Combined experimental and TD-DFT/DMOl3 investigations, optical properties, and photoluminescence behavior of a thiazolopyrimidine derivative

We present here the FT-IR, DFT computation, XRD, optical, and photophysical characterization of a heterocyclic compound with thienopyrimidine and pyran moieties. TD-DFT/DMOl3 and TD-DFT/CASTEP computations were used to study the geometry of isolated and dimer molecules and their optical behavior. The indirect (3.93 eV) and direct (3.29 eV) optical energy bandgaps, HOMO–LUMO energy gap (3.02 eV), and wavelength of maximum absorption (353 nm) were determined in the gas phase with M062X/6-31+G (d, p). A thin film of the studied molecule was studied using XRD, FT-IR, and UV–Vis spectroscopy. The average crystallite size was found as 74.95 nm. Also, the photoluminescence spectroscopy revealed that the compound exhibited different emission bands at the visible range with different intensities depending on the degree of molecular aggregation. For instance, solutions with different concentrations emitted blue, cyan, and green light. On the other hand, the solid-state material produced a dual emission with comparable intensities at λmax = 455, 505, and 621 nm to cover the entire visible range and produce white emission from a single material with CIE coordinates of (0.34, 0.32) that are very similar to the ideal pure white light. Consequently, these findings could lead to the development of more attractive new luminous materials.

Purely organic light-emissive materials have potential biomedical and optoelectronic applications [1][2][3] . Multicolor emissive systems, particularly dual emitters, have many potential applications in data encryption 4 , sensor 5 , bioimaging 6 , anticounterfeiting 4 , and inexpensive efficient white organic light-emitting diodes (WOLEDs) 7 . Generally, the construction of worlds requires covering the entire visible light spectrum from 400 to 700 nm by the use of two (blue and yellow) or three (blue, green, and red) color emitters [8][9][10] . Complex device designs, time-consuming fabrication procedures, and high costs are usually required to accommodate several emissive materials 11 . Single-molecular systems emitting white light have various benefits over multi-fluorescent molecule systems: Single-molecular systems eliminate the drawbacks of degradation, color aging, and phase separation; they also result in increased reproducibility and stability, as well as simplified device fabrication [12][13][14] . Recently, we have developed several single-component light-emitting materials [15][16][17][18][19] . Nevertheless, designing pure organic molecules with a dual emission that is appropriate for useful WOLEDs needs a clear understanding of the dual emission mechanism. Owing to the complexity of engineering the dual photoluminescence combinations, studying the mechanism of dual emission is primarily essential and complicated as it requires taking into account both the molecule and its surroundings in solution, crystalline form, or thin layer 20 . This conformational isomerism, obtained by restricting covalent interactions at distinct dihedral angles, produces diverse spatial configurations of the atoms of the molecule 21 . Photophysical features of the conformers are particularly distinguishable because of alterations in electron interactions and distribution, which result in unique light emissions from each conformer 22,23 . Additionally, supramolecular structures and noncovalent interactions between molecules in some materials can affect the excited state and, as a result, the emission properties 24 . Thus, controlling these Computational study of the isolated molecule [ThiPy-3,8-Dc] Iso . It is well-known that timedependent density functional theory (TDDFT) yields substantial errors for the excitation energies of chargetransfer (CT) excited states, when approximate standard exchange-correlation (xc) functionals are used, for example, B3LYP 29 . The Materials Studio 7.0 program on TDDFT/DMol 3 was used to optimize the molecular structure and perform the frequency calculations for the crystal models and isolated molecules 30 . Without computing, the excited states, the frequency of the simulated IR dependence of a time-dependent electrical field perturbation excitation energies, and transition probabilities are calculated. The TDDFT algorithm's performance is dependent on the approximation exchange and correlation functional (xc-functional) and the basis set. Numerous functionals have been proposed for TDDFT calculations. The well-known and broadly used Becke3-Lee-Yang-Parr hybrid functional (B3LYP) was favored in this study over the parameter-free Perdew-Burke-Enzerhof hybrid functional (abbreviated as PB0 or PBE1PBE). The PB0-functional was recommended by Jaquemin et al. in studies of classical dyes. In critical cases, however, both functionals were used in this study 31,32 . We employed the correlation consistent polarized valence basis sets with added diffuse functions, double and triple-zeta Pople-type basis sets (6-31+G*) with different amounts of polarization and diffuse functions, and polarization consistent basis sets with diffuse functions. Smaller basis sets are generally less suited. The standard basis set 6-31+G* was generally employed in the calculation of the optimum geometry and the spectral excitation and more extended basis set only if the results were unsatisfactory 33 . According to former studies the mean absolute error (MAE) of the calculated TDDFT excitation energies without consideration of the solvent effect amounts to about 0.21 eV 34 . Reflex simulates X-ray tools/TD-DFT, neutron, and electron powder diffraction patterns based on models of crystalline materials. Reflex Plus offers a complete package for the determination of crystal structures from medium-to high-quality powder diffraction data 35 .
