Synthesis, Crystal Structure and Two-photon Absorption Properties of 4′-(N,N-di(4-hydroxymethyl phenyl) amino)phenyl-2,2′:6′,2?-terpyridine①

LIU JieWANG Hui LI Dn-Dn ZHOU Hong-Ping TIAN Yu-Peng WU Jie-Ying

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Synthesis, Crystal Structure and Two-photon Absorption Properties of 4′-(N,N-di(4-hydroxymethyl phenyl) amino)phenyl-2,2′:6′,2?-terpyridine①

LIU Jiea, bWANG HuiaLI Dan-DanaZHOU Hong-PingaTIAN Yu-PengaWU Jie-Yinga②

a(230039,)b(236037)

A novel donor-acceptor (D-A)triphenylamino terpyridine derivative L was facilelysynthesized and fully characterized, and its single crystals were obtained and determined by X-ray diffraction analysis. It crystallizes in triclinic, space groupwith= 11.760(5),=12.516(5),= 12.850(5) ?,= 67.141(5),= 65.284(5),= 75.876(5)o,M= 621.54,= 1575.6(11) ?3,= 2,D= 1.310 g/cm3,= 0.245 mm?1,(000) = 648, the final= 0.0671 and= 0.1869 for 11328 observed reflections with> 2()Linear and nonlinear optical properties of terpyridine derivative L were systematically investigated. The maximum two-photon cross-section of L was 382.5 GM (Goeppert-Mayer), measured by two-photon excited fluorescence (TPEF) method. This result demonstrates that the increase of intramolecular charge transfer (ICT) leads to enhanced two-photon absorption (2PA), which could be achieved by introducing additional electron-donor groups to the molecular framework.

terpyridine derivatives, crystal structure, photophysical properties, two-photonexcited fluorescence;

1 INTRODUCTION

Organic small moleculeswith striking two- photon absorption (2PA) effect and large 2PA cross- section (2PA) values have gained much attention over the past decades due to their potential applica- tions in the areas of three-dimensional (3D) optical memory and data storage[1, 2], multi-photon fluore- scence imaging[3-7], 3D lithographic micro-fabrica- tion[8-11], photodynamic therapy[12-15], two-photon up-conversion lasing and optical power limi- ting[16-18]. Along with the promising applications, numbers of strategies are employed to design and synthesize new materials for achieving large 2PA cross-section values. According to our previous experimental results[19], increasing intramolecular charge transfer (ICT), which could be realized by introducing additional electron-donor/acceptor groups, is one of the most effective ways of tuning the value of2PA.

Based on the above considerations, as well as the continue work of our group, herein, we designed a novel D-A structural model of terpyridine derivative L with triphenylamine moiety as the electron donor (D) and 2,2′:6′,2?-terpyridine, as the acceptor (A). Moreover, in order to increase the ICT within the molecule and solubility of chromophore, two hydroxymethyl substituents (-CH2OH) are introdu- ced to the triphenylamine moiety. The target mole- cule was conveniently synthesizedformylation and reduction in satisfied yields. Apart from that, its photophysical properties, such as linear absorption, single- and two-photon excited fluorescence in various solvents, were also systematically studied.

2 EXPERIMENTAL

2. 1 Materials and instruments

All chemicals were purchased as analytical grade and all solvents were purified by conventional methods before use.1H- and13C-NMR spectra were obtained on a Bruker Avance 400 MHz spectro- meter at 25 ℃ (TMS as internal standard in NMR). Mass spectrum was determined with a Micro-mass GCT-MS (EI source). X-ray diffraction data of single crystals were collected on a CCD diffrac- tometer. The determination of unit cell parameters and data collection were performed with Mo-radiation (= 0.71073 ?).

UV-vis absorption spectra were recorded on a UV-265 spectrophotometer. Fluorescence measure- ments were performed at room temperature using a Hitachi F-7000 fluorescence spectrophotometer. Two-photon excited fluorescence (2PEF) spectra were measured using femtosecond laser pulse and Ti: Sapphire system (680～1080 nm, 80 MHz, 140 fs, Chameleon II) as the light source.

2. 2 Synthesis

Scheme 1. Synthetic route for dye L

Compounds L0and L1were prepared according to the methods reported in literatures[20,21]. The synthesis of compound L is described below.

ChromophoreL1(0.53 g, 1.0 mmol) was dissolved in 200 mL EtOH. And then 0.19 g (5.0 mmol) NaBH4was added slowly. The reaction was monitored by TLC. After the completion of the reaction, the solvent was removed under reduced pressure. The solid L was recrystallized from EtOH and obtained as a slightly yellow solid. Yield 91.4% (0.49 g). FT-IR (KBr,,cm?1): 3435 (OH, s), 3052 (Ar-H, s), 2922 (HOCH-H, s), 1591, 1511, 1489 (Benzene skeleton vibration, s), 1117 (C–O, m).1H NMR (DMSO-6, 400 MHz)(ppm): 8.75 (t, 4H), 8.59 (d, 2H), 7.94 (d, 4H), 7.80 (d, 4H), 7.50 (t, 2H), 7.32 (d, 2H), 7.21 (d, 4H), 5.25(s, 2H), 4.66(s, 4H). MS:/(%) = 536.2 (100%).

