Solid-State Luminescent Materials Containing Both Indole and Pyrimidine Moieties: Design, Synthesis, and Density Functional Theory Calculations.

Heterocyclic compounds with effective solid-state luminescence offer a wide range of uses. It has been observed that combining pyrimidine and indole moieties in a single molecule can enhance material behavior dramatically. Here, different heterocyclic compounds with indole and pyrimidine moieties have been synthesized effectively, and their structures have been validated using NMR, IR, and mass spectroscopy. The photoluminescence behavior of two substances was investigated in powder form and solutions of varying concentrations. After aggregation, one molecule displayed a redshifted luminescence spectrum, whereas another homolog showed a blueshift. Thus, density functional theory calculations were carried out to establish that introducing a terminal group allows modifying of the luminescence behavior by altering the molecular packing. Because of the non-planarity, intermolecular interactions, and tiny intermolecular distances within the dimers, the materials demonstrated a good emission quantum yield (Φem) in the solid state (ex. 25.6%). At high temperatures, the compounds also demonstrated a stable emission characteristic.

Introduction

Since Tang and Vanslyke published their outstanding work in 1987, organic light-emitting devices (OLEDs) have received the researchers’ attention.[1] OLEDs offer a variety of applications[2−5] in storage media,[6] sensors,[7] optics,[8] electronics,[9] and biological science.[10] Emitting materials in most of the lighting systems on the market are inorganic.[11] While organic emitting materials were anticipated to be widely used in the future, they seem not to have been significantly utilized in the industry.[12,13] Compared to inorganic compounds, organic luminescent chromophores offer superior luminescence characteristics and lower industrial costs.[14]

Applications in biology, such as sensors, biomedical imaging, and bacterial imaging, benefit from materials with both luminous and antimicrobial features.[15,16] Recently, solid-state luminous compounds with antimicrobial effects containing indole[17,18] or pyrimidine[19,20] moieties have been produced. Furthermore, earlier research has shown that combining two or more active structural moieties might improve the activity.[21,22] As a result, combining pyrimidine and indole moieties in a single molecule can dramatically boost biological activity.[23] Several heterocyclic and non-heterocyclic luminogens have recently been described, and their luminescence was regulated by molecular aggregation.[24−27]

As a response to the challenges outlined above in addition to what we have already accomplished,[28−35] we present here a synthetic strategy for several novel indole–pyrimidine hybrids. Some substances’ photophysical characteristics in solid and solution forms were investigated. Density functional theory (DFT) simulations were used to understand the relationship between the luminescence behavior and molecular structure.

Results and Discussion

Molecular Design and Characterization

The synthetic pathway for synthesizing the materials containing both indole and pyrimidine moieties is depicted in Schemes and 2. As shown in Scheme , The amino carboxamide derivative (10) was effectively synthesized similar to our previously published technique with some changes,[17] where compound 10 was prepared by treating the uncyclized compound 9 with sodium ethoxide as an organic base rather than its direct formation from the mercaptocarbnitrile derivative 8.[36,37] Compound 8 was obtained through a series of reactions starting from the formation of pyrimidine block 4, which was obtained through a multi-component reaction of 3-chloro-1H-indole-2-carbaldehyde (1), thiourea (2), and ethyl cyanoacetate (3). The obtained compound 10 was then reacted with different reagents to synthesize the target compounds. Condensation of 10 with cyclohexanone or triethyl orthoformate in glacial acetic acid followed by a cycloaddition reaction afforded the corresponding compounds 11 and 12, respectively. The synthesized compounds 9–12 were successfully characterized by IR and NMR spectra. The IR spectrum of the S-alkylated compound 9 featured stretching vibration bands at 3455, 3389, and 3317 cm–1 for NH2, 2209 cm–1 for the CN group, and 1651 cm–1 for the C=O group, whereas the 1H NMR spectrum exhibited singlet signals at 7.90 and 3.94 ppm belonging to two NH2 groups. Also, the IR spectrum of compound 11 showed bands at 3162 and 3420 cm–1 for the stretching vibration of the NH and OH tautomer. Meanwhile, its 1H NMR spectrum showed multiplet signals in the range of 0.89–1.97 ppm for the 10 aliphatic protons of the cyclohexanone moiety. In a similar manner, the structure of compound 12 was confirmed by spectral data, where the IR spectrum revealed the appearance of bands at 3288 cm–1 for NH and 1663 cm–1 for amidic C=O. The 1H NMR spectrum of substance 12 displayed a singlet signal at 8.12 ppm, which proved the other fused pyrimidine ring formation. Furthermore, the reaction of compound 10 with chloroacetyl chloride under a neat condition followed by treatment with sodium carbonate solution gave the chloromethyl compound 13. The formation of compound 13 can be explained as follows: The amino group in C5 of compound 10 was acylated by chloroacetyl chloride. The amidic NH2 group was then nucleophilically added to the carbonyl group in the newly acylated NHCOCH2Cl followed by the elimination of a water molecule. The spectral analyses elucidated that the structure of 13, bands at 3357 and 3472 cm–1 for NH2 groups in the starting compound 10 disappeared from the IR spectrum. Also, new bands at 3256 and 3372 cm–1 appeared for the formed two NH groups. Also, the 1H NMR spectrum exhibited the disappearance of signals of NH2 groups in compound 10 and the appearance of a new singlet signal at δ = 4.43 ppm for the CH2 group. IR, 1H NMR, and 13C NMR spectra of compounds 9–12 are presented in the electronic Supporting Information (ESI) as Figures S1–S9.