Characterization. Table 1 shows the instruments used to characterize the compound under investigation.

Results and discussions
FT-IR Spectroscopy. As shown in Fig. 2 42 . A wide dispersion spectrum in XRD patterns is dependent especially on the arrangements of atoms and crystallite size of particles in the unit cell 43 . At 5 0 ≤ 2θ ≤ 45 0 , the FWHM ( β hkl ) and D Av were determined. The interplanar distance (d-spacing) between the greatest diffraction peaks and D Av was calculated using Bragg' equation: D Av = 0.9 /(β hkl )cosθ where (λ is the X-ray wavelength = 0.1541874 nm), θ is the matching 2θ, and the β hkl is FWHM (in radians) 44 are recorded in Table 2.  There are slight changes in the intensities and position of certain peaks between the experimental and calculated XRD patterns; however, we will only concentrate on the major areas of similarity between them in this section 36 . Microstructural aspects of thin film samples may impact the experimental XRD pattern in addition to the features of instruments and data collecting methods. The slight changes include crystallite size, shape, and orientation distribution. The observed and calculated XRD patterns for both polymorphs match well when compared qualitatively, indicating that the synthesized material XRD patterns are correct 45

Geometry study of [ThiPy-3,8-Dc] Iso and molecular electrostatic potential (MEP). Using
M062X/6-31+G(d,p) calculations, the HOMO and LUMO of the most stable conformer in the ground gaseous state were determined (Fig. 4). The energy difference between the frontier molecular orbitals (FMOs) determines the equilibrium state of the molecule, which is essential for calculating electrical conductivity and understand-   Fig. 4b. The negative electrostatic potential of the macro-cyclical plane is symmetrically distinct in all computations 50 , and the geometry of the mutually positive and mutually negative sections varies per base group. Figure 4c illustrates how the source range (DNP) is enlarged irrelevantly with the base folder (4.4), SCF lenience (0.0001), and maximum. The most stable molecular orbitals (HOMO and LUMO), in the ground gaseous state, were determined using M062X/6-31+G(d,p) computations, Fig. 5. The energy difference between the fragment molecular orbitals (FMOs) establishes the molecule equilibrium state, which is critical for estimating electrical conductivity and comprehending electricity transit. If the entropy values of isolated substances are negative, they are stable 51 . The observed FMOs can be used to determine the electrophilic sites of an aromatic molecule. When the number of dimer molecule bonds (DMB) increases and the bond length decreases, the Gutmannat variance approach is used in the DMB sites to increase the HOMO energy ( E H ) 52 . These properties were determined by examining the optimized energy gap ( E Opt g ), and the molecular system reactivity and stability. The most crucial aspects in determining stability and responsiveness are the material softness and hardness 53,54 . Table 3 Table 3.
