2H-Pyrano[3,2-c]chromene-2,5(6H)-diones: Synthesis, Characterization, Photophysical and Redox Studies for Potential Optoelectronic Applications

Herein, we report the preparation of 2H-pyrano[3,2-c]chromene-2,5(6H)-diones 3a-x by reacting 4-hydroxycoumarins 1a-b with Baylis–Hillman adducts 2a-w having electron releasing or electron withdrawing groups on benzyl ring of the pyranochromene moiety and study of their photophysical properties. The study of optical and electrochemical properties of the prepared compounds reveals that the electron releasing and electron withdrawing groups has not much impact on ground and excited state electronic behavior on pyranochromene moiety. The density functional theory suggests the highest occupied molecular orbital and lowest unoccupied molecular orbitals spread on coumarin moiety of pyranochromene unit. Further, these compounds are thermally stable (up to 200 °C) and lead to blue or green emission that should facilitate the development of organic light emitting diodes (OLEDs).

However, synthesis of the 2H-pyranochromene-2,5-diones are restricted by the lack of enough available methods and evaluation of their optical and electronic properties are not much explored to the best of our knowledge. For instance, Siva Hariprasad Kurma and Ramya Somanaboina are equally contributed to this work.

Materials and Methods
Melting points were determined in open glass capillary tubes on a Stuart melting point apparatus and are uncorrected. Cyclic-and differential pulse voltammetric measurements were performed on a PC-controlled CH instruments model CHI 620C electrochemical analyzer. Cyclic voltammetric experiments were performed on 1 mM solution of respective compound in dichloromethane at scan rate of 100 mV/s using 0.1 M tetrabutyl ammoniumperchlorate (TBAP) as supporting electrolyte. The working electrode is glassy carbon, standard calomel electrode (SCE) is reference electrode and plantinum wire is an auxillary electrode. After a cyclic voltammogram (CV) had been recorded, ferrocene was added, and a second voltammogram was measured. UV-visible spectra were recorded with a Shimadzu spectrophotometer (Model UV-3600). Concentration of the samples used for these measurements ranged from about 5 × 10 -4 M. Steady state fluorescence spectra were recorded using a Spex model Fluoromax-3 spectrofluorometer for solutions having optical density at the wavelength of excitation (λ ex ) ≈ 0.11. Fluorescence lifetime measurements were carried on a picosecond time-correlated single photon counting (TCSPC) setup (FluoroLog3-Triple Illuminator, IBH Horiba JobinYvon) employing a picosecond light emitting diode laser (NanoLED, λ ex = 345 nm) as the excitation source. A Mettler Toledo TGA/SDTA 851e instrument was used for the thermogravimetric measurements at a heating rate of 10 °C min −1 with 10 mg of sample.
All the calculations have been carried out using a Gaussian 09 package on a personal computer. The Ground state geometry of compounds 5a-x were optimized using density functional theory (DFT), while time-dependent DFT (TDDFT) was employed for the estimation of ground to excited-state transitions. The obtained geometries were to be genuine global minimum structures, using B3LYP hybrid functional and 6-31G (d, p) basis set, and were used as the input for further calculations. The geometries were then used to obtain frontier molecular orbital's (FMOs).

Synthesis
General Procedure for the Preparation of Baylis-Hillman Adducts (2a-w) Baylis-Hillman adducts 2a-w were prepared by reacting substituted benzaldehydes and aliphatic aldehydes with methyl acrylate in presence of DABCO as per the literature reported procedures. The structures of the compounds 2a-w were depicted in supporting information (Table S1).
Having optimized the reaction conditions, then, series of target compounds 3b-r were prepared by reacting 4-hydroxy-2H-chromen-2-ones 1a-b with electron donating and electron withdrawing groups present on Baylis-Hillman adducts 2a-q (Table 1) under optimized reaction conditions. The electron donating groups such as CH 3 , OCH 3 , C 2 H 5 are present at benzyl ring of pyranochromenes 3b-f were provided in 79-87% yields under optimal reaction conditions. Especially, the election donating substituents present at p-position of benzyl ring 3e-f were obtained in higher yields when compared to other positions 3b-d. The electron withdrawing groups such as F, Cl, Br, CF 3 and CN present at benzyl ring of pyranochromene 3 g-q were obtained in 33-84% yields. Further, the reaction was investigated with methyl substituted 4-hydroxy-2H-chromen-2-one 1b with methyl 2-(hydroxy(phenyl)methyl)acrylate 2q under optimized reaction conditions provided compound 3r in 51% yield (Table 1).
In order to expand the scope of the reaction, next, the reactions were performed between 4-hydroxy-2H-chromen-2-one 1a with aliphatic Baylis-Hillman adducts 2r-w under optimized reaction conditions. All the reactions were preceded well and afforded the corresponding compounds 3S-X in 72-84% yields ( Table 2). All the prepared compounds were characterized by IR, 1 H-NMR, 13 C-NMR, and HRMS spectroscopies (see ESI).