Scheme 1

Synthetic Route for Obtaining the Amino Carboxamide (10) and Pyrimidothienopyrimidine Compounds (11–13)

Scheme 2

Reactions of Indolylpyrmidothienopyrimidine Derivatives 12 and 13

Compound 13 underwent nucleophilic substitution reactions with primary and secondary amines in dioxane and afforded compounds 14 and 15 (Scheme ). The structures of the obtained compounds were affirmed based on IR and 1H NMR spectra. The IR spectrum of compound 14 demonstrated stretching vibration bands at 3245 and 3185 cm–1 for NH and 1643 cm–1 for the C=O group. Similarly, compound 15 demonstrated bands at 3473 and 3327 cm–1 for the two NH groups and 1674 cm–1 for the C=O group. Moreover, the 1H NMR spectrum of 15 exhibited signals at 2.72, 2.73, 3.78, 3.79, and 3.80 ppm, characteristic of the morpholine moiety, in addition to the singlet signal at 3.33 ppm for the CH2 group. Furthermore, when compound 12 was treated with a mixture of phosphorus oxychloride and phosphorus pentachloride, it delivered the chloropyrimidine derivative compound 16. The latter compound was reacted in dioxane with thiourea to give the analogous thioxopyrimidine compound 17. The structure of the obtained compound 16 was confirmed by the IR spectrum that showed the disappearance of C=O bands in 12. Also, the IR spectrum of compound 17 showed the appearance of new bands at 3434 and 1268 cm–1, characteristic of NH and C=S, respectively. Moreover, the compound 171H NMR spectrum displayed a new singlet signal at 14.28 ppm, which belonged to the NH group.

The nucleophilic substitution of the chlorine atom in compound 16 with different nucleophiles like hydrazine hydrate and aniline produced hydrazinopyrimidothienopyrimidine (18) and anilinopyrimidiothienopyrimidine (19), respectively. The IR spectrum of compound 18 showed new absorption bands at 3287 and 3395 cm–1 for the amino group, while its 1H NMR spectrum showed a singlet signal at δ = 4.89 ppm for the NH2 group. When compound 18 was allowed to react with triethyl orthoformate in the presence of a catalytic amount of acetic acid, it gave compound 9-(p-tolylamino)-7-(3-chloro-1H-indol-2-yl)[1,3,4]triazolo[1″,5″:1′,6′] pyrimido[4′,5′:4,5]thieno[2,3-d]pyrimidine (20). Bands at 3202, 3287, and 3395 cm–1 of NH and NH2 groups in the compound 18 IR spectrum disappeared with 20. The 1H NMR spectrum of 20 showed the appearance of a singlet signal at δ = 6.04 ppm for CH pyrazole and the disappearance of the singlet signal of the NH2 group in 18. On the other hand, compound 18 reacted with acetylacetone in dioxane to produce compound 4-(3,5-Dimethylpyrazol-1-yl)-7-morpholino-9-phenylpyrimido [4′,5′:4,5]thieno[2,3-d]pyrimidine (21). The IR spectrum of compound 21 exhibited the disappearance of NH and NH2 bands in 18. There were two singlet signals at δ = 2.38 and 2.75 ppm in the 1H NMR spectrum for 2CH3 groups, as well as a singlet signal at 6.28 ppm, which indicated the synthesis of the pyrazolyl ring in compound 21. IR, 1H NMR, 13C NMR, and mass spectra of compounds 13–21 are presented in the ESI as Figures S10–S37.

