Luminescent properties of newly synthesized thioxanthone-polypyridyl derivatives and their metal-organic complexes

Accepted Manuscript Luminescent properties of newly synthesized thioxanthone-polypyridyl derivatives and their metal-organic complexes Dong-Mei Ma, Aishun Ding, Hao Guo, Meng Chen, Dong-Jin Qian PII:

S0022-2313(18)31672-7

DOI:

https://doi.org/10.1016/j.jlumin.2019.04.010

Reference:

LUMIN 16405

To appear in:

Journal of Luminescence

Received Date: 12 September 2018 Revised Date:

3 April 2019

Accepted Date: 5 April 2019

Please cite this article as: D.-M. Ma, A. Ding, H. Guo, M. Chen, D.-J. Qian, Luminescent properties of newly synthesized thioxanthone-polypyridyl derivatives and their metal-organic complexes, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.04.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Luminescent properties of newly synthesized thioxanthone-polypyridyl

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derivatives and their metal-organic complexes

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Dong-Mei Ma et al

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Luminescent properties of newly synthesized thioxanthone-polypyridyl derivatives

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and their metal-organic complexes Dong-Mei Ma,1 Aishun Ding,2 Hao Guo,1* Meng Chen,3 Dong-Jin Qian1*

Department of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200438, China 2

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1

State Key Laboratory of Molecular Engineering of Polymers and Department of

Department of Material Science, Fudan University, 220 Handan Road, Shanghai 200433, China

Corresponding Authors

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3

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Macromolecular Science, Fudan University, Shanghai 200433, China

E-mail: [email protected] (Dong-Jin Qian)

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ORCID: 0000-0001-8050-867X Tel: +86-21-31249210

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E-mail: [email protected] (Hao Guo)

Abstract

Thioxanthones (TXs) are not only important photoinitiators for free radical polymerization but also efficient light-harvesting units for organic light emitting diodes. Here, we reported synthesis and photophysical properties of new TXs with 2,2'-bipyridyl (BPy) and 2,2':6',2''-terpyridyl (TPy) substituents, TXOBPy and TXOTPy, as well as those of their complexes with some

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transition metal ion (Zn2+, Fe2+, Ni2+, Eu3+ and Tb3+) solution in diverse solvents and solid powders. Absorption spectra revealed mainly two groups of bands at approximately 250–290 and 366 nm, attributed to the π–π* and n–π* electron transfer of the TXs. A broad luminescent

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emission was recorded and centered at approximately 422 nm for these TXs and their metal complexes, its relative intensity was solvent and concentration dependent. Further, it was observed that this emission band red shifted to approximately 470 nm in their solid powders. For

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the TXs and their Zn/Fe/Ni-complexes in the methanol solutions, the quantum efficiency (QE) of TX rings was about 0.04−0.11, and the fluorescent lifetime (τ) was about 0.5–1.2 ns. On the

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other hand, for the Fe- and Ln-complexes, the QE of TX rings was below 0.01, which was attributed to the reason that the excited energy of the TX rings was quenched by ligand-Fe2+ charge transfer or by transferring the energy to the central Ln3+ (Eu3+ and Tb3+) ions. Thus, the Ln-TXOBPy and Ln-TXOTPy complexes gave off strong and sharp Eu3+/Tb3+ emissions at the

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wavelengths between 480 and 750 nm. The fluorescent emission lifetime of the central Ln3+ ions was about 0.3–0.6 ms. Thus, we can suggest that the TXOBPy and TXOTPy photoinitiators can act as potential candidates for the development of efficient emitters with tunable light emissions

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and luminescent sensors.

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Keywords: Thioxanthone; Metal−organic complex; Luminescence; Bipyridyl; Terpyridyl

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1. Introduction Thioxanthone (TX) and its derivatives have attracted much attention recently not only because

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they are important photo-initiators for the free radical polymerization [1,2], mediators for the development of antitumoric, antiparasitic and anticarcinogenic agents [3,4], but also because they are important light-harvesting units for the efficient luminescent emitters and organic light

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emitting diodes (OLED) [5,6]. It has been revealed that the energy gap between the first singlet and triplet excited state of TX is lower than 0.3 eV, resulting in facilitating the reverse

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intersystem crossing from triplet to singlet state, the feature of which is benefit for the development of highly efficient luminescent emitters [7]. The color of the OLED devices (emission wavelengths) is dependent on the type of the emissions, for instance, the orange and red phosphorescent OLEDs can be prepared based on the phosphorescent emission of TX derivatives [5], while blue and yellow ones can be done on their fluorescent emissions [6]. That

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is, the device color is closely related to the photo-physical properties of the TX derivatives, thus a clear understanding of the electron transfer processes in the excited state is important for the

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practical applications of the devices.