Theoretical Studies
It is very essential to understand the optical and electrochemical properties of materials for optoelectronic applications. For this we have adopted density functional theory (DFT) initially, using Gaussian 09 package with a functional basis set of the B3LYP/6-31G (d, p) to ascertain the orbital energy levels and electron distribution of pyranochromene-dione derivatives in vacuum in order to understand the electrondonating/accepting ability. Figure 2 illustrates energy level diagrams of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of representative investigated molecules. From the energy levels it suggests that both HOMO and LUMO lying on the coumarin   & 3d) at meta position of phenyl ring depict the lowest band gap of 3.335 eV due to + I effect assign the influence of electron withdrawing and electron releasing propensity on the band gap of pyranochromene-dione derivatives as shown in energy level diagram. The electron releasing or electron withdrawing groups on benzyl ring has not much effect on HOMO-LUMO energy levels (see Fig. S73). In contrast, electron releasing methyl substitution on phenyl ring of coumarin has destabilized the LUMO level and appeared at -2.242 eV.

Optical Properties
UV-Visible optical absorption spectral studies were carried at room temperature in dichloromethane solvent at a concentration of 0.5 × 10 -4 M. Absorption spectra of 3ax revealed in Figs. 3 and S73 and respective absorption maxima along with logarithmic of molar extinction coefficient were presented in Table 3. All the molecules are having absorption maxima at ~ 275 nm and ~ 346 nm, that are assigned to the localized aromatic π-π* transitions.
The presence of either electron releasing or electron withdrawing group/s on benzyl ring has not much effected on absorption maxima. Further, the molar extinction coefficient (ε) has also not much effected on substitution using either electron withdrawing or electron releasing groups. However, the substituents have effect on emissive properties. On the other hand, the absorption maxima bathochromically shifted to 351 nm in case of compound 3r where the electron releasing methyl substituent on aromatic ring of coumarin moiety.   derivatives 3a, 3b, 3d, 3f, 3 g, 3 k, 3 l, 3n, 3p, 3r, 3w, and 3 × in dichloromethane at a concentration of 0.5 × 10 -4 M Table 3 Photophysical, electrochemical, and band gap calculations of pyranochromene-dione derivatives 3a-x a Solvent CH 2 Cl 2 . Error limits: λ max , ± 1 nm; log ε, ± 10% b Solvent Error limits: λex, ± 2 nm; λem, ± 1 nm c All lifetimes are in nanoseconds (ns); Error limits of τ ∼10%. Values in parentheses are relative amplitudes of the corresponding decay components d Singlet state energies as estimated by the intersection between the absorption and the emission spectra; Error limits: ± 0.05 e CH 2 Cl 2 , 0.1 M TBAP. Glassy carbon working electrode; standard calomel electrode is reference electrode, Pt electrode is auxiliary electrode. Error limits, E1/2 ± 0.03 V f HOMO and LUMO energy level calculations using the Gaussian 09 package with a functional basis set of the B3LYP/6-31G (d, p) g Band gap values calculated from the optimized dye structures using E HOMO -E LUMO  S74 illustrates the emission spectra of compounds 3a-x in dichloromethane solvents at room temperature by exciting at ~ 345 nm and the corresponding emission maxima presented in Table 3. The parent compound i.e., 3a has emission maxima at 406 nm, whereas its methoxy (3b) and bromo substituted (3 k and 3 l) has not much effected on emission maxima. In contrast, all other compounds are having two emission maxima at ~ 375 nm and ~ 495 nm. In general, the para substitution on benzyl ring (3e, 3f, 3 h, 3p and 3q) has bathochromically shifted the emission maxima irrespective of electron withdrawing or electron releasing substituents. The emission maxima has not much effected when substitutents are directly connected to dione unit of 2H-pyranochromene-dione. The singlet state energies (band gap, E 0-0 ), were estimated from excitation and emission spectra (Table 3), which were in the range of ~ 3.20 ± 0.05 eV for investigated compounds of the study. Eventually, the photoluminescence decay studied for the pyranochromene-dione derivatives using time-correlated single photon counting (TCSPC) experiments in dichloromethane solvent performed using 345 nm LED source and emission monitored at ~ 495 nm exhibit a bi exponential decay profiles except in compounds 3a, 3 k, and 3w where a mono exponential decay profiles were observed (Fig. 5). The obtained TCSPC data presented in Table 3 and values are obtained typically in ns regime. The parent 2H-pyranochromene-dione was found 3.101 ns.

