Synthesis and Photophysical Properties of Colorful Salen-Type Schiff

Article pubs.acs.org/JPCC

Synthesis and Photophysical Properties of Colorful Salen-Type Schiff Bases Jinghui Cheng, Kaiyi Wei, Xiaofeng Ma, Xiangge Zhou, and Haifeng Xiang* Institute of Homogeneous Catalysis, College of Chemistry, Sichuan University, Chengdu 610041, China S Supporting Information *

ABSTRACT: A series of colorful Salen-type Schiff bases derived from the different diamine bridges including 1,2ethylenediamine (Et1−Et3), 1,2-cyclohexanediamine (Cy1−Cy3), 1,2-phenylenediamine (Ph1−Ph4), dicyano-1,2-ethenediamine (CN1−CN3), phenazine-2,3-diamine (Phen1−Phen3), and naphthalene-2,3-diamine (Naph1 and Naph2) have been designed and prepared. The presence of electron-accepting substituents, electron-donating substituents, donor−acceptor (DA) systems, and/or π-extended systems leads to not only full absorption and emission spectra (300−700 nm) in the visible region but also high fluorescence quantum yields up to 0.83 in solution. The experimental results and density functional theory (DFT) calculations have proved that the highest occupied molecular orbital (HOMO) levels, the lowest unoccupied molecular orbital (LUMO) levels, and consequently the energy gaps of these Salen ligands can be well tuned. The LUMO levels of these Salen ligands are mainly affected by the diamine bridges, whereas both the HOMO and LUMO levels are influenced by the phenol fragments. Adding Cu2+ and Co2+ to CN1 solution leads to a drastic color change from pink into brownish red and purple, respectively, which is useful not only for the ratiometric detection but also for rapid visual sensing even by the naked eye. Moreover, the properties of the cis−trans isomer of CN1 are examined. The Salen ligands have coordination chemistry similar to other well-known tetradentate porphyrin ligands as well as much easier preparation and rich photophysical properties.

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INTRODUCTION The condensation of an amine with an aldehyde, forming what is called a Schiff base, is one of the oldest reactions in chemistry.1−8 Especially, Salen, a particular class of tetradentate [O∧N∧N∧O] chelating bis-Schiff base (Figure 1), can be synthesized by the condensation of 1,2-diamine with 2 equiv of salicylaldehyde.9−13 Salen ligands containing two covalent and two coordinate covalent sites in a planar array can coordinate to many metal ions, leaving the two axial sites open for ancillary ligands. In this regard, they are very much like other well-known tetradentate [N∧N∧N∧N] chelating porphyrin ligands. Our previous work focused on the synthesis and optoelectronic properties of porphyrin ligands and their associated metal complexes.14−18 However, Salen ligands, unlike porphyrin ligands, are easy to prepare and inexpensive.11 Due to their easy preparation, reasonable stabilities, biological activities, and rich photophysical properties, Salen ligands and their associated metal complexes have attracted much attention in many fields including catalysts,2,4,9−11 DNA cleavage,19,20 optical materials,1,3,6 magnetic materials,6,12 supramolecular materials,21−27 chemical probes,5,28−44 and cell imaging.45,46 Recently, Salen ligands47,48 and their Zn(II),47,49−52 B(III),53−55 Al(III),56−58 and Pt(II)59−64 complexes with good stabilities and decent emission quantum yields (Φ) have been successfully applied as emitters in organic light-emitting diodes (OLEDs). Moreover, Salen ligands usually containing π-conjugated tetradentate [O∧N∧N∧O] chelating system can coordinate to various metal ions, resulting in the changes of optical properties that © 2013 American Chemical Society

can report the interaction between the ligands and metal ions, and thus they are widely used as optical probes for many metal ions, such as Zn2+,30−34 Mg2+,36 Cu2+,32,37,42 Al3+,37,38 La3+,29 and Pt2+.43 In the present work, we systematically designed and synthesized some new Salen ligands with electron-accepting substituents, electron-donating substituents, donor−acceptor (DA) systems, and/or π-extended systems, as shown in Figure 1. The photophysical properties of this series of Salen ligands with different bridges of 1,2-ethylenediamine (Et1−Et3), 1,2-cyclohexanediamine (Cy1−Cy3), 1,2-phenylenediamine (Ph1−Ph4), dicyano-1,2-ethenediamine (CN1−CN3), phenazine-2,3-diamine (Phen1−Phen3), and naphthalene-2,3-diamine (Naph1 and Naph2) were fully investigated by UV/visible absorption and fluorescent emission spectra and density functional theory (DFT) calculations. The presence of DA systems and/or π-extended systems in these Salen ligands ensures not only full absorption and emission spectra (300−700 nm) in the visible region but also high fluorescence quantum yields up to 0.83 in solution. Then, the colorimetric and fluorescent responses of CN1 toward various metal ions were fully examined. Among many metal ions, only Cu2+ and Co2+ lead to a drastic color change from pink into brownish red and purple, respectively, which is useful not only for the ratiometric detection Received: April 16, 2013 Revised: June 20, 2013 Published: July 22, 2013 16552

dx.doi.org/10.1021/jp403750q | J. Phys. Chem. C 2013, 117, 16552−16563

The Journal of Physical Chemistry C

Article

Figure 1. Chemical structures of Salen ligands used in this work.

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but also for rapid visual sensing even by the naked eye. Moreover, the cis- (CN1-Z) and trans-isomer (CN1-E) of CN1 were obtained, which exhibit similar photophysical properties and isomer tautomerism from CN1-Z into CN1-E under the irradiation of an UV lamp.