Quantum-Chemical Computations

DFT calculations were presented to reveal the structures of molecules 11 and 12 and their dimers. Calculations were carried out on compounds 11 and 12 exclusively since the former emits in both the solid and solution phases, while the latter exhibits blueshifted emission after aggregation. A 46.6° dihedral angle between the indole and pyrimidine rings of molecule 11 is seen in Figure a, while molecule 12 (Figure b) has a dihedral angle of 16.2°. Figure c,d shows the dimers of molecules 11 and 12, respectively. The most stable dimer structures were chosen from a variety of binding modes. The structures in Figure c,d are dominated by π–π stacking. Dimerization of molecule 11 results in a binding energy of −35.4 kcal/mol, whereas that of molecule 12 is 43.4 kcal/mol. These high binding energies indicate that the dimers of 11 and 12 are stable. Within these dimers, the average intermolecular distance was found less than 3.3 Å. This proximity prohibits the rings from rotating around single bonds. To elucidate the electron distribution in molecules 11 and 12, their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) along with their energies computed with the B3LYP-D3BJ/6-31+G(d,p) level of theory (Figure e,f). From these calculations, we may gain intensive information regarding the orbital distribution. Both HOMO and LUMO spread on the complete molecule (for both 11 and 12). The computed band gaps in 11 and 12 were 3.32 and 3.20 eV, respectively. The critical changes in structural parameters of the monomers and dimers are shown in Figure a,b. The structural shifts between the excited and ground states of the dyes were estimated to illustrate their aggregation-induced emission feature. At the same level of theory, the structure of the initial excited state was optimized. The key structural differences between the excited and ground states of molecules 11 and 12 are depicted in Figure c,d. In general, we noticed a greater deviation from planarity in the excited state, and the rings are more twisted. The maximum absorption wavelengths (λabs) of the studied compounds 11 and 12 computed with B3LYP/6-31+G(d,p) for the monomers and dimers are presented in Figure S38 (ESI).[38−46] The energy gaps of 3.32 and 3.20 eV between the HOMO and LUMO of compounds 11 and 12 gave λabs = 381 and 479 nm for monomers as compared to 376 and 515 nm for dimers, respectively. The monomers and dimers exhibited several absorption bands, the short-wavelength bands can be attributed to the delocalized π–π* transitions, while absorptions at the long wavelengths may result from the prolonged conjugation along the molecule.

Figure 1

(a) 11 and (b) 12 geometry of the ground state. (c) 11 and (d) 12 geometry of the most stable dimers optimized using B3LYP-D3BJ/6-31+G(d,p). For both dimers, specific intermolecular distances between some atoms and the center of the nearest ring (in Å) are presented. Color: blue = N; gray = C; yellow = S; red = O; green = Cl. (e) 11 and (f) 12 HOMO and LUMO and the energies calculated at the B3LYP-D3BJ/6-31+G(d,p) level of theory.

Figure 2

Key changes in the structural parameters calculated at the B3LYP-D3BJ/6-31+G(d,p) level of theory between the (a) monomer and dimer of molecule 11, (b) monomer and dimer of molecule 12, (c) excited and ground states of molecule 11, and (d) ground and excited states of molecule 12.

Photophysical Properties

As illustrated in Figure and Table , the optical behavior of compounds 11 and 12, as representative samples, was investigated for powders and DMSO solutions. Only compounds 11 and 12 exhibited solid-state emission under UV irradiation and hence were chosen for photophysical analysis. The two dyes showed absorption until 420 nm but seemed transparent at longer wavelengths, a desirable feature for emissive molecules. The absorption spectra of these compounds are similar, implying that their solution ground states are comparable as a result of their similar electronic structures. On the other hand, solutions emit from a variety of excited states as specified by their diverse excitation spectra. Additionally, because solids and solutions have distinct ground and excited states, their excitation spectra differ from their corresponding absorption spectra, as seen in Figure a,b. These findings demonstrate that aggregation and substitution influence excited and ground states.

Figure 3

Photophysical behavior of the powders (dashed lines) and DMSO solutions (solid lines). Absorption (blue, 3 × 10–6 mol L–1) and excitation spectra (red, λem = 520 nm for 11 and 464 nm for 12) of (a) 11 and (b) 12. Emission spectra at λex = 300 nm of (c) 11 and (d) 12 (violet: powder, blue: 1 × 10–3 mol L–1, black: 3 × 10–6 mol L–1, and red: 1 × 10–7 mol L–1). CIE diagrams of the emission colors of (e) 11 and (f) 12. Photos under UV irradiation at λex = 360 nm of (g) 11 and (h) 12.

Table 1

Photophysical Data of the Examined Compounds in the Powder and Different Solution Concentrations

		11	12
λmaxb (nm)	powder	520	466
	1 × 10–3 M	492	477
	3 × 10–6 M	332 (w), 373 (s), 466 (m)	373 (w), 472 (s)
	1 × 10–7 M	332 (s), 373 (m)	332 (w), 373 (s), 477 (m)
CIE (x, y)a	powder	0.22, 0.59	0.17, 0.23
	1 × 10–3 M	0.20, 0.42	0.16, 0.28
	3 × 10–6 M	0.17, 0.23	0.15, 0.25
	1 × 10–7 M	0.17, 0.18	0.15, 0.21
Φem (%)b	powder	25.6	9.7
τ (ns)c	powder	3.7 (36%) + 5.8 (64%)	2.3 (30%) + 5.3 (70%)
Kr (× 107 s–1)d	powder	4.4	1.8
Knr (× 107 s–1)e	powder	12.8	17.1

CIE coordinates for the emission at λex = 300 nm.

Φem: emission quantum yield.

τ: emission lifetime.

Kr: radiative decay rate constant for the major component (Kr = Φem/τ).

Knr: non-radiative decay rate constant for the major component (Knr = (1/τ) – Kr).

