to their emission spectra recorded in solution. The supramolecular arrangement in the solid state of indenopyrone 6a is ensured by a number of non-covalent interactions such as intermolecular H-bonding, C-H···π and π···π contacts.

Supporting Information
Supporting Information File 1 Experimental.