Preparation and photophysical properties of quinazoline-based fluorophores

The donor–acceptor design is a classic method of synthesizing new fluorescent molecules. In this study, a series of new fluorescent compounds (1–10) were synthesized based on 2-(3,5-bis(trifluoromethyl)phenyl)-quinazoline acceptor and various amino donors. The fluorescent emissions of 1–10 cover the spectrum from 414 nm to 597 nm in cyclohexane solutions with various amino donors on 4- or 7-positions of quinazoline. Ultimately, compounds 1 and 2 presented the highest photoluminescence quantum yield (QY) over 80%, while compound 10 provided the largest Stokes shift (161 nm) in cyclohexane. Most of them have strong emissions in aggregated states such as in nanoparticles, in powders, in crystals and in films. Mechanochromic properties were observed for compounds 1, 2, 4 and 7. Furthermore, blue OLEDs were fabricated by using compound 2 or 7 as the active layer.


Introduction
Quinazoline is one of the most attractive heterocycles in alkaloids.][3][4][5] For instance, some quinazoline-based compounds exhibited effective and selective CYP1A2 inhibition and might function in cancer chemoprevention. 6CID 9998128 (2,4-disubstituted quinazoline) has been revealed as a good candidate for Alzheimer's disease (AD) treatment. 7][13][14] Recently, quinazoline was successfully functioned as an anchor to prevent the stacking between uorophores and turned aggregation-caused quenching (ACQ) to aggregation-enhanced emission (AEE). 15Covalently bonded with coumarin, the coumarin-quinazoline dyad displayed solvatochromic and ACQ properties.Taking advantage of this disaggregation-induced emission (DIE), a turn-on uorescent probe was designed and was capable of probing parallel G4 DNA topologies over other G4 DNA structures. 16However, in contrast to frequently used electron-decient benzothiadiazole (BTD), 17 the uorescent compounds based on quinazolines largely remained immature.The same goes for the uorescent structure-property relationship (FSPR) 18,19 studies which are instructive in the development of new uorophores with a unique application.
The multicomponent reaction provided a new avenue in the construction of quinazolines with various substituents in a single step and made the investigation and establishment of FSPR practical and possible. 20For instance, assisted by CuCl 2 , 2,4-diarylquinazolines could be effectively prepared from 2methylquinoline, 2-aminobenzophenome, ammonia acetate and oxygen in a single step. 212,3-Dihydroquinazolin-4(1H)-ones could be efficiently prepared by three component reaction from isatoic anhydride, aniline and arylaldehyde in the presence of Al 2 (SO 4 ) 3 with a variety of functional groups. 22In the presence of Cu(OTf) 2 , a [2 + 2 + 2] cascade annulation of diaryliodonium salts with two nitriles produced 2,4-disubstituted quinazolines in excellent yields. 23Mediated by 4-hydroxy-TEMPO radical 24 or iodine, 25 2-aryl-quinazolines could be obtained from arylmethanamines, 2-aminobenzoketones and oxidant.Here, we report the preparation procedure for 2,4-diarylquinazolines and 2,4,7-triarylquinazolines and their uorescent properties as well as their applications in OLED.

Synthesis of compounds 1-10
The preparatory routes leading to compounds 1-10 were shown in Fig. S1 † and structures of intermediates A and B as well as the structures of compounds 1-10 were presented in Scheme 1. Key intermediates A and B were synthesized according to the literature reporting palladium-catalyzed three components reaction starting from arylboronic acids, cyanoanilines and arylaldehydes. 26In this way, A and B were prepared in yields of 60% and 70%, respectively.Subsequent Buchwald-Hartwig coupling of A RSC Advances PAPER or B with various secondary aromatic amines afforded target molecules 1-10 in yields varying from 84% to 99%.Structures of these synthesized compounds (A, B, 1-10) were conrmed by 1 H NMR, 13 C NMR and 19 F NMR (Fig. S2-S37 †).Further conrmation was made by HRMS and IR as well as single crystal analysis of compounds 1-4 (Table S1 †).These compounds are thermally stable (Fig. S38 † and Table 1).The decomposition temperatures were determined to be 369 C, 380 C, 382 C, 363 C, 383 C, 377 C, 342 C, 379 C, 351 C and 393 C for compounds 1-10, respectively.
Compounds 1-10 are highly uorescent in dilute solutions.Compound 1 emits light at 414 nm with Stokes shi of 45 nm (2982 cm À1 ) and quantum yield of 84.67%.As the amino group Scheme 1 Key intermediates A, B and compounds 1-10.