Redox Properties
The redox properties of pyranochromene-dione derivatives (3a-x) were studied by using cyclic voltammetric technique in dichloromethane solvent with 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. Figure 6 illustrates the cyclic voltammogrammes of investigated compounds and respective redox potentials are presented in Table 3. Wave analysis suggested that the reduction step of Fig. 4 a Emission spectra of 2H-pyranochromene-dione derivatives 3a, 3b, 3 k, and 3 l in dichloromethane solvent. b Emission spectra of 2Hpyranochromene-dione derivatives 3e, 3 h, 3j, 3n, 3p, 3q 3a, 3c, 3e, 3 g, 3 h, 3i, 3 k, 3 m, 3o, 3p, 3q, 3 s, 3u and 3w derivatives in dichloromethane solvent at an excitation wavelength of 345 nm LED source each pyranochromene-dione is reversible (i pc /i pa = 0.9-1.0) and diffusion-controlled (i pc /v 1/2 ) constant in the scan rate (v) range50-500 mV/s) one-electron transfer (ΔE P = 60-70 mV; ΔE P = 65 ± 3 mV for ferrocenium/ferrocene couple) reactions. On the other hand, the oxidation process is irreversible [53]. The electron withdrawing and electron releasing groups on phenyl ring has not showed much effect on redox potentials of pyranochromene-dione moiety. For instance, the oxidation potential of compounds 3e, 3 k, 3 l, 3 s, 3w, and 3 × anodically shifted and appeared at ~ 1.80 V vs. SCE, when compared to the oxidation potential of its pristine compound 3a. On the other hand, the compound 3r in which the methyl substitutent present on benzene ring of the coumarin has cathodically shifted and appeared at 1.01 V vs. SCE. Similarly the reduction potential values are also not much effected except 3p and 3q, where the reduction potentials are cathodically shifted to -1.49 and -1.45 V vs. SCE, respectively due to the presence of electron withdrawing groups on phenyl ring. Therefore, it clearly suggest that substituent on benzene ring of coumarin has much effect on redox potentials rather than benzyl group.

Thermogravimetric Analysis
The thermal stability of organic compounds is also very essential for optoelectronic applications. For this reason, we have carried out thermal stability of 2H-pyranochromene-dione derivatives using thermogravimetric analyses as shown in Fig. 7. It is known in literature that coumarin derivatives are thermally stable up to 200 °C. [54] Almost no mass loss is observed for the samples 3b, 3e, and 3q below 200 °C whereas in case of 3t it is up to 150 °C (Fig. 7). The decomposition of samples starts at ~ 250 °C and the onset of decomposition is above 300 °C in case of 3b and 3e. This indicates the compounds are thermally stable up to 200 0 C which are essential and prerequisite for opto-electronic applications. Optical, and electronic properties of 2H-pyranochromene-dione derivatives suggests that they are potential either blue or green emitters for organic light emitting diodes applications. Further, the electronic properties suggest that these compounds are also potential for field effect transistors.
In conclusion, we have prepared 2H-pyrano[3,2-c] chromene-2,5(6H)-diones having either electron releasing or electron withdrawing groups. Furthermore, the optical and redox studies exploited to understand the nature of electrophilicity/necleophilicity in pyranochromene-dione derivatives. DFT and optical studies indicated that the pyranochromene-dione derivatives having electron donor/acceptor substituents such as -CH 3 , -OCH 3 , -C 2 H 5 , -CN, -CF 3 has not much effected on HOMO and LUMO energy levels, electron density distribution and band gap except in compound 3r. This leads to emission in blue or green region observed. The redox properties are also not much effected when substituents present on benzyl ring of the 2H-pyranochromene-dione derivatives. The optical, electrochemical, and thermal properties of these derivatives encourage the development of OLEDs, filed effect transistors, and biological applications.