2. 3 X-ray structure determination

A slightly yellow crystal of the title compound L with dimensions of 0.23mm × 0.22mm × 0.22mm was selected and mounted on a glass fiber. The single-crystal X-ray diffraction data were collected on a Bruker Smart CCD diffractometer equipped with a graphite-monochromatic Moradiation (= 0.71073 ?) situated in the incident beam by using an-2scan mode at 298(2) K. In the range of 1.32<<27.24o (–13≤≤13, –14≤≤14, –15≤≤15), a total of 11328 reflections were collected, of which 5486 were independent (int= 0.0190). Unit cell dimensions were obtained with least-squares refinements, and all structures were solved by direct methods using SHELXS-97[22]. The final refinement was performed by full-matrix least-squares methods with anisotropic thermal parameters for the non- hydrogen atoms on2. The hydrogen atoms were located theoretically and refined with riding model position parameters as well as fixed isotropic ther- mal parameters. The final= 0.0779,= 0.2143 (=1/[2(F2) + (0.068)2], where= (F2+ 2F2)/3),(Δ/)max= 0.000,= 1.086, (Δ)max= 1.084 and (Δ)min= –0.423 e/?3. The relevant crystal data and structural parameters are listed in Table 1.

Table 1. Selected Bond Lengths (?), Bond Angles (°) and Dihedral Angles of L

2. 4 TD-DFT computational studies

The structure of L was optimized by time depen- dent density functional theory (TD-DFT) using a TD-DFT/B3LYP/6-31G(d) basis set. The structure optimization and energy calculations were perfor- med with the GAUSSIAN 03 program[23].

3 RESULTS AND DISCUSSION

3. 1 Crystal structure analysis

The crystal structures about the ORTEP plot of molecule L is shown in Fig. 1. The selected bond lengths, bond angles and the parameters P1-P6 for L are summarized in Table 1. It can be seen that the terpyridine derivativeL consists of triphenylamine (donor) and terpyridyl (acceptor) moieties. At the triphenylamine donor partial, the central nitrogen and its three adjacent carbon atoms are basically coplanar (defined as the P6 plane), with the sum of three C–N–C angles (360.0°) and almost the same C–N bond length. However, due to the stero repulsion, the three phenyl ring planes are arranged in a propeller-like fashion, which is favorable to avoid molecule aggregation and to form a more highly soluble amorphousness. At the other partial of terpyridyl acceptor, the dihedral angles among the central pyridine (named as P3) and the three planes of two pyridines (P4 and P5) and a phenyl (P2) are 31.99°(P3,4),16.09°(P3,5) and 37.01°(P2,3), respectively. The result indicates that the terpyridyl group is seldom coplanar.

Fig. 1 . Single molecular structure of compound L

The packing of molecule L in the crystal lattice is given in Fig. 2. The two-dimensional structure along theandaxes is generated by two kinds of hydrogen bonds listed in Table 2, which are N(3)···H(2C)–O(2) and N(1)···H(1C)–O(1).And then, due to the distance of 3.616(3) ? between rings of P4 with approximate parallel, strongstaking interactions are formed along the-axis which may stabilize the three-dimensional sheet framework (shown in Fig. 2).

Fig. 2 . Three-dimensional chain of the title compound, showing intermolecular N(3)···H(2C)–O(2) (pink lines),N(1)···H(1C)–O(1) (green lines) hydrogen bonds and intermolecular π (ring P4)··· π (ring P4) stacking (bright green lines) along the b axis. Most H atoms have been omitted for clarity

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Symmetry transformations used to generate the equivalent atoms: (i)–1,,+1; (ii),–1,; (iii) –, –+1, –+2

3. 2 Linearabsorption, TD-DFT studies and single-photon exited fluorescence (SPEF)

The linear absorption (UV-Vis) and single-photon excited fluorescence (SPEF) spectra of the chro- mophore L in seven different solvents with the same measured concentration of 1.0 × 10-5mol L-1are shown in Fig. 3.

As shown in Fig. 3(a), it is obvious that L exhibits dual bands in the 270～500 nm range. The shorter wavelength band near 290 nm is attributed to the absorption of triphenylamine moiety and the longer absorption peak corresponds to the ICT or-* transition of the whole molecule. Additionally, little solvatochromism was observed in spite of increasing the peripheral solvents’ polarity, indicating little difference in dipoles between ground and excited states of the chromophore. These findings will be further corroborated by TD-DFT calculation as follows.