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changed to better electron-donating groups, compounds 2, 3, 4 and 5 emit lights at 450, 500, 514 and 575 nm with Stokes shis of 47 (2592), 85 (4079), 94 (4354) and 131 nm (5152 cm À1 ), respectively (Fig. 1a).The highest emission quantum yield (87.59%) was found for compound 2 with MPA occupied on the 4-position of quinazoline.Similar results were observed for compounds 6-10.These compounds emit lights at 412, 458, 519, 533 and 597 nm with Stokes shis of 15 (930), 58 (3153), 108 (5063), 77 (3168) and 161 nm (6185 cm À1 ), respectively (Fig. 1b).Among this series, compound 7 presented the highest emission quantum yield (43.32%).To both series, emission spectra t the single exponential function with uorescence lifetime varying from 3.29 to 8.36 ns except for compounds 4 and 9 with POZ substituted (Fig. S41 †).The decay trace is biexponential with lifetimes of 1.32 ns (40.4%) and 9.71 ns (59.6%) for the emission band at 514 nm of compound 4. The average lifetime was calculated to be 7.62 ns.The average life is calculated using the literature method. 27or compound 9, lifetimes of 1.18 ns (52.9%) and 9.06 ns (47.1%) were recorded and the average lifetime was calculated to be 5.98 ns.That both 4 and 9 exhibited emissions with bicomponents might be the respective decays from S 1 and T 1 due to the phenoxazine-caused steric repulsion between hydrogens at donor-acceptor linkage and the resulted large dihedral angle between donor and acceptor. 28Tuning the donor on the 4-position of quinazoline affected the absorption spectra to a greater extent, while the larger Stokes shis were observed for those compounds with the donor on 7-position except for compounds 6 and 9. Finally, compound 10 provided the largest Stokes shi with a value of 161 nm in cyclohexane.

Absorption and emission of 1-10 in various solvents
These compounds are soluble in normal organic solvents, such as cyclohexane (CH), toluene (TOL), dioxane (DIO), tetrahydrofuran (THF), dichloromethane (DCM) and acetonitrile (MeCN).Blue-shied absorption and red-shied emission of these D-A compounds were observed as the solvent polarity increased from cyclohexane to toluene, dioxane, tetrahydrofuran, dichloromethane and acetonitrile (Fig. S39, S40 and Table S2 †).Meanwhile, rainbow emissions were recorded and could be clearly photographed under UV light as the solvent polarity changes.As a typical example, compound 1 absorbs light at 338/369, 340/363, 339/358, 339/354, 340/354 and 339 nm (Fig. 2a), while emits light at 414, 453, 461, 495, 501 and 548 nm in CH, TOL, DIO, THF, DCM and MeCN, respectively (Fig. 2b).The absorption at the lower energy gap of 1 disappeared as the solvent changed to MeCN.Finally, the largest Stokes shi (209 nm) was found for 1 in MeCN. 29As the solvent polarity increases, the HOMO/LUMO levels of these D-A compounds decrease through the salvation of compounds both in ground states and in excited states.However, in a more polar solvent, a larger energy gap was found for D-A compounds in ground state because of the more stabilization of the HOMO level, while a smaller energy gap for D-A compounds in excited states was observed because of the more stabilization of the LUMO level.Thus, blue-shied absorption and red-shied emission were presented for these D-A compounds as the solvent polarity increases.