Emission spectra of compounds 11 and 12 were recorded for powders and solutions with varying DMSO concentrations (from 1 × 10–3 to 1 × 10–7 mol L–1), as shown in Figure c,d. The color of the emission was further validated by the Commission Internationale de l’Eclairage (CIE) chromaticity diagrams and digital photos under UV light (Figure e–h). Compound 11 gave emission with maximum wavelengths (λmax) at 332 nm (strong) and 373 nm (medium), resulting in CIE coordinates of (0.17, 0.18). Under UV light, these CIE coordinates created blue emission that can be seen with the naked eye. Using a more concentrated solution (3 × 10–6 mol L–1) diminished the high energy emission at 332 nm compared to that at 373 nm, and a new wide emission band with λmax = 466 nm developed. Increasing the solution concentration to 1 × 10–3 mol L–1 led to a significant quenching of the emissions at <425 nm, resulting in a significant emission band mainly at long wavelengths (λmax = 492 nm). The luminescence spectrum of dye 11 powder with λmax = 520 nm was redshifted compared to that of the 1 × 10–3 mol L–1 solution. This redshift in emission is normal for organic components as molecules gather in powders and concentrated solutions, where π–π interactions between neighboring molecules strengthen, enabling the redshift in emission.[47−49] The emission color on the CIE diagram changed from blue to light blue, then cyan, and eventually greenish-yellow as the molecular aggregation increased from 1 × 10–7 to 3 × 10–6 then 1 × 10–3 mol L–1 followed by the powder state. Because the emissions at 332 and 373 nm were only significantly observed in diluted solutions, these bands can be attributed to monomer emission (emission from non-interacted molecules due to the solvation effect). However, bands at >420 nm were released by molecular aggregation, and these bands were observed using concentrated solutions and powder.

The increased solution concentration of dye 12 suppressed emission at shorter wavelengths (332 and 373 nm) while improving luminescence intensity at longer wavelengths (477 nm). However, the powder emission of compound 12 (λmax = 466 nm) displayed a blueshift compared with its solutions (λmax = 477 nm). This strange behavior has been documented previously.[20,50] The blueshift denotes a reduction in π–π interactions in dye 12 powder aggregates compared with its solutions. Intermolecular interactions between dye 12 aggregates may also shorten the conjugation and induce the blueshift. Therefore, we may conclude that adding cyclohexyl as a terminal group to compound 11 changed the molecular packing and, thus, the optical characteristics of the solid-state luminescent material. The decay profiles and lifetimes (τ) were also evaluated, and the results are displayed in Figure and Table . τ of dyes 11 and 12 are expressed on a nanosecond scale, and then the resulting emissions are fluorescence. The decay characteristics of both materials were monoexponential. The emission quantum yields (Φem) of dyes 12 and 13 in the solid state were determined to be 25.6 and 9.7%, respectively. These reasonable values of Φem, as verified by DFT calculations, can be attributed to the small intermolecular distances within the dimers (less than 3.3 Å), which prohibits the rings from rotating around single bonds. Non-planarity, in conjunction with intermolecular interactions, can also open up new routes for radiative decay in the solid state. Additionally, Table summarizes the observed Φem and the radiative decay rate constant (Kr) and non-radiative decay rate constant (Knr). Compound 11 has a greater Kr value and a lower Knr value than compound 12. The increased non-planarity may result in the blockage of non-radiative channels by further inhibiting molecule stacking, hence increasing emission efficiency. Thus, we hypothesize that the increased Φem of 11 (25.6%) over 12 (9.7%) is due to the presence of a non-planar cyclohexyl group in the molecular structure of 11, which increases Kr and decreases Knr. The photoluminescence behavior of dye 12 powder was examined by heating and cooling the sample at various temperatures (Figure ). Because of thermally stimulated molecular motions, the emission intensity decreases with heating. So, cooling the sample increased the intensity once again. Since λmax was not shifted upon heating, it can be suggested that the emission from this compound is due to the aggregation-induced emission enhancement. Even at 150 °C, the material emits effectively, and its spectral structure is substantially identical as before heating. As a result, this dye may be used in applications that need extreme conditions, such as elevated temperatures. In comparison to our previously published compounds,[17] the present research demonstrated the influence of non-planar groups on the emission behavior, lifetime, and decay rate constants, where the presence of the cyclohexyl group enhanced emission efficiency and altered the characteristics of the excited state relative to the ground state, as shown above by comparing molecular orbitals and absorption, excitation, or emission spectra.

Figure 4

(a) 11 and (b) 12 emission decay profiles in the air at room temperature excited at 340 nm; green: decay, blue: IRF, and red: fitting.

Figure 5

Photoluminescence spectra of 12 at λex = 300 nm under various temperatures: (a) first heating and (b) first cooling.