Fig. 4 gives straight-forward representations of the electron density distribution for L using the G03 software, and the energies and compositions of some frontier orbitals are listed in Table 3. According to the calculated results, there exist two transition bands atmax= 384.67 nm,= 0.4764 (HOMO → LUMO) andmax=303.00 nm and= 0.1872 (HOMO-2 → LUMO), which are in good agreement with the experimental electronic transitions. In ad- dition, compared to the energy gap of 4′-(4-(4- phenyl-4′-hydroxymethyl phenyl) amino)phenyl- 2,2′:6′,2?-terpyridine (3.63 ev)[19], that of L occu- pying two ethoxyl units decreases to 3.22 ev, which indicates that the electron-donor group (-CH2OH) may raise the HOMO level and lead to small gap.

Fig. 3. (a) Linear absorption and (b) SPEF spectra of L in seven different solvents with a concentration of 1.0 × 10?5 mol L?1,respectively

Table 3. Calculated Linear Absorption Properties (nm), Excitation Energy (eV), Oscillator Strengths and Major Contribution for L

However, the SPEF spectra of the chromphoreLpresent remarkable bathochromic shifts with increasing polarity of the solvents.The maximum emission wavelength of L is shifted by 59 nm from 473 nm in benzene to 532 nm in DMF. We can also find that the higher polarity the organic solvent had, the strongerthe fluorescence intensitywas.

3. 3 Two-photon excited fluorescence (2PEF)

As presented in Fig. 5, there is no linear absorp- tion in the range of 600～880 nm for L, which indicates that there is no energy level corresponding to an electron transition in the spectral range. Therefore, upon excitation from 600 to 880 nm, it is impossible to produce single-photon excited up- converted fluorescence[24]. The linear dependence on the square of input laser power (slope = 1.96) suggests a two-photon excitation mechanism at 750 nm for the molecule (Fig. 6).

Fig. 5 . Two-photon ?uorescence spectra of L in DMF pumped by femtosecond laser pulses at 500 mW at different excitation wavelength

Fig. 6 . (a) TPEF spectra of Lunder different pumped powers at 750 nm, with c = 1.0 × 10-3 mol L-1 in DMF; (b) Output fluorescence (Iout) vs. the square of input laser power (Iin) for L

Fig. 7 . 2PA cross-section (δ) of L in DMF vs. excitation wavelengths of identical energy of 500 mW

The 2PA cross sections () are determined by comparing their two-photon excited fluorescence to that of fluorescein in the NaOH solution (aq, 0.1 mol L?1) (pH = 11) according to the following equation[25, 26]:

s= δr·s·r·C·r/(r·s·C·s) Eq (1)

In this equation, the subscriptsrefers to the reference molecule, which is fluorescein.is the 2PA cross-section value,is the concentration of the solution,is the refractive index of the solution,is the 2PEF integral intensities of the solution emitted at the exciting wavelength, andis the fluorescence quantum yield. The δvalue of re- ference equals to 38, which is from the literature[27]. Fig. 7 shows the 2PA cross-section () of the chrom- phore L at different excitation wavelength in DMF. The maximummaxvalue of L at excitation wave- length is 382.5 GM, which increases by 17.8 GM compared to that of 4′-(4-(4-phenyl-4′-hydroxy- methyl phenyl) amino)phenyl-2,2′:6′,2?-terpyri- dine[19]. It implies that the introduction of two weak electron-donor groups (-CH2OH) can enhance the degree of ICT, which is helpful for increasing the value of.

4 CONCLUSION

A novel donor-acceptor (D-A)triphenylamino terpyridine derivative L was convenientlysynthe- sizedformylation and reduction in high yields and fully characterized by1H NMR, MS and single-crystal X-ray diffraction. The SPEF spectra of the terpyridine derivativeL showed obvious positive solvatochromism in different polar solvents mainly owing to the ICT. The result of 2PEF indicates that the EDG and -CH2OH can increase the ICT, which is benefit to enhance the 2PA cross-section. Therefore,it was found that rela- tively larger 2PA cross section may be obtained by subtle modification rather than employing com- plicated organic synthesis. Furthermore, the ter- pyridine derivativeL may act as a ligand to coordinate with metal ions to form functional complexes. The work is going on in our group.

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9 September 2014; accepted 26 November 2014 (CCDC 1014826)

① This project was supported by the National Natural Science Foundation of China (51372003), Natural ScienceFoundation of Anhui Province (1208085MB22), and Natural Science Foundation of Fuyang Normal School (2014FSKJ07)

. Wu Jie-Ying (1957-), professor, engaged in coordination chemistry. E-mail: jywu1957@163.com

10.14102/j.cnki.0254-5861.2011-0503