RESULTS AND DISCUSSION

Synthesis and Characterization. The general method of preparation of Schiff bases is quite straightforward and consists of the condensation reaction of primary amines with an aldehyde precursor usually in alcoholic solution or toluene and sometimes 16553



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CN1-Z and CN1-E. On the other hand, the 1H NMR signals belonging to the OH protons are readily distinguishable (CN1-Z: δ = 12.26 ppm, CN1-E: δ = 11.71 ppm in CDCl3). The large downfield shift of the OH proton also supports strong intramolecular hydrogen bonding for both CN1-Z and CN1-E. The UV/visible absorption and emission spectra of both isomers of CN1 were acquired in MeCN at room temperature and are depicted in Figure S3 (Supporting Information). Evidently, despite the similarity in the lowest-lying transition (S0−S1) in the region of 565 nm, salient differences were observed around 300−500 nm, in which CN1-Z exhibits much stronger highenergy absorption bands at about 431 and 375 nm than CN1-E. Moreover, both isomers emit similar strong red fluorescence with Φ of 0.83 and 0.80 for CN1-Z and CN1-E in MeCN, respectively. Under the irradiation, the isomer tautomerism was observed from CN1-E into CN1-Z within about half an hour and 5 h by UV lamp and room light, respectively (Figure S4, Supporting Information). No such isomer tautomerism was found in solid samples. All these experimental phenomena about the isomers are consistent with Lin’s previous report.65 Using the same method, we failed to isolate the cisand trans-isomer of CN2 and CN3. However, since either cis- or trans-isomers that are photothermally instable and isomerized have similar photophysical properties, we would subsequently discuss their photophysical properties without the distinction of cis- and transisomers. With the sulfonate groups, Ph4 has a good stability and solubility in water and was fully studied in our previous report.42 Electronic Absorption Spectroscopy. The UV/visible absorption spectral data of all the Salen ligands are listed in Table 1. As examples, the normalized absorption spectra of Et3, Cy3, Ph3, Ph2, Cy2, CN3, Phen2, Phen3, and Naph2 in MeCN are given in Figure S5 (Supporting Information). Et3, Cy3, and Ph3 containing the same phenol and different bridge of 1,2ethylenediamine, 1,2-cyclohexanediamine, and 1,2-phenylenediamine, respectively, have relatively intense absorption bands centered at 320−350 nm, which are assigned to the π → π* transition involving molecular orbitals essentially localized on the CN group and the benzene ring. The lower intensity absorption bands in the 380−420 nm region are assigned to the n → π* transition involving molecular orbitals of the CN chromophore and the benzene ring.30,32,37,42,43,71−73 It is well-known that the π-conjugation enhancement should lead to a considerable decrease in the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), resulting in a red-shifting absorption and emission spectrum. Et3 and Cy3 containing the similar π-nonconjugated bridges exhibit similar absorption spectra, but both Ph3 containing the π-conjugated bridge and Cy2 containing the π-nonconjugated bridge and naphthalenol have a stronger and red-shifted absorption spectrum compared with Et3 and Cy3. Further extending the π-conjugated system, the absorption peaks of Ph2 and Naph2 are red-shifted up to 465 and 475 nm, respectively. Compared with Ph3, CN3 containing the π-conjugated bridge of dicyano-1,2ethenediamine shows the red-shifted absorption due to the effect from the strong electron-accepting substituents of the −CN group. A similar red shift is also observed in the presence of strong electron-donating substituents of the −NEt2 group in Ph1 (Figure 3). Moreover, Phen2 containing the big π-conjugated bridge of phenazine-2,3-diamine exhibits an obvious red-shifted absorption spectrum as expected. All these data indicate that extending the π-conjugated system of Salen ligands is a more efficient way to red shift the absorption spectrum than introducing the electron-accepting or electron-donating substituents.

under reflux conditions, as shown in Figure S1 in the Supporting Information. The reaction is acid catalyzed, but catalysts are not generally required when aliphatic amines are involved. To achieve high yield, the water produced in the reversible reaction can be removed by azeotropic distillation, when conducting the synthesis in toluene solution. Finally, the condensation of primary 1,2-ethylenediamine with 2 equiv of salicylaldehyde precursor in ethanol under refluxing conditions was adopted for the preparation of Salen ligands in this work. Usually, to avoid the degradation of Schiff bases during the purification step through hydrolysis, it is better to purify Schiff bases by recrystallization (e.g., CH2Cl2/hexane or CH2Cl2/EtOH) than chromatography. Unlike many free Schiff bases that are not always stable in solution and need to be prepared by template synthesis in the presence of a metal ion, most of the Salen ligands in this work are stable either in solution or in the solid state under air for synthesis, characterization, and application. In theory, the Salen ligands (CN1−CN3) containing the maleonitrile CC double bond have cis- and trans-isomers, but there are still rare studies on these cis- and trans-isomers in the literature, probably due to the difficulty in the isolation of isomers with similar physical and chemical properties and the photothermal instability of either the cis- or trans-isomer. Recently, Lin and co-workers65 first demonstrated that the cis- and transisomer of a Salen ligand, 2,3-bis[4-(di-p-tolylamino)-2-hydroxybenzylideneamino maleonitrile, can be prepared under room light and dim red light, respectively. For CN1, the low temperature (room temperature) and absence of room light are in favor of forming CN1-Z; on the other hand, the high temperature (reflux in EtOH) and presence of room light are in favor of forming CN1-E. However, when recrystallizing CN1 (CN1-Z or CN1-Z) under or keeping out of room light at room temperature, we simultaneously obtained two kinds of mixed crystals. One is dark green crystals (CN1-Z) (single-crystal X-ray diffraction structure, Figure S2, Supporting Information), and the other is dark violet crystals (CN1-E), similar to Lin’s report65 but contrary to many other reports.47,48,66−70 Further recrystallization was consequently carried out but also failed to obtain either of the pure isomers. Fortunately, both the cis- and trans-isomer of CN1 were stable enough on silica to be isolated, for the first time, by chromatography66 using CH2Cl2 or CH2Cl2/EtOAc (3:1) as eluent. To identify the cis- and trans-isomer of CN1, 1H nuclear magnetic resonance (NMR) and UV/visible absorption and emission spectra were recorded for both isolated CN1-Z and CN1-E. As shown in Figure 2, the 1H NMR signals belonging to

Figure 2. 1H NMR spectrum of CN1-Z and CN1-E in CDCl3.

the aromatic protons are complicated and similar for the two isomers, thus they are not convincing enough to distinguish 16554



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Table 1. Photophysical Data of the Salen Ligands at Room Temperature ligand CN1-E