Emission of 1-10 in aggregates
Nanoparticles were prepared by quickly pouring water into their dilute THF solutions and their emission spectra were recorded accordingly (Fig. S42 and S43 †).The shape and size were photographed by TEM (Fig. S44 †) and recorded by DLS as well.Emissions of aggregates could be classied into two categories in general.For those compounds with strong uorescence in dilute solutions, such as compounds 1, 2, 6 and 7, aggregationcausing quenching (ACQ) was rstly observed and followed by aggregation-induced emission enhancement (AIEE) as the water fraction increases.As the water fraction increase to 60%, the emission of compound 2 red-shied to 564 nm and the emission intensity decreased to 5.1% of its original value (Fig. 3a).ACQ was observed in this stage.Further increment of water fraction to 95%, a steady blue-shi emission as well as the increment of the emission intensity was observed.Nanoparticles formed in ball shape with the diameter about 100 nm, determined by DLS (Fig. 3b) and conrmed by TEM (Fig. 3c).A similar situation was observed for compounds 1, 6 and 7. To those compounds with weak or non-uorescent in dilute THF solutions (3-5, 8-10), increment of water fraction induced emission enhancement signicantly.Compound 3 presented the typical aggregation-induced emission (AIE). 30As the water fraction increased to 60%, the uorescence of compound 3 was turned on.Beyond this point, a steady increment of emission intensity was recorded.As the water fraction reached 90%, the emission intensity was magnied 21.6 times of its initial.

Emission of 1-10 in powders
These compounds are also strongly uorescent in powders (Fig. 4 and Table 2).As the electron-donating group on 4- position of quinazoline changes from CZ to PA, MPA, and POZ, the maximum emission wavelengths in powders were determined to be 450, 495, 558 and 539 nm, respectively (Fig. 4a).
When DMPA occupied on the 4-position of quinazoline, the maximum emission wavelength of 5 in powder was determined to be 467 nm with a shoulder peak (620 nm) at a larger energy gap.A colourful solid emission was photographed under UV light, altering the colour from bright blue to green, orange, yellow and nally to red, as the secondary aromatic amine changed on the 4-position of quinazoline (Fig. 4c).Emission quantum yields were determined to be 56.55%,49.91%, 28.03%, 27.51% and 3.08% with the lifetimes of 4.34, 2.86, 5.44, 15.04 and 2.45 ns for compounds 1-5, respectively.For the series of electron-donating compounds on the 7-position of quinazoline, compounds 6-9 exhibited similar emissions with the colour changed from bright blue to red, but with relatively lower quantum yields, accordingly (Fig. 4b and c).As for compound 10, the emission presented weak and cannot be detected.

Emission of 1-10 in lm
The transparent lms of these compounds were fabricated by dipping their dilute methylene chloride solutions onto quartz plates and let it evaporate to dry aerwards.For the series of amines occupied on the 4-position of quinazoline, emissions of compounds 1-5 in lms were red-shied in contrast to emissions of these compounds in powders, while quantum yields decreased accordingly too (Table 2).Film emission of compound 4 was bi-exponential with the combination of 0.48 ns (60.8%) and 3.94 ns (39.2%).The average lifetime was calculated to be 2.62 ns.Film of compound 5 presented weak uorescence and the quantum yield was not detectable.For the series of the substituent on 7-position of quinazoline, lms of compounds 6 and 7 exhibited larger quantum yields with respect to those in powders.Film emission of compound 9 also showed bi-exponential with the combination of 0.65 ns (6.5%) and 5.28 ns (93.5%).The average lifetime was calculated to be 4.50 ns.

Emission of 1-4 in crystalline
Four single crystals (1-4) were obtained by slow evaporation of the dilute solutions of dichloromethane and hexane.The single crystal data were shown in Table S1.† Four single crystals are all granular, of which compound 1, 3 and 4 are clusters of small particles, and compound 2 are large regular crystals.)

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As shown in Fig. 5, the dimers of 1, 4 are stacked together in an anti-parallel manner that might be the nature of the planarity of carbazole in 1 and phenoxazine in 4. Intermolecular p-p stacking between carbazole and quinazoline in molecule 1 was observed at a distance of 3.690 A. A shorter distance (3.545 A) in 4 was measured between phenoxazine and quinazoline which might be the dislocated alignment arising from the larger dihedral angle (72.71 ) between phenylene and phenoxazine in comparison with that in 1 between phenylene and carbazole (40.84 ).Finally, multiple C-H/p and C-H/F interactions hold molecules together.The stacking of molecule 4 was additionally assisted by the methylene chloride solvent in which C-H/Cl interactions were seen.Single crystal 1 emits light at 468 nm which is similar to its lm emission, while single crystal 4 emits light at 532 nm which is red-shied in comparison with its lm in 506 nm.
Because of the propeller structure of triarylamine, intermolecular p-p stacking between donor and acceptor were not seen in the cases of 2 and 3.The main interactions are intermolecular C-H/p and C-H/F interactions.Thus, both of them showed blue-shied emissions in comparison to their lm emissions.Single crystal 2 emits light at 491 nm while single crystal 3 emits light at 539 nm (Table 2).