Experimental Section

Synthesis

The solvents and reagents utilized in this study were of analytical grade and were utilized as provided. A Fisher-Johns instrument was used to determine the melting points. On a Pye-Unicam Sp-100 spectrophotometer, IR spectra were recorded using the KBr disk method. Bruker BioSpin GmbH spectrometers (1H: 400 MHz; 13C: 101 MHz) were used to produce 1H and 13C NMR spectra with DMSO-d6 as a solvent and tetramethylsilane (TMS) as a reference. A JEOL JMS-600 spectrometer was used to obtain electron impact mass spectra. Using UV light, preparative and analytical TLC were done on silica gel plates (Fluka 70643-50EA. SIGMA-ALDRICH, Germany). All of the chemical reactions occurred in the atmosphere. Compounds 4–8 were previously synthesized according to reported procedures.[17,28]

2-((6-(3-Chloro-1H-indol-2-yl)-5-cyano-2-(p-tolylamino)pyrimidin-4-yl)thio)acetamide (9)

A mixture of compound 8 (3.9 g, 10 mmol), fused sodium acetate (1.64 gm, 20 mmol), and chloroacetamide (0.94 gm, 12 mmol) in 50 mL of ethanol was refluxed for 3 h. The solid precipitate formed during reflux was filtrated off, washed with water and ethanol, and dried in air to afford the opened structure compound 9 as off-white crystals in 95% yield. This compound was employed in the next step without additional purification. m.p.: 150–152 °C. IR: ν (cm–1) 3455, 3389, 3317, 2209, 1651. 1H NMR (400 MHz, DMSO) δ = 11.96 (s, 1H, NH indole), 10.28 (s, 1H, NH p-tolyl), 8.13–6.87 (10H, Ar–H and CONH2), 3.94 (s, 2H, NH2), 2.33 (s, 3H, CH3). 13C NMR (101 MHz, DMSO) δ = 171.30, 169.57, 147.14, 136.87, 133.28, 127.48, 124.17, 122.69, 119.11, 118.59, 113.51, 110.52, 92.81, 44.96, 20.62.

5-Amino-4-(3-chloro-1H-indol-2-yl)-2-(p-tolylamino)thieno[2,3-d]pyrimidine-6-carboxamide (10)

To an ethanolic solution of compound 9 (4.48 gm, 10 mmol), sodium ethoxide (0.5 M) was added in a dropwise manner, and the mixture was stirred for 30 min at ambient temperature. The produced yellow crystals were collected, filtrated off, and dried in air to afford the desired product in 80% yield. m.p.: 223–225 °C, which is in agreement with the data reported for this compound.[17]

9-(3-Chloro-1H-indol-2-yl)1,3-dihydro-7-(p-tolylamino)-4-oxo-2,2-pentamethylenepyrimido [4′,5′:4,5]thieno[2,3-d]pyrimidine (11)

A mixture of amino carboxamide compound 10 (2 gm, 5 mmol) and cyclohexanone (0.6 gm, 6 mmol) was fused in the presence of few drops of acetic acid for 30 min. The solid product that formed on the cold was filtered off, dried, and recrystallized from dioxane as a pale-yellow crystal in 80% yield, m.p.: 274–276 °C. IR: ν (cm–1) 34 20, 3162, 3046, 2926, 1654. 1H NMR (400 MHz, DMSO): δ 11.96 (s, 1H, NH indole), 10.09 (s, 1H, NH p-tolyl), 7.74 (d, J = 6.8 Hz, 2H, Ar–H), 7.69 (m, 3H, Ar–H and NHCO), 7.63 (m, 2H), 7.14 (d, J = 8.0 Hz, 2H, Ar–H), 4.66 (s, 1H, NH), 2.27 (s, 3H, CH3), 1.95 (d, J = 11.4 Hz, 2H, cyclohexyl), 1.44 (dd, J = 27.3, 14.2 Hz, 5H, cyclohexyl), 0.99 (dd, J = 69.4, 9.6 Hz, 3H, cyclohexyl). 13C NMR (101 MHz, DMSO) δ = 172.03, 164.39, 161.43, 157.88, 142.81, 137.64, 137.15, 131.70, 130.55, 129.50, 129.01, 128.86, 120.03, 113.54, 99.83, 69.97, 36.05, 24.63, 21.77, 20.89.

9-(3-Chloro-1H-indole-2-yl)-4-oxo-7(p-tolylamino)-(3H)-pyrimido[4′,5′:4,5]thieno[2,3-d]pyrimidine (12)

A mixture of amino carboxamide derivative 10 (3.0 gm, 7 mmol) dissolved in triethyl orthoformate (20 mL) and some drops of glacial ethanoic acid was refluxed for 2 h, and the crystals that formed while heating were filtrated off, washed with ethanol, and dried. Then, ethanol was used for recrystallization to produce yellow crystals in 81% yield, m.p.: 352–354 °C. IR: ν (cm–1) 3288, 3057, 2963, 1663. 1H NMR (400 MHz, DMSO) δ = 11.91 (s, 1H, NH indole), 10.16 (s, 1H, NH p-tolyl), 8.12 (s, 1H, NHCO), 7.86 (d, J = 6.8 Hz, 2H, Ar–H), 7.72 (d, J = 8.3 Hz, 2H, Ar–H), 7.63–7.40 (m, 3H, Ar–H and CH pyrimidine), 7.15 (d, J = 8.2 Hz, 2H, Ar–H), 2.28 (s, 3H, CH3). 13C NMR (101 MHz, DMSO) δ = 173.05, 158.45, 154.62, 151.48, 131.11, 130.78, 129.54, 127.69, 120.28, 115.33, 66.82, 43.15, 20.92.