CN1-Z CN2 CN3 Phen1

Phen2 Phen3 Ph1

Ph2 Ph3 Ph4 Et1

Et2 Et3 Cy1

Cy2 Cy3 Naph1

Naph2

λabs/nm (ε /dm3 mol−1 cm−1)

medium MeCN DMF CH2Cl2 MeCN MeCN MeCN solid MeCN DMF CH2Cl2 MeCN MeCN MeCN DMF CH2Cl2 solid MeCN solid MeCN solid H2O MeCN DMF CH2Cl2 solid MeCN solid MeCN solid MeCN DMF CH2Cl2 solid MeCN solid MeCN MeCN DMF CH2Cl2 MeCN

339(1.01 × 10 ); 565 (4.38 × 104) 338 (8.37 × 103); 439 (5.63 × 103); 571 (4.87 × 104) 437 (9.33 × 103); 570 (8.85 × 104) 374 (1.12 × 104); 433 (1.01 × 104); 564 (4.63 × 104) 520 (1.90 × 104) 324 (2.03 × 104); 374 (2.95 × 104) 4

317 (1.34 × 104); 451 (2.28 × 104) 330 (8.05 × 103); 488 (1.69 × 104) 476 (2.04 × 104) 420 (1.99 × 104) 403 (2.18 × 104) 370 (3.59 × 104); 442 (2.20 × 104) 445 (1.94 × 104); 490 (1.28 × 104) 413 (3.58 × 104); 442 (3.32 × 104) 310 (1.72 × 104); 366 (1.58 × 104); 429 (1.01 × 104) 335 (1.06 × 104); 414 (1.69 × 103) 301 (8.42 × 103); 322 (8.42 × 103); 337 (7.71 × 103); 371 (4.31 × 103) 339 (1.17 × 104) 353 (2.35 × 104) 344 (3.21 × 104) 294 (4.61 × 103); 396 (4.42 × 103); 408 (8.84 × 103) 352 (2.45 × 103); 396 (1.16 × 103) 310 (4.9 × 104); 376 (6.0 × 104) 336 (2.19 × 104); 410 (1.20 × 104) 327 (3.62 × 104); 403 (1.03 × 104) 296 (2.18 × 104); 397 (9.82 × 103) 275 (2.10 × 104); 356 (2.86 × 103); 400 (2.01 × 103) 329 (9.34 × 103); 403 (2.45 × 104) 340 (9.86 × 103); 416 (2.44 × 104) 329 (1.01 × 103); 407 (2.91 × 104); 447 (1.28 × 103) 321 (2.38 × 104); 376 (2.64 × 104); 449 (1.2 × 104); 474 (1.06 × 104)

λem/nm

Φ

609 618 612 606 560; 625 (sh) 553 555 551 599 568 525 501 502 573 535 562 496 571 458 555 426 415 433 432 467 466 479 461 548 (max); 593; 649 (sh) 426 432 431 468 471 462 435 512 516 539 509

0.83

0.80 0.37 0.017 0.21

0.041 0.34 0.22

0.30 0.21 0.56 0.13

0.12 0.053 0.068

0.078 0.069 0.044

0.042

(Supporting Information), a similar trend was observed in λabs with Et1 ≈ Cy1 < Ph1 < Naph1 < Phen1, consistent with the tendency in extending the π-conjugated system of these Salen ligands. However, even for Phen1, the λabs was found to be 460 nm responding for the yellow color of Phen1 in MeCN, which needs further red shift for the purpose of full-color dyes. Notably, by CN1 functionalization of a DA system based on the π-conjugated backbone it is facile to induce intramolecular charge transfer, resulting in the λabs up to 565 nm and red color. This indicates that the DA system is another efficient strategy of red shifting the absorption band.74,75 By carefully modifying chemical structures, colorful Salen ligands were demonstrated, as shown in Figure 4. Fluorescence Spectroscopy. The fluorescent emission data of the Salen ligands in MeCN and the solid state are listed in Table 1. At room temperature, the Salen ligands in the present work emit red, green, or blue (RGB) light upon the UV excitation, as shown in Figure 4. As examples, the normalized fluorescence emission spectra of Et3, Cy3, Ph3, Ph2, CN3,

Figure 3. Normalized absorption spectra of the Salen ligands with the different bridges and same diethylamino-phenol in MeCN.

The absorption spectra of CN1, Phen1, Naph1, Ph1, Cy1, and Et1 that contain the same diethylamino-phenol and different bridges in MeCN are depicted in Figure 3. Like in Figure S5 16555



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Figure 6. Normalized emission spectra of the Salen ligands with the different bridges and diethylamino-phenol in MeCN at room temperature (excited at 540, 410, 370, 380, 340, and 380 nm for CN1, Phen1, Naph1, Ph1, Cy1, and Et1, respectively).

Figure 4. Photographs (top: under sunlight; bottom: under 360 nm UV light) of the selected Salen ligands in MeCN at room temperature.

Phen3, and Naph2 in MeCN are given in Figure 5. These Salen ligands emit a broad fluorescence band in the region from 400 to