There were numerous conformers studied for the ground state geometry in quantum-chemical calculations and the conformer with the lowest energy was selected, which was validated by the harmonic vibrational frequency. The dimers binding energies were adjusted for the basis set superposition error using the counterpoise correction technique BSSE. The binding energies of [ThiPy-3,8-Dc] dimers and single molecules are + 5795.644 and + 11,584.712 kcal/mol, respectively 56 . Dimers binding energies ( E b ) were assessed at the same level of theory using the following formula: E b = E dimer − 2E monomer = 6.575eV. www.nature.com/scientificreports/ Recognizing the nature of noncovalent interactions between small nonpolar molecules is not only intriguing from a theoretical standpoint, but also essential for practical applications. Using the quantum chemistry computations and energy decomposition analyses, the interaction mechanism of [ThiPy-3,8-Dc] dimers is explored. Even though the monomers have zero dipole moment, it is demonstrated that their configuration choices are governed mostly by the electrostatic component and not the dispersion effect. These configuration choices can also be understood directly by analyzing the electrostatic potential map 57,58 . TDDFT/DMOl 3 method was done on the studied molecules and their dimers to get a better understanding of the nature of intermolecular interactions 59 . Intermolecular hydrogen bonding interactions C-H…O, and C-H…N are shown in Fig. 6 for the examined molecule. The lengths of C-H…O and C-H…N hydrogen bonds are 2.547 Å and 3.48 Å, respectively. On the other hand, the dimer centroid lengths are 3.75 Å and 4.89 Å. The intermolecular distance between the two dimers is less than 3.50 Å, preventing the rotation of the rings of both molecules about the single bonds. While the dimer's centroid length exceeds 3.50, the molecule rings rotate around the centroid point 60 . The dihedral angles of the single molecule and the dimer are 166.52° and 127.43°, respectively. According to the type of atom, the dihedral angle will shift from 166.52° to 127.43° when two isolated molecules are joined (as in the case of polymerization) via hydrogen and π bonding in the 2-amino-4-phenyl-4H-pyran-3-carbonitrile moiety.
Optical properties. Analyzing optical characteristics enables discussion of material energy gaps, band structure, and optical transitions.   g ) is estimated from the straight portion of (αhν) 2 a nd (αhν) 0.5 versus (hν ) plot at α = 0 , as shown in Fig. 8. The estimated values of (E Opt. g ) are presented in Table 3.  (Fig. 8 inset), the value of E Opt. g = 3.02 eV. suggesting that the TD-DFT functional reproduces the experimental values more accurately 64,65 .
The refractive index is critical in the design of optical equipment and communication systems.  On the other hand, the value of k(λ) is less than the refractive index "n(λ)" with a ratio equal to n( )/k( ) = 7.8 × 10 7 . This result indicates that k(λ) is the key structural parameter controlling many canopy functions like radiation and water interception, radiation extinction, and water or gas exchange 67 . The CASTEP/ DFT simulated technique was used to evaluate n(λ) and k(λ) and compared with the experimental values, a good similarity was obtained 68 . The frequency dependence of the optical dielectric constant ( ε ) was supposed as an effective technique for elucidating electronic excitations in materials. The real and imaginary components of the complex permittivity characterize the dielectric features. ε is defined as follows: ε = ε 1 − iε 2 , where ε 1 is the real part (the dielectric constant) and ε 2 is the imaginary part (dielectric loss). The real and imaginary parts can be written in terms of k( ) and n( ) as follows: 69 . It is seen, from Fig. 10a, that with increasing photon energy, the ε Re and ε Imag values increase and then decrease at the higher values of photon energy. The maximum values of ε Re and ε Imag are 3.61 and 9.2 × 10 -8 (Fm −1 ) at hν = 4.22eV . The CASTEP/ DFT technique is used to estimate ε real and ε imag values of [ThiPy-3,8-Dc] Iso as an isolated state as a shown in Fig. 10b, the values of ε Re (λ) and ε Imag (λ) in the isolated state are varied in the range of ≅ 0-3 (Fm −1 ) at various photon energies (eV) ≅ 0-10. Reflecting that the experimental and calculated data illustrate similar behavior.