2-Chloromethyl-3H-7-(p-tolylamino)-4-oxo-9-(3-chloro-1H-indol-2-yl)pyrimido [4′,5′:4,5]thieno[2,3-d]pyrimidine (13)

A mixture of amino carboxamide derivative 12 (3.50 gm, 7 mmol) and chloroacetyl chloride (5 mL) was heated at reflux in dioxane on a water bath for 3 h, then poured into cold water (100 mL), and neutralized with sodium carbonate solution (10%) to be just alkaline. The solid product was filtrated off, dried, and recrystallized from dioxane as pale-yellow crystals in 60% yield, m.p.: 355–357 °C. IR: ν (cm–1) 3372, 3256, 3056, 2974, 1668. 1H NMR (400 MHz, DMSO) δ = 13.15 (s, 1H, NH), 12.08 (s, 1H, NH indole), 10.24 (s, 1H, NH p-tolyl), 7.93–7.16 (m, 8H, Ar–H), 4.43 (s, 2H, CH2), 2.29 (s, 3H, CH3). 13C NMR (101 MHz, DMSO) δ = 172.82, 165.75, 158.30, 151.99, 148.03, 137.52, 137.13, 131.81, 130.73, 130.44, 129.45, 129.15, 127.79, 120.13, 117.55, 115.64, 56.53, 20.89. Mass spectrum m/z = 506.6.

9-(3-Chloro-1H-indole-2-yl)-4-oxo-7-(p-tolylamino)-2-phenylmethylpyrimido[4′,5′:4,5]thieno[2,3-d]pyrimidine (14)

For 5 min, a mixture of chloromethyl derivative 13 (4.13 gm, 10 mmol) and aniline (20 mmol) was fused followed by the addition of ethanol (20 mL). After 2 h of refluxing, the solid product produced on heat was filtered off, dried, and recrystallized as yellow crystals from dioxane in an 80% yield, m.p.: 301–302 °C. IR: ν (cm–1) 3245, 3185, 3026, 2969, 1643. 1H NMR (400 MHz, DMSO) δ = 12.68 (s, 1H, NH), 11.98 (s, 1H, NH indole), 9.92 (s, 1H, NH p-tolyl), 7.39–6.57 (m, 13H, Ar–H), 6.16(s, 1H, NH anilino), 4.21 (s, 2H, CH2), 2.32 (s, 3H, CH3).

9-(3-Chloro-1H-indole-2-yl)-4-oxo-7-(p-tolylamino)-2-morphlinomethylpyrimido [4′,5′:4,5]thieno[2,3-d]pyrimidine (15)

A mixture of chloromethyl derivative 13 (4.13 gm, 10 mmol) and morpholine (20 mmol) was fused for 5 min, and then ethanol (20 mL) was added. The mixture was refluxed for 2 h, the solid product that formed on heat was filtrated off, dried, and recrystallized from dioxane as yellow in 81% yield, m.p.: 310–311 °C. IR: ν (cm–1) 3473, 3327, 3057, 2961, 1674. 1H NMR (400 MHz, DMSO) δ = 12.65 (s, 1H, NH), 11.94 (s, 1H, NH indole), 10.21 (s, 1H, NH p-tolyl), 7.97–6.93 (m, 8H, Ar–H), 3.80–3.78 (t, 4H, 2CH2 morpholine), 3.33 (s, 2H, CH2), 2.74–2.72 (t, 4H, 2CH2 morpholine), 2.33 (s, 3H, CH3). 13C NMR (101 MHz, DMSO) δ = 173.20, 157.80, 155.92, 155.05, 150.54, 149.95, 139.02, 136.85, 132.24, 131.95, 129.75, 129.75, 127.62, 126.03, 124.51, 121.47, 119.81, 118.86, 118.86, 117.84, 112.90, 112.55, 66.82, 66.82, 57.83, 52.76, 52.76, 21.13.