CN1 containing the DA system exhibits unexpectedly high fluorescence quantum yield up to 0.83, and thus the CN1 solution gives bright red emission even in room light (Figure 4). Electronic Structure Calculations. To gain further insight into the nature of the excited states, DFT calculations were carried out for all the Salen ligands with the Gaussian 03 program package (B3LYP 6-31G(d,p)).77 For geometry optimization, the singlecrystal X-ray diffraction structures of selected ligands (phenol-imine form) are required, which were obtained or modified from published data.42,43,78 The spin multiplicities and charges of the Salen ligands were set equal to 1 and 0 (except Ph442), respectively. The theoretical modeling was performed in the isolated molecule approximation ignoring the effect of the aggregation state or solvent. The optimized structures of CN1 (CN1-Z), Phen1, Naph1, Ph1, Cy1, Et1, Ph2, and Ph3 are shown in Figure 7, and other Salen ligands are shown in Figure S6 (Supporting Information). The optimized geometries of CN1, Ph3, Cy3, and Et3 remain similar to their single-crystal structures.42,43,78 Diagrams of the LUMO+1, LUMO, HOMO, and HOMO−1 for the ground states and the energies of frontier molecular orbitals of the selected Salen ligands are shown in Figures 7 and 8 and listed in Table S1 (Supporting Information), respectively, and those of other Salen ligands are demonstrated in Figure S6 and Table S1 (Supporting Information), respectively. For Et1 and Cy1 with the π-nonconjugated bridge of 1,2-ethylenediamine and 1,2cyclohexanediamine, respectively, the HOMO and LUMO are not composed primarily of the bridges but π-functions on the two phenolic rings (Figure 7), resulting in that Et1 and Cy1 have similar HOMO and LUMO levels (Figure 8), UV/visible absorption spectra, and emission spectra. On the contrary, for Ph1 with the π-conjugated bridge of 1,2-phenylenediamine, the HOMO and LUMO are uniformly distributed in the whole molecule. The HOMO level of Ph1 (−4.80 eV) is a little higher than that of Et1 (−5.02 eV) and Cy1 (−4.99 eV), but the LUMO level of Ph1 (−1.28 eV) is much lower than that of Et1 (−0.82 eV) and Cy1 (−0.71 eV). Therefore, the energy gap between HOMO and LUMO of Ph1 (3.52 eV) is lower than that of Et1 (4.20 eV) and Cy1 (4.28 eV), which is in agreement with experimental data observed in UV/vis absorption and emission spectra. The UV/vis absorption and emission spectra of Ph1 show red shifts compared with those of Et1 and Cy1. A similar trend is observed in Ph3, Cy3, and Et3. Further extending π-conjugated systems in Phen1, Naph1, and CN1 leads to lower energy gaps compared with Ph1.

Figure 5. Normalized emission spectra of the Salen ligands with the different bridges and diethylamino-phenol in MeCN at room temperature (excited at 330, 320, 330, 355, 480, 380, and 360 nm for Et3, Cy3, Ph3, Ph2, CN3, Phen3, and Naph2, respectively).

600 nm, mostly attributed to π → π* transition.30,32,37,42,43,71−73 A trend was observed in λem with Cy3 < Et3 < Ph3 < Ph2 ≈ Phen3 ≈ Naph2 < CN3, which is consistent with the tendency to extend the π-conjugated system of these Salen ligands on the whole but is a little different from the tendency in their absorption spectra (Figure S5, Supporting Information). To classify the relationship between the emission properties and chemical structures, the emission spectra of CN1, Phen1, Naph1, Ph1, Cy1, and Et1 containing the same diethylaminophenol and different bridges in MeCN are depicted in Figure 6. Like in Figure 3, a similar trend was observed in λem with Et1 ≈ Cy1 < Ph1 ≈ Naph1 < Phen1 < CN1, consistent with the tendency to extend the π-conjugated system and the presence of DA systems in these Salen ligands. All fluorescence quantum yields of these Salen ligands in solution were measured by the optical dilute method of Demas and Crosby76 with a standard of quinine sulfate (Φr = 0.55, quinine in 0.05 mol dm−3 sulfuric acid) and are listed in Table 1. The fluorescence quantum yield of CN1 (CN1-Z), Phen1, Naph1, Ph1, Cy1, and Et1 in MeCN is 0.83, 0.21, 0.044, 0.22, 0.068, and 0.13, respectively. Naph1, Cy1, and Et1 with π-nonconjugated bridges have lower fluorescence quantum yields than CN1, Phen1, and Ph1 with π-conjugated bridges. 16556



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is very important not only for well tuning absorption and emission bands but also for fabricating OLEDs because the LUMO and HOMO levels of the light-emitting layer should match with those of the neighboring active layers, such as the electron-transporting layer and hole-transporting layer, to reduce the hole and electron injection barrier, thus lowering the device driving voltages and improving the device performances.79 As depicted in Figure 7 and Figure S6 (Supporting Information), in the frontier molecular orbitals of CN1 (CN1Z or CN-E), the −NEt2 donor is mostly contributed to HOMO and HOMO−1 orbitals, whereas the −CN acceptor is mostly contributed to LUMO and LUMO+1 orbitals, clearly indicating the intense intramolecular charge transfer from the donor end groups to the central acceptor fragment. This DA charge transfer is useful for red shifting absorption and emission bands74,75 and rendering a core structure for designing photoactivatable fluorophores, according to a photoinduced electron transfer mechanism.80,81 It is unexpected that Phen1 containing the bridge of phenazine-2,3-diamine has the biggest π-conjugated system in appearance but blue-shifted absorption and emission spectra compared with CN1. The phenazine comprises a nitrogen-containing heterocyclic ring, so the nature of high electron affinity and rigid π-conjugated backbone of phenazine should be taken into account.82−84 In contrary to Naph1 and Ph2 in which the acenes efficiently and uniformly expand the π-functions of both HOMO and LUMO orbitals in the whole molecule, Phen1 only exhibits the partial expansion effect in the phenazine fragment of the LUMO orbital and the phenol fragments of the HOMO orbital. This phenomenon of π-breakage is also observed in Phen2 and Phen3, consequently owing to the strong electron-deficient effect of phenazine. Of course, both Phen1 and CN1 show DA charge transfers (Figure 7); nonetheless, their difference is obvious. The DA charge transfer in CN1 is built on the π-conjugated backbone of the whole molecule, while that in Phen1 is built on the partial π-conjugated backbone, which leads to red shifts of absorption and emission bands for CN1. The above combined results provide unequivocal insight into the inherent relationship between photophysical properties and chemical structures and the useful tools for tuning absorption and emission bands. Colorimetric Analysis. Ratiometric absorption and/or emission probes, widely used in biological and sensory materials chemistry,85−88 have different colors in absorption and/or emission induced by the analyte, which can be useful not only for the ratiometric method of detection but also for rapid visual sensing. Our previous studies42,43 demonstrated that Ph4 and Ph3 are colorless or slight yellow in solution and can be used as Cu2+ and Pt2+ probes. CN1 (CN1-Z) with strong red color in absorption and emission might have a useful application in ratiometric detection. The colorimetric analysis of CN1 in MeCN toward various metal ions, such as Na+, Li+, Zn2+, Mg2+, Ca2+, Pb2+, Mn2+, Cu2+, Co2+, Ni2+, Cd2+, Cd2+, Fe3+, Al3+, Ce3+, Cr3+, and Sr3+, was studied. The color change of solutions was visualized by naked eye analysis, and optical response was studied by UV/visible absorption spectroscopy, as shown in Figure 9. Monitored by the naked eye, the addition of 2 equiv of most of the metal ions, except Cu2+ and Co2+, to the red solution of CN1 did not show any detectable color changes when detected by the naked eye. However, drastic changes were found by UV/visible absorption spectroscopy (Figure 9a), indicating the complexation of the metal ion and CN1. Upon addition of Cu2+ or Co2+ to the CN1 solution, remarkable color changes were observed not only by the naked eye (Figure 9b) but also by UV/vis absorption spectroscopy. The absorption band of CN1 at 565

Figure 7. Frontier molecular orbitals for the selected Salen ligands calculated at B3LYP 6-31G(d,p) level of theory.