Researches on the photo-physical properties of TXs have revealed that their luminescence is

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closely dependent on the composition, chemical structure and the nature of the solvents they are dissolved [8,9]. For instance, Arsu and coworkers synthesized several TXs containing benzothiophene and anthracene substituents, these products displayed a bathochromic shifted absorption up to about 460 nm; with high quantum yields for intersystem crossing to generate sufficient amount of triplet states and efficient polymerization behaviors [10-12]. Su and coworkers prepared some TXs containing an electron donor substituent of carbazone, thus produced a donor-acceptor or donor-acceptor-donor structure, which displayed thermally

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activated delayed fluorescence emitters for the fabrication of OLEDs with high external quantum efficiency [13]. The TX chromophores can also be bound with other organic and polymeric species such as carboxylic acids [14], poly(vinylchloride) and chitosan [15-17], all products

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displayed enhanced UV and/or visible absorption ability as well as improved efficiency for various photopolymerization reactions.

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Besides the TXs’ composition and chemical structure, solvent polarity affects also strongly on their luminescent behaviors [18,19]. An earlier work has revealed that emission quantum yield of

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TXs in apolar solvents such as benzene is smaller than 0.001, while it can rise to 0.4−0.6 in the polar or protic solvents [20-22]. The phenomena have been attributed to the reason that the nonradiative photophysical processes of the excited TXs are solvent dependent [22]. In details, the energy of the (3n, π *) state is very close to that of the (1π, π*) one in the nonpolar solvents, which facilitates the non-radiative deactivation of the TXs’ excited state that results in a weak

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fluorescence emission. While in the polar solvents, the energy of the (3n, π*) state becomes greater than that of the (1π, π*) one, leading to a poor intersystem crossing from (1π, π*) to (3n,

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π*), as a result, the fluorescence emission is increased. These experimental results have been recently confirmed by Marian and coworkers with the use of quantum chemical methods to

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investigate the photophysics of TXs in the vacuum and various solvents [23]. Based on such a solvent

effect,

Su

and

coworkers

recently

developed

a

fluorescent

probe

for

solvatofluorochoromism and qualitative and quantitative detection of low-level water content in various solvent media [24].

From the chemical structure of TXs in the literatures[1-6,25,26], it can be found that, to prepare novel TXs, a convenient route is introducing a hydroxyl or acetic acid substituent in the TX

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aromatic rings, followed by a substitution reaction or others to bind with functional groups. In the present work, we synthesized new TXs containing 2,2'-bipyridyl (BPy) and 2,2':6',2''terpyridyl (TPy) substituents; both of them have been widely used to coordinate with transition

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metal ions (Mn+) such as Zn2+, Fe2+, Ni2+, Eu3+ and Tb3+ to produce metal-organic complexes with interesting optical and electrochemical properties [27,28]. Because both TX aromatic rings and polypyridyl substituents of BPy and TPy are good light harvesting units, the present

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TXOBPy and TXOTPy compounds can act not only as the luminescent emitters, but also as an antenna to absorb light energy and then transfer the energy to the lanthanide (Ln3+) ions to

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produce typical Ln3+ sharp fluorescent emissions [29,30]. Steady and transient state fluorescence studies revealed that the emission quantum efficiency (QE) and fluorescence lifetime (τ) of these TXs and their metal complexes are solvent and concentration dependent. Particularly, strong luminescent emissions of TXs can be recorded in the methanol solution, and typical Ln3+ ions’

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emissions recorded in the acetonitrile solution. These observations revealed that the TXOBPy and TXOTPy can act as potential candidates for the blue emitters, luminescent sensors and as

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efficient light antenna of the Ln3+ ions for tunable luminophors.

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2. Experimental section 2.1. Materials

Thiosalicylic acid, 3-methoxyphenyl iodide and boron tribromide were purchased from Energy Chemical (Shanghai, China). Oxalic dichloride, 1-(bromomethyl)-4-methyl-benzene, benzoyl peroxide，bromosuccinimide and zinc trifluoromethanesulfonate were from J&K Scientific Ltd. 4-Methyl-4'-(bromomethyl)-2,2'-bipyridine was from TCI Chemical Industry Co., Ltd. Iron(II)

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trifluoromethanesulfonate,

zinc

trifluoromethanesulfonate

and

nickel(II)

trifluoromethanesulfonate were from Aladdin Industrial Co. (Shanghai, China). Europium

from Alfa Aesar. 4'-(4-(Bromomethyl)phenyl)-2,2':6',2''-terpyridine

and

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trifluoromethanesulfonate, terbium trifluoromethanesulfonate and 2-phenyl-benzimidazol were

3-methoxy-9H-thioxanthen-9-one

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(TXOMe) were synthesized according to literature methods [31,32].