4-Chloro-7-(p-tolylamino)-9-(3-chloro-1H-indol-2-yl)pyrimido[4′,5′:4,5]thieno[2,3-d]pyrimidine (16)

A mixture of compound 12 (2.0 gm, 4 mmol) in phosphorylchloride (10 mL) and phosphorus pentachloride (2.0 gm) was refluxed for 3 h on a water bath, after which it was allowed cooling to room temperature before being poured into a 100 mL mixture of ice-cold water and neutralized with sodium carbonate solution. The solid product was filtered out, dried, and purified by recrystallization from dioxane to obtain white crystals with a yield of 62.5%, m.p.: 228–230 °C. IR: ν (cm–1) 3396, 3069, 2967. 1H NMR (400 MHz, DMSO) δ = 11.92 (s, 1H, NH indole), 10.52 (s, 1H, NH p-tolyl), 8.87 (s, 1H, CH pyrimidine), 7.92 (d, J = 7.3 Hz, 2H, Ar–H), 7.60 (m, 4H, Ar–H), 7.18 (d, J = 8.1 Hz, 2H, Ar–H), 2.29 (s, 3H, CH3). 13C NMR (101 MHz, DMSO) δ = 166.80, 155.08, 145.89, 143.32, 142.75, 130.48, 129.60, 128.11, 127.88, 120.72, 120.23, 20.94.

4-Mercapto-7-(p-tolylamino)-9-(3-chloro-1H-indol-2-yl)pyrimido[4′,5′:4,5]thieno[2,3-d]pyrimidine (17)

For 6 h, a mixed solution of compound 16 (2.0 gm, 4 mmol) and thiourea (0.4 gm, 6 mmol) was refluxed in 30 mL of ethanol. The yellow thiouronium salt generated during the heating process was filtered out and washed with dioxane, then treated with 10% NaOH, and acidified with dilute HCl. The yellow crude product was filtered, washed repeatedly with water, dried in the air, and recrystallized as yellow crystals from dioxane in an 89% yield, m.p.: 297–299 °C. IR: ν (cm–1) 3434, 3025, 2964, 1268. 1H NMR (400 MHz, DMSO) δ = 14.28 (s, 1H, NH), 11.98 (s, 1H, NH indole), 10.34 (s, 1H, NH p-tolyl), 8.30 (s, 1H, CH pyrimidine), 7.86 (d, J = 7.0 Hz, 2H), 7.78–7.45 (m, 4H), 7.16 (d, J = 7.8 Hz, 2H), 2.28 (s, 3H). 13C NMR (101 MHz, DMSO) δ = 175.07, 174.08, 167.02, 158.44, 147.48, 146.93, 137.26, 136.88, 132.26, 131.96, 130.66, 129.54, 127.97, 120.43, 115.45, 20.93. Mass spectrum m/z = 474.3.

4-Hydrazino-7-(p-tolylamino)-9-(3-chloro-1H-indol-2-yl)pyrimido[4′,5′:4,5]thieno[2,3-d]pyrimidine (18)

A mixture of chloropyrimidine derivative 16 (3.0 gm, 6 mmol) was dissolved in hot dioxane (20 mL), and then hydrazine hydrate (0.5 mL, 10 mmol) was added. The mixture was refluxed for 2 h. The solid product formed after cooling was filtrated off, dried, and recrystallized from dioxane as white crystals in 93% yield, m.p.: 280–282 °C. IR: ν (cm–1) 3395, 3287, 3202, 3050, 2918. 1H NMR (400 MHz, DMSO) δ = 11.78 (s, 1H, NH indole), 10.11 (s, 1H, NH p-tolyl), 9.00 (s, 1H, NH hydrazinyl), 8.20 (s, 1H, CH pyrimidine), 7.91–7.16 (m, 8H, Ar–H), 4.89 (s, 2H, NH2), 2.29 (s, 3H, CH3). 13C NMR (101 MHz, DMSO) δ = 175.19, 165.14, 160.70, 158.69, 154.83, 154.58, 137.89, 137.71, 131.51, 130.72, 130.28, 129.48, 127.71, 119.97, 114.36, 106.68, 20.90. Mass spectrum m/z = 472.2.

7-(p-Tolylamino)-9-(3-chloro-1H-indol-2-yl)-4-phenylaminopyrimido[4′,5′:4,5]thieno[2,3-d]pyrimidine (19)

A mixture of chloropyrimidine derivative 16 (3.0 gm, 6 mmol) and aniline (0.6 mL, 6 mmol) was fused for 5 min, and then ethanol (20 mL) was added. The mixture was refluxed for 2 h. The solid product formed on heat was filtrated off, dried, and recrystallized from dioxane as yellow crystals in 80% yield, m.p.: 260–262 °C. IR: ν (cm–1) 3207, 3154, 3098, 3053, 2923. 1H NMR (400 MHz, DMSO) δ = 11.93 (s, 1H, NH indole), 9.61 (s, 1H, NH p-tolyl), 9.05 (s, 1H, NH anilino), 8.63 (s, 1H, CH pyrimidine), 7.58–6.84 (m, 13H, Ar–H), 2.33 (s, 3H, CH3).