Figure 8. HOMO and LUMO levels and energy gaps of the selected Salen ligands.

Interestingly, for CN1 (CN1-Z), Phen1, Naph1, Ph1, Cy1, and Et1 containing the same diethylamino-phenol and different bridges, the HOMO levels were observed to remain constant circa −4.9 eV, whereas the LUMO levels took place in the range from −2.48 to −0.71 eV (Figure 8). This reveals that the LUMO levels of the Salen ligands are mainly affected by the bridges. However, for Ph1, Ph2, and Ph3 with the same bridge and different phenols, both HOMO and LUMO levels were found to vary in the region from −4.80 to −5.63 eV and from −1.28 to −1.90 eV, respectively, indicating that the HOMO and LUMO of the Salen ligand levels are influenced by the phenol fragments. Compared with Ph3, Ph1, and Ph2, owning the electrondonating diethylamine and extending the π-conjugated system, respectively, efficiently expand the π-functions in HOMO and LUMO orbitals (Figure 7), which lead to not only some higher levels of LUMO but also much higher levels of HOMO (Figure 8), and thus Ph1 and Ph2 have lower energy gaps and red-shifted absorption and emission spectra. Understanding the nature of controlling LUMO and HOMO levels of Salen ligands 16557



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stoichiometry of the complex formed between CN1 and Cu2+ was 1:1. Fluorescence Emission Analysis. Copper is a widely used pollutant in industry and our daily life which can cause severe risks for the ecosystems. Consequently, much attention has been drawn to the development of sensitive Cu2+ probes for biological and environmental applications. The fluorescence titration of CN1 with Cu2+ was presented in Figure 11. The fluorescence

Figure 11. Emission spectra of CN1 (1.0 × 10−6 mol dm−3 in MeCN, excited at 540 nm) upon addition of 0−1.0 equiv of Cu2+. Inset: a plot of (I − I0)/I as a function of Cu2+ concentration.

intensity of CN1 is highly sensitive to Cu2+, which will be reduced 91% and 93% upon the addition of 1 and 2 equiv of Cu2+, respectively. It is well-known that the paramagnetic Cu2+ center has a pronounced quenching effect on fluorescent ligands. Copper in solution has two common oxidation states: Cu+ (d10) and Cu2+ (d9). Clearly, the complete filling of d orbitals prevents d−d metal-centered electronic transitions in Cu(I) complexes. On the contrary, such transitions are exhibited by d9 Cu(II) complexes and lead to deactivation via ultrafast nonradiative processes, resulting in that the most luminescent Cu complexes are not Cu(II) complexes but Cu(I) complexes.42,89 The fluorescence intensity of CN1 exhibits gradual reduction upon the addition of 0−1.0 equiv of Cu2+ and saturation upon the addition of 1.0−5.0 equiv of Cu2+ (Figure S7, Supporting Information), revealing that the stoichiometry of the complex formed between the ligand and Cu2+ ions is 1:1, which is consistent with the absorption analysis and the former reported result.32,37,42 As shown in the inset photograph of Figure 11, a good linearity (correlation coefficient R2 = 0.994, n = 10) of the fluorescence intensity as a function of the concentration of Cu2+ between 0 and 1.0 × 10−6 mol dm−3 is established. The detection limit, based on the definition by IUPAC (CDL = 3 Sb/m),90 was found to be 6.0 nmol dm−3 (0.39 ppb) from 10 blank solutions. To the best of our knowledge, this value is one of the most sensitive fluorescence probes for Cu2+, indicating that CN1 is suitable to be a highly sensitive Cu2+ chemosensor. The selectivity behavior is obviously one of the most important characteristics of a probe, that is, the relative probe response for the primary ion over other ions present in solution. To evaluate the selectivity of CN1, 16 other different kinds of metal ions (Na+, Li+, Zn2+, Mg2+, Ca2+, Pb2+, Mn2+, Co2+, Ni2+, Cd2+, Cd2+, Fe3+, Al3+, Ce3+, Cr3+, and Sr3+), which probably interfere with the detection of Cu2+, were performed under similar conditions: the concentrations of CN1 were all kept at 1.0 × 10−6 mol dm−3 in MeCN, and 2 equiv of metal ion was added. When excited by 360 nm (UV lamp) or 565 nm (fluorescence spectrophotometer), as depicted in Figure 9b and 12,

Figure 9. (a) Absorption spectra CN1 (1.0 × 10−5 mol dm−3 in MeCN) upon addition of 2 equiv of different metal ions. Inset: Job plot of CN1 with addition of Cu2+; (b) photographs (top: under sunlight; bottom: under 360 nm UV light) of CN1 treated with different metal ions.

nm was red-shifted to 590 and 598 nm upon the addition of Cu2+ and Co2+, resulting in a color change from pink into brownish red and purple, respectively. Therefore, CN1 can be potentially used as a simple and rapid colorimetric receptor for sensing Cu2+ and Co2+ (Figure 10). Job plot studies (Figure 9a) revealed that the

Figure 10. Plot of (Abs0 − Abs)/Abs0 (at 565 nm) for CN1 as a function of Cu2+ concentration. 16558