Optical conductivity σ ( ) explains the substance response to the electromagnetic wave. The optical conductivity real ( σ Re ) and imaginary ( σ Imag ) parts are calculated from 70 : σ Re (ω) = ωε Imag ε 0 and σ Imag (ω) = ωε Re ε 0 where the real part ( σ Re ) represents the in-phase current, while the imaginary part ( σ Imag ) represents the π/2 out-of-phase inductive current, ω is the angular frequency (ω = 2πν), ε 0 is the free space dielectric constant, ε Re is the real (normal) dielectric constant, and ε Imag is the imaginary (absorption associated with radiation by free carrier) parts of the dielectric constants. ε Re and ε Imag are given by ε Re = n 2 − k 2 and ε imag = 2nk , respectively 71 . The dependence of σ Re and σ Imag on the incident photon energy ( hν ) is displayed in Fig. 10. The values of the imaginary part are larger than that of the real part of optical conductivity by a factor of σ Imag /σ Re = 4 × 10 7 (Fig. 11a). The values of σ Re and σ Imag are increased with the photon energy increasing till reaching the maximum value then start to decrease. At hν = 4.60 eV, the conductivity is nearly constant and does like a plateau region.  Table 4. These different emission colors may be due to the different molecular arrangements or packings. On the other hand, the solid-state material (powder) produced a dual emission with comparable intensities at 505 and 621 nm beside a shoulder at 455 nm to cover the entire visible range extending from 400 to 750 nm. CIE plot of this spectrum www.nature.com/scientificreports/ presented white-light emission from a single material with CIE coordinates of (0.34, 0.32) that are so close to the ideal coordinates of the pure white emission (0.33, 0.33). As concluded from the DFT simulations, a dimer of the studied molecule exhibited various kinds of molecular interactions with different molecular arrangements and tetrahedral angles from the isolated molecule. Thus, we speculate that at different molecular packings (solid or different solution concentrations), the noncovalent interactions (such as C-H···π, hydrogen bonding, and π···π stacking) stimulate various stacking arrangements with different emission behaviors 24 . In other words, the conformational heterogeneity relies mainly on the packing forces and molecular organization as well as the rigidity of the media. For example, the four states of the studied material (solid state and three different solution concentrations) showed various emission characteristics based on the ability to form specific intermolecular interactions and molecular arrangements which determined their photophysical properties. To explore the origin of the long-wavelength emissions, the excitation spectra were measured at different emission wavelengths (λ em = 505 nm for the solution, 505 and 621 nm for the solid), Fig. 13d. Also, the photoluminescence spectra of [ThiPy-3,8-Dc] were measured for the solution and powder at different excitation wavelengths, λ ex = 300, 350, 375, and 400 nm (see Fig. 13b,c. The obtained different excitation spectra imply different excited states. The emissions at different excitation wavelengths are not similar, thus proposing that these emissions should not have a common excitation pathway, to confirm the possibility of forming various fluorophores. Accordingly, controlling the degree of molecular aggregation can control the relative emission intensities of the three bands at 455, 505, and 621 nm to tune the luminescence color as seen from the Commission Internationale de l'Eclairage (CIE) chromaticity diagrams in Fig. 14. The high-energy emission was assigned to the single molecules interchromophore π-π* transitions. Whereas the low-energy bands would result from the π⋯π stacking of the aggregated molecules. As concluded from the DFT simulations and emission spectra at different excitation wavelengths, the dimer of the studied molecule exhibited various molecular interactions and arrangements. Thus, we suppose that at different molecular packings, the noncovalent interactions (such as C-H···π, hydrogen bonding, and π···π stacking) promote various stacking arrangements with different emission behaviors. In other words, the conformational heterogeneity relies mainly on the packing forces and molecular organization as well as the rigidity of the media. For example, solutions of [ThiPy-3,8-Dc] emitted blue, cyan, and green light. On the other hand, the powder material produced a dual emission with comparable intensities that covered the entire visible range. CIE plot of this spectrum presented white-light emission from a single material with CIE coordinates of (0.34, 0.32) that are similar to the ideal coordinates of the pure white emission (0.33, 0.33). The four states of the studied material (solid and three different solution concentrations) showed various emission characteristics based on the ability to form specific intermolecular interactions and molecular arrangements.  Table 4. CIE coordinates of the emission colors for the spectra in Fig. 13.