7-(3-Chloro-1H-indol-2-yl)-9-(p-tolylamino)-[1,3,4]triazolo[1″,5″:1′,6′]pyrimido[4′,5′:4,5]thieno[2,3-d]pyrimidine (20)

A few drops of acetic acid was added into a mixture of hydrazinyl compound 18 (1.5 gm, 3 mmol) in triethyl orthoformate (5 mL). The solution was refluxed for 1 h. The pale yellow produced on heating was filtered off, dried, and recrystallized as pale-yellow crystals from acetic acid in a 75% yield, m.p.: 310–312 °C. IR: ν (cm–1) 3435, 3080, 2974. 1H NMR (400 MHz, DMSO) δ = 12.05 (s, 1H, NH indole), 10.25 (s, 1H, NH p-tolyl), 8.12 (s, 1H, CH pyrimidine), 7.87–7.03 (m, 8H, Ar–H), 6.04 (s, 1H, CH pyrazole), 2.22 (s, 3H, CH3). 13C NMR (101 MHz, DMSO) δ = 172.90, 165.76, 158.52, 158.23, 156.71, 151.93, 140.13, 136.69, 131.14, 130.76, 130.60, 129.05, 127.82, 127.46, 122.79, 119.97, 115.64, 114.79, 21.64.

9-(3-Chloro-1H-indole-2-yl)-4-(3,5-dimethylpyrazol-1-yl)-7-(p-tolylamino)-pyrimido [4′,5′:4,5]thieno[2,3-d]pyrimidine (21)

A mixture of hydrazinyl compound 20 (2.0 gm, 4 mmol) and acetyl acetone (0.5 mL, 5 mmol) was refluxed in dioxane (20 mL) for 3 h. After cooling the reaction mixture, the solid precipitate produced was collected, filtered out, dried, and recrystallized in 75% yield from dioxane as white crystals, m.p.: 220–222 °C. IR: ν (cm–1) 3465, 3061, 2926. 1H NMR (400 MHz, DMSO) δ = 11.87 (s, 1H, NH indole), 10.28 (s, 1H, NH p-tolyl), 8.64 (s, 1H, CH pyrimidine), 7.87–7.03 (m, 8H, Ar–H), 6.28 (s, 1H, CH pyrazole), 2.75 (s, 3H, CH3), 2.38 (s, 3H, CH3), 2.24 (s, 3H, CH3). 13C NMR (101 MHz, DMSO) δ = 169.21, 165.91, 162.33, 158.71, 156.81, 152.06, 147.60, 143.01, 140.44, 137.58, 135.18, 131.09, 129.14, 127.56, 122.73, 120.05, 116.21, 116.03, 111.81, 105.31, 21.69, 13.36, 12.12.

Quantum-Chemical Calculations

To acquire information about the structure of molecules 11 and 12, quantum-chemical computations were done. The ground states were optimized via DFT with B3LYP functional, D3BJ correction, and the basis set 6-31+G(d,p).[51,52] This dispersion correction is needed to explain non-covalent and long-range interactions such as π–π stacking and hydrogen bonding, which are critical in this work and are required to achieve realistic intermolecular interactions.[53] For the ground state geometry, many conformers were investigated, and the one with the minimum energy was chosen. Harmonic vibrational frequency validated this global minimum. The binding energies (ΔEb) of dimers were estimated using the relationship: ΔEb = Edimer – 2Emonomer.

UV–vis absorption spectra were detected using a JASCO V-550 absorption spectrometer. A Hitachi F-7000 fluorescence spectrometer equipped with a Hamamatsu R928 photomultiplier detector was used to record the steady-state photoluminescence spectra. Φem were measured in powder form using a calibrated integrating sphere (Hitachi). The powder luminescence behavior at various temperatures was investigated by placing it in a thin quartz cell (1 × 10 × 20 mm) set on a homemade heating stage. The decay patterns and emission lifetimes of 11 and 12 were measured at λ = 545 and 477 nm, respectively, using a Quantaurus-Tau photoluminescence lifetime measurement device (C1136-21, Hamamatsu) with λex = 340.

Conclusions

Different heterocyclic compounds containing both indole and thienopyrimidine moieties have been successfully synthesized from 5-amino-4-(3-chloro-1H-indol-2-yl)-2-(p-tolylamino)thieno[2,3-d]pyrimidine-6-carboxamide (10). Spectral analysis was used to confirm the structures of all novel compounds. Due to the dependence of the luminescence spectrum on the aggregate structure, the photoluminescence behavior of two homologous dyes displayed opposing emission behavior upon aggregation of molecules. DFT simulations indicated that introducing a terminal group may substantially modify molecular packing and hence the emission spectrum by encouraging or suppressing the development of specific intermolecular interactions such as π–π interactions, and as a result of which, the emission spectrum exhibits a redshift or blueshift. Because of the short intermolecular distance among the dimers of the two materials (less than 3.3 Å), which makes it difficult for aromatic rings to rotate around single bonds, the materials demonstrated excellent Φem in the solid state (25.6 and 9.7%). Non-planarity and intermolecular interactions can also open up new routes for radiative decay in the solid state. At high temperatures, the compounds also demonstrated a stable emission characteristic.