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contained in 10.0 mm path length quartz cuvettes (3.5 mL volume) and a quartz tube, respectively. CN2,91 CN3,92 Phen3,93 Ph1, Et1,20 Ph2,94 Et2,95 Naph2,96 Cy1, Cy2, Et3, Cy3, Ph3,42 and Ph443 were synthesized according to previously reported methods. Measurement of Fluorescence Quantum Yield (Φ). Φ was measured by the optical dilute method of Demas and Crosby76 with a standard of quinine sulfate (Φr = 0.55, quinine in 0.05 mol dm−3 sulfuric acid) calculated by Φs = Φr(Br/Bs)(ns/nr)2(Ds/Dr), where the subscripts s and r refer to the sample and reference standard solution, respectively; n is the refractive index of the solvents; and D is the integrated intensity. The excitation intensity B is calculated by B = 1 − 10−AL, where A is the absorbance at the excitation wavelength and L is the optical path length (L = 1 cm in all cases). The refractive indices of the solvents at room temperature are taken from a standard source. Errors for Φ values (±10%) are estimated. Measurement of Metal Ion Sensing. Each metal ion titration experiment was started with CN1 (CN1-Z) (3 mL) of known concentration (1.0 × 10−6 mol dm−3 in MeCN). Various metal salts (1.0 × 10−4 mol dm−3 in H2O) were used for the titration. All types of fluorescent measurement (excited at 540 nm) were monitored 5 h after addition of the metal salt to the ligand solutions under room light at room temperature. Computational Details. Calculations were carried out using the Gaussian 03 software package (B3LYP 6-31G(d,p)). For the atoms of Salen ligands, the standard split-valence basis sets 631G(d,p) augmented with polarization d-functions for the nonhydrogen atoms and p-functions for the hydrogen atoms were used. Full geometry optimization of all the Salen ligands corresponding to the minima on the potential energy surface (PES) was conducted until a gradient of 10−5 au. The spin multiplicities and charges of the Salen ligands (except Ph442) were set equal to 1 and 0, respectively. The other parameters were set to default values. Cyclic Voltammetry (CV). CV was carried out on a CHI voltammetric analyzer at room temperature in nitrogen-purged anhydrous acetonitrile with tetrabutylammonium hexafluorophosphate as the supporting electrolyte at a scanning rate of 200 mV/s. Platinum disk and Ag/Ag+ were used as the working electrode and reference electrode, respectively. Ferrocene was used for potential calibration. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. Synthesis of CN1-Z. Diaminomaleonitrile (108 mg, 1 mmol) and 4-(diethylamino)salicylaldehyde (405 mg, 2.1 mmol) were stirred for 2 days at room temperature in 100 mL of absolute ethanol containing one drop of sulfuric acid as a catalyst. A dark green precipitate was filtered and washed with ethanol and 3 h later washed with diethyl ether. An impurity was removed by chromatography on silica, using CH2Cl2 as eluent. All procedures were carried out in the absence of room light (32% yield). 1H NMR: (CDCl3) 1.21 (t, 12H), 3.39−3.45 (q, 8H), 6.16 (d, 2H), 6.3 (dd, 2H), 7.19 (d, 2H), 8.34 (s, 2H), 12.29 (s, 2H). Anal. Calcd (found): C, 68.10 (67.91); H, 6.59 (6.87); N, 18.34 (18.17). EI-MS, m/z 458. Synthesis of CN1-E. Diaminomaleonitrile (108 mg, 1 mmol) and 4-(diethylamino)salicylaldehyde (386 mg, 2 mmol) were refluxed for 19 h at 78 °C in 100 mL of absolute ethanol containing one drop of sulfuric acid as a catalyst. A dark violet precipitate was filtered and washed with ethanol. An impurity was removed by chromatography on silica, using CH2Cl2:EtOAc = 3:1 as eluent. All procedures were carried out under room light

Figure 12. Emission spectra of CN1 (1.0 × 10−6 mol dm−3 in MeCN, excited at 540 nm) upon addition of 2 equiv of different metal ions.

several metal ions, such as Na+, Mg2+, and Zn2+, gave almost 20−50% quenching of the fluorescence intensity at 606 nm, but Co2+ and Cu2+ showed fluorescence quenching over 90%, indicating that the fluorescence detection has much worse selectivity than absorption detection.

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CONCLUSIONS We have systematically synthesized and studied the photophysical properties of a series of Salen-type Schiff bases, which contain different diamine bridges, electron-accepting substituents, electron-donating substituents, DA systems, and/or πextended systems and lead to tunable RGB light absorption and emission. The experimental results and DFT calculations have well proved that DA systems and/or π-extended systems can decrease the energy gap and red shift absorption and emission bands. DFT calculations also reveal that the LUMO levels of theses Salen ligands are mainly affected by the diamine bridges, whereas both the HOMO and LUMO levels are influenced by the phenol fragments. Both Phen1 and CN1 show DA charge transfers; nonetheless, the DA charge transfer is built on the π-conjugated backbone of the whole and partial molecule in CN1 and Phen1, respectively, which leads to red shifts of absorption and emission bands for CN1. These results are very useful not only for well tuning the LUMO and HOMO levels and absorption and emission bands but also for fabricating OLEDs. In addition, ratiometric detection and cis−trans isomer properties based on CN1 are examined. In conclusion, Salen ligands have coordination chemistry similar to other well-known tetradentate porphyrin ligands as well as much easier preparation and rich photophysical properties. Further studies on the application of these colorful Salen ligands and their complexes are currently underway in our laboratory.

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EXPERIMENTAL SECTION Materials and Instrumentation. All reagents were purchased from commercial suppliers and used without further purification. All the ligands were prepared according to previous reports.42,43,66 1H NMR (400 MHz) spectra were recorded in CDCl3 or DMSO-d6. Chemical shifts are reported in parts per million using tetramethylsilane as internal standard. UV/vis absorption spectra were recorded using a UV 765 spectrophotometer with quartz cuvettes of 1 cm path length. Fluorescence spectra were obtained using an F-7000 fluorescence spectrophotometer (Hitachi) at room temperature. The slit width was 5.0 nm for both excitation and emission. The photon multiplier voltage was 400 V. Samples in solution and powder were 16559