Mechanochromic properties of 2 and 4
Some of them showed mechanochromic property, such as compounds 1, 2, 4 and 7 (Fig. 6 and S45 †).While grinding the crystalline of compound 2, the maximum emission wavelength changed from 494 nm to 505 nm and emission colour under UV light changed from blue to green (Fig. 6a).Aer methylene chloride fumigation, maximum emission wavelength changed back to 492 nm while the colour was turning back to blue (Fig. 6a).The cycle of blue and green was reproducible without the loss of the emission intensity (Fig. 6b).SAXS analysis indicated that the phase alternation existed during the grinding and fumigation process (Fig. S46 †).Grinding crystalline of compound 4 altered the maximum emission wavelength from   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 30297-30303 | 30301 brown (Fig. 6a).The colour could be tuned back to yellow by fuming the sample with DCM vapour (Fig. 6b).

Analysis of HOMO/LUMO orbitals of 1-10
Three methods were used to evaluate the HOMO/LUMO orbitals, including UV (Fig. S39 and S40 †) and CV measurements (Fig. S47 †) as well as the theoretical calculation (Fig. 7a) for comparison.Results were listed in Table S3.† Based on the calculation, donor and acceptor are clearly separated by HOMO and LUMO orbital distributions.Energy gaps obtained from CV and UV measurements are in good accordance with the results from calculation (Fig. 7b).Gradually decreased energy gaps from 1 to 5 as well as from 6 to 10 explained the full spectrum emissions of these compounds in solutions (Fig. 1) and in solids (Fig. 4c).

OLED performance
Considering the quantum yields of solids, the atness of the lms and the HOMO/LUMO levels, we chose a pair of compounds 2 and 7 for the OLED fabrication (Fig. S48 †).The devices were structured of ITO/PEDOT: PSS (40 nm)/ compound (65 nm)/TBPI (40 nm)/LiF (1.5 nm)/Al (50 nm) and the device performances were listed in Table S4.† When the pure compound 2 was sandwiched in a thickness of 65 nm, device A emitted light at 509 nm with the EQE of 0.47% and the turn-on voltage of 4.5 V.By doping compound 2 in PVK (2 : PVK ¼ 1 : 4) as emitter, device B emitted light at 513 nm.Increased EQE (1.09%) and decreased turn-on voltage (4.2 V) were recorded.As the ratio of 2 : PVK changed from 1 : 4 to 1 : 8 (device C), the EQE value was further increased to 1.35%.Device D, fabricated with the emitter of 7 in PVK (7 : PVK ¼ 1 : 4) emitted light at 510 nm with the EQE of 0.83% and the turn-on voltage of 4.8 V.

Conclusions
In this work, 4-or 7-donor substituted quinazolines were synthesized for the investigation of the uorescent structureproperty relationship of quinazoline-based donor-acceptor compounds.These compounds are thermally stable and could be dissolved in normal organic solvents which showed the potential applications in devices for the solution-processible.Moreover, these quinazolines-based D-A compounds emit bright light in different states with excellent quantum yields.The emission colour could be nely tuned by the intrinsic substituent, the extrinsic solvent polarity and grinding.Moreover, some of them might be used as the emissive material in the fabrication of OLEDs.
Excited at 380 nm(9), 400 nm(5) and 365 nm (1-4, 6-8 and 10).b According to uorescence decay traces.c Quantum yields were obtained from an integrating sphere.d Calculated by QY ¼ sk r ¼ k r /(k r + k nr ).e Films were prepared by spin-coating a dilute DCM solution on a quartz plate.f Excited at 365 nm.g Emission is too weak to be detected.h No single crystal was obtained.30300 | RSC Adv., 2020, 10, 30297-30303 This journal is © The Royal Society of Chemistry 2020