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(12% yield). 1H NMR: (CDCl3) 1.21 (t, 12H), 3.39−3.45 (q, 8H), 6.16 (d, 2H), 6.3 (dd, 2H), 7.19 (d, 2H), 8.34 (s, 2H), 11.75 (s, 2H). EI-MS, m/z 458. Synthesis of CN2. Diaminomaleonitrile (108 mg, 1 mmol) in 15 mL of ethanol was added to a stirred solution of 2-hydroxy1-naphthaldehyde (362 mg, 2 mmol) in 25 mL of absolute ethanol. The mixture turned red-brown immediately and was refluxed overnight then cooled to room temperature, and a brown precipitate formed. The product was collected by filtration as a brown solid. Recrystallization from chloroform by slow evaporation afforded orange needles (64% yield). 1H NMR: (DMSO-d6) 7.0 (d, 2H), 7.43 (t, 2H), 7.6 (t, 2H), 7.8−7.9 (m, 2H), 8.0 (d, 2H), 8.63 (d, 2H), 9.26 (s, 2H), 11.94 (s, 2H). Anal. Calcd (found): C, 71.88 (72.20); H, 4.18 (4.10); N, 12.90 (12.97). EI-MS, m/z 416. Synthesis of CN3. Diaminomaleonitrile (108 mg, 1 mmol) and salicylaldehyde (256 mg, 2.1 mmol) were refluxed for 19 h at 78 °C in 100 mL of absolute ethanol. The mixture turned yellow immediately. The product was collected by filtration as a paleyellow solid after the mixture cooled to room temperature. Recrystallization from absolute ethanol (87% yield). 1H NMR: (DMSO-d6) 6.17 (td, 2H), 6.21 (d, 2H), 7.22 (td, 2H), 7.43 (dd, 2H), 9.26 (s, 2H), 11.9 (s, 2H). Anal. Calcd (found): C, 68.35 (68.01); H, 3.82 (3.80); N, 17.71 (17.62). EI-MS, m/z 316. Synthesis of Phen1. 2,3-Diaminophenazine (210 mg, 1 mmol) and 4-(diethylamino)salicylaldehyde (405 mg, 2.1 mmol) were refluxed for 19 h at 78 °C in 100 mL of absolute ethanol. The mixture turned brown immediately. The product was collected by filtration as a brown solid after the mixture cooled to room temperature. Recrystallization from absolute ethanol (63% yield). 1 H NMR: (DMSO-d6) 1.2 (t, 12H), 3.43 (q, 8H), 7.56, 7.64, 7.91, 8.12 (m, 12H), 8.43 (s, 2H), 13.84 (s, 2H). Anal. Calcd (found): C, 77.39 (77.45); H, 6.86 (6.82); N, 10.03 (9.98). EI-MS, m/z 560. Synthesis of Phen2. 2,3-Diaminophenazine (210 mg, 1 mmol) and 2-hydroxy-1-naphthaldehyde (380 mg, 2.1 mmol) were refluxed for 19 h at 78 °C in 100 mL of absolute ethanol. The mixture turned yellow-brown immediately. The product was collected by filtration as a yellow-brown solid after the mixture cooled to room temperature. Recrystallization from absolute ethanol (57% yield). 1H NMR: (DMSO-d6) 7.0, 7.43, 7.56, 7.6, 7.64, 7.91, 8.12 (m, 18H), 8.43 (s, 2H), 13.8 (s, 2H). Anal. Calcd (found): C, 83.70 (83.91); H, 4.68 (4.65); N, 5.42 (5.39). EI-MS, m/z 518. Synthesis of Phen3. 2,3-Diaminophenazine (210 mg, 1 mmol) and salicylaldehyde (256 mg, 2.1 mmol) were refluxed for 19 h at 78 °C in 100 mL of absolute ethanol. The mixture turned brown immediately. The product was collected by filtration as a brown solid after the mixture cooled to room temperature. Recrystallization from absolute ethanol (65% yield). 1H NMR: (DMSO-d6) 6.76, 7.12, 7.43, 7.45, 7.56, 8.12 (m, 14H), 8.41 (s, 2H), 13.75 (s, 2H). Anal. Calcd (found): C, 80.75 (80.27); H, 4.84 (4.81); N, 6.73 (6.69). EI-MS, m/z 418. Synthesis of Ph1. 4-(Diethylamino)salicylaldehyde (405 mg, 2.1 mmol) and o-phenylenediamine (108 mg, 1 mmol) were placed into a round-bottom flask (100 mL) and dissolved in 60 mL of absolute ethanol. The pH of the mixed reaction was adjusted to 4−5 with a few drops of acetic acid, and then the mixture was refluxed at 60 °C for 24 h. After the reaction was complete, the solvent was removed from the reaction mixture in vacuo. The residue was dissolved in 25 mL of methanol, and the mixture was poured into 30 mL of distilled water with vigorous stirring and yellowish precipitate produced. The precipitate was filtered and recrystallized from methanol (72% yield). 1H NMR: (CDCl3) 1.2

(t, 12H), 3.43 (q, 8H), 6.2 (s,2 H), 6.25 (d, 2H), 6.75 (d, 2H), 7.0 (d, 2H), 7.16 (d, 2H), 8.39 (s, 2H), 13.43 (s, 2H). Anal. Calcd (found): C, 73.33 (72.76); H, 7.47 (7.48); N, 12.22 (12.81). EI-MS, m/z 458. Synthesis of Ph2. o-Phenylenediamine (108 mg, 1 mmol) and 2-hydroxy-1-naphthaldehyde (380 mg, 2.1 mmol) were refluxed for 19 h at 78 °C in 100 mL of absolute ethanol in the presence of a few drops of acetic acid. The mixture turned orange immediately. The product was collected by filtration as orange crystals after the mixture cooled to room temperature. Recrystallization from absolute ethanol (70% yield). 1H NMR: (CDCl3) 6.83−8.40 (m, 16H), 9.46 (s, 2H), 14.07 (s, 2H). Anal. Calcd (found): C, 80.74 (80.24); H, 4.81 (4.78); N, 6.73 (6.80). EI-MS, m/z 416. Synthesis of Et1. 4-(Diethylamino)salicylaldehyde (405 mg, 2.1 mmol) in ethanol (25 mL) was added dropwise to 1,2diaminoethane (60.1 mg, 1 mmol) in ethanol (25 mL) at room temperature. The reaction mixture was gradually heated to 60 °C and maintained at this temperature for 4 h. Upon removal of the solvent and cooling, a yellow precipitate was collected and recrystallized from ethanol (69% yield). 1H NMR: (CDCl3) 1.2 (t, 12H), 3.4 (q, 8H), 3.76 (s, 4H), 6.09 (s, 2H), 6.13 (d, 2H), 6.95 (d, 2H), 8.39 (s, 2H), 12.72 (s, 2H). Anal. Calcd (found): C, 70.21 (69.99); H, 8.35 (8.34); N, 13.65 (13.52). EI-MS, m/z 410. Synthesis of Et2. 1,2-Diaminoethane (60 mg, 1 mmol) and 2-hydroxy-1-naphthaldehyde (380 mg, 2.1 mmol) were refluxed for 12 h at 78 °C in 100 mL of absolute ethanol. The mixture turned pure yellow immediately. The product was collected by filtration as a yellow solid after the mixture cooled to room temperature. Recrystallization from absolute ethanol (68% yield). 1H NMR: (DMSO-d6) 3.81 (s, 4H), 6.09 (s, 2H), 6.15 (d, 2H), 6.89 (d, 2H), 7.2 (d, 2H), 7.63 (d, 2H), 8.42 (s, 2H), 7.8 (m, 2H), 12.97 (s, 2H). Anal. Calcd (found): C, 78.26 (78.46); H, 5.43 (5.62); N, 7.61 (7.61). EI-MS, m/z 368. Synthesis of Cy1. 1,2-Diaminocyclohexane (114 mg, 1 mmol) and 4-(diethylamino)salicylaldehyde (405 mg, 2.1 mmol) were refluxed for 12 h at 78 °C in 100 mL of absolute ethanol. The mixture turned deep yellow immediately. The product was collected by filtration as a deep-yellow solid after the mixture cooled to room temperature. Recrystallization from absolute ethanol (78% yield). 1H NMR: (CDCl3) 1.14 (t, 12H), 1.40 (m, 2H), 1.63 (m, 2H), 1.80−1.96 (m, 4H), 3.14 (m, 2H), 3.31 (q, 8H), 6.02 (dd, 2H), 6.07 (d,2 H), 6.88 (d, 2H), 7.92 (s, 2H), 13.16 (broad, 2H). Anal. Calcd (found): C, 72.38 (71.29); H, 8.68 (8.39); N, 12.06 (12.25). EI-MS, m/z 464. Synthesis of Cy2. 1,2-Diaminocyclohexane (114 mg, 1 mmol) and 2-hydroxy-1-naphthaldehyde (380 mg, 2.1 mmol) were refluxed for 12 h at 78 °C in 100 mL of absolute ethanol. The mixture turned yellow immediately. The product was collected by filtration as a yellow solid after the mixture cooled to room temperature. Recrystallization from absolute ethanol (75% yield). 1 H NMR: (CDCl3) 1.52 (m, 2H), 1.91 (m, 2H), 2.2 (m, 2H), 2.81 (m, 2H), 4.03 (m, 2H), 7.19−7.82 (m, 12H), 8.43 (s, 2H), 12.97 (s, 2H). Anal. Calcd (found): C, 79.59 (79.15); H, 6.20 (6.16); N, 6.63 (6.59). EI-MS, m/z 422. Synthesis of Naph1. 2,3-Diaminonaphthalene (158 mg, 1 mmol) and 4-(diethylamino)salicylaldehyde (405 mg, 2.1 mmol) were stirred for 2 days at room temperature in 100 mL of absolute ethanol containing one drop of sulfuric acid as a catalyst. A dark yellow precipitate was filtered and washed with ethanol and 3 h later washed with diethyl ether. Recrystallization from absolute ethanol (43% yield). 1H NMR: (DMSO-d6) 1.2 (t, 12H), 3.43 (q, 8H), 6.15−6.19 (m, 4H), 7.31 (d, 2H), 7.43−7.47 (m, 2H), 7.86−7.9 (m, 2H), 8.22 (s, 2H), 9.01 (s, 2H), 12.97 16560



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(s, 2H). Anal. Calcd (found): C, 75.56 (75.11); H, 7.13 (7.09); N, 11.01 (10.94). EI-MS, m/z 508. Synthesis of Naph2. 2,3-Diaminonaphthalene (158 mg, 1 mmol) and 2-hydroxy-1-naphthaldehyde (380 mg, 2.1 mmol) were refluxed for 12 h at 78 °C in 100 mL of absolute ethanol. The mixture turned yellow immediately. The product was collected by filtration as a yellow solid after the mixture cooled to room temperature. Recrystallization from absolute ethanol (57% yield). 1H NMR: (DMSO-d6) 6.96−7.68 (m, 18H), 8.48 (s, 2H), 12.75 (s, 2H). Anal. Calcd (found): C, 68.35 (68.01); H, 3.82 (3.80); N, 17.71 (17.62). EI-MS, m/z 466.

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ASSOCIATED CONTENT

* Supporting Information S

Figures giving general methods for the preparation of Schiff bases and Salen ligands, single-crystal X-ray diffraction structure of CN1-Z, absorption and emission spectra of CN1-Z and CN1-E in MeCN at room temperature, absorption spectra of CN1-Z under the irradiation of UV lamp, normalized absorption spectra of the selected Salen ligands in MeCN, frontier molecular orbitals for the selected Salen ligands calculated at the B3LYP 631G(d,p) level of theory, a plot of (I − I0)/I (at 610 nm) for CN1 as a function of Cu2+ concentration, cyclic voltammetry plots of CN1 (CN1-Z), Phen1, Naph1, Ph1, Cy1, and Et1, partial cyclic voltammetry plots of CN1 (CN1-Z), Phen1, Naph1, Ph1, Cy1, and Et1, partial cyclic voltammetry plots of Ph1, Ph2, and Ph3, IR spectrum of solid-state CN1-Z, and IR spectrum of solid-state CN1-E, and table giving the energy gap and energy (eV) of frontier molecular orbitals in the selected ligands and LUMO and HOMO levels of the selected ligands. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 28-8541-2291. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (no. 21172160) and Ministry of Education of China (no. 20100181120039).

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REFERENCES

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Synthesis and Photophysical Properties of Colorful Salen-Type Schiff

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