2.2. Synthesis

Fig. 1 shows the chemical structure of the TXs used in the present work. We synthesized first 3-hydroxy-9H-thioxanthen-9-one (TXOH) according to the method reported by Nielsen and co-

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workers [33]. Then, the other TXs were synthesized by the substitution reaction of TXOH with

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various bromobenzyl derivatives.

2.2.1. 3-Hydroxy-9H-thioxanthen-9-one

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To a cooled solution of TXOMe (2.18 g, 9 mmol) in dry dichloromethane, boron tribromide (6.1 ml, 65 mmol) was added dropwise via glass syringe [33]. The mixture was stirred at room temperature under nitrogen atmosphere overnight, and then quenched by addition of a saturated NaHCO3 solution. The organic phase was separated with the aqueous solution and extracted by ethyl acetate. The organic solution obtained was dried over MgSO4. After removal of the solvents, the crude product was purified by silica gel chromatography (petroleum ether/ethyl

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acetate = 1:1) to afford TXOH as yellow solid powders (1.62 g, 79%). Anal. 1H NMR (400 MHz, DMSO-d6): 7.10-7.00 (m, 2 H), 7.56 (t, 1H), 7.80-7.70 (m, 2 H), 8.43 (d, J = 2.4 Hz, 1 H), 8.35 (d, J =8.8 Hz, 1 H);

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C NMR (400 MHz, DMSO-d6): 110.35, 116.31, 121.00, 126.27,

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126.59, 128.54, 128.93, 131.74, 132.51, 138.80, 136.21, 161.47, 177.71; IR (KBr, cm-1): 3176, 1630, 1590, 1240.

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Figure 1

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2.2.2. 3-((4-Methylbenzyl)oxy)-9H-thioxanthen-9-one (TXOBenMe)

To an acetonitrile solution of TXOH (187 mg, 0.82 mmol), 1-(bromomethyl)-4-methylbenzene (166 mg, 0.9 mmol) and K2CO3 (135 mg, 0.98 mmol) were added [34]. The mixture was stirred at 80 °C under nitrogen atmosphere for 24 h, then cooled to room temperature and poured into ethyl acetate. The organic solution was washed with H2O/brine and dried over MgSO4. After

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removal of the solvents, the crude product was purified by flash chromatography (EtOAc/petroleum ether) to produce TXOBenMe as white solid powders (241 mg, 89%). Anal. 1

H NMR (400 MHz, CDCl3): 2.38 (s, 3 H), 5.13 (s, 2 H), 7.05 (d, J = 2.4 Hz, 1 H), 7.08 (dd, J =

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C NMR (400 MHz, CDCl3): 21.19, 70.32, 109.08, 115.52, 123.11, 125.68,

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8.55 (m, 2 H);

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8.8, J = 2.4 Hz, 1 H), 7.2 (d, J = 8 Hz, 1 H), 7.34 (d, J = 7.6 Hz, 2 H), 7.76-7.52 (m, 3H), 8.61-

126.16, 127.66, 129.38, 129.67, 131.83, 131.93, 132.68, 136.88, 138.21, 139.43, 161.68, 178.94; IR (KBr, cm-1): 3010, 2920, 1630, 1600, 1240.

2.2.3. 3-((4-([2,2':6',2''-Terpyridin]-4'-yl)benzyl)oxy)-9H-thioxanthen-9-one (TXOTPy)

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A similar process to that of TXOBenMe was used to prepare TXOTPy, with TXOH (569 mg, 2.5 mmol) and 4'-(4-(bromomethyl)phenyl)-2,2':6',2''-terpyridine (1.0 g, 2.5 mmol) as the starting materials. The crude product was purified by recrystallization in mixed solvents of DCM

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and CH3OH to afford TXOTPy as solid powders (1.2 g, 88%). Anal. 1H NMR (400 MHz, CDCl3): 5.23 (s, 2 H), 7.07 (s, 1 H), 7.12 (d, J = 8.8 Hz, 1 H), 7.34 (t, J = 6.4 Hz, 2H ), 7.59-7.44 (m , 5 H), 8.59 (t, J =8.0 Hz, 2 H), 8.66 (d, J = 8.0 Hz, 2 H), 8.75-8.70 (m, 4 H); 13C NMR (400

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MHz, CDCl3): 70.04, 109.32, 115.52, 118.84, 121.35, 123.41, 123.82, 125.74, 126.23, 127.71, 127.96, 129.38, 129.75, 131.89, 132.11, 136.65, 136.82, 138.65, 139.54, 149.13, 149.65, 156.03,

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156.03, 156.20, 161.58, 179.02; IR (KBr, cm-1): λ 3061, 1628, 1588, 1244.

2.2.4. 3-((4'-Methyl-[2,2'-bipyridin]-4-yl)methoxy)-9H-thioxanthen-9-one (TXOBPy)

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A similar process to that of TXOBenMe was used to prepare TXOBPy, with TXOH (187 mg, 0.82 mmol) and 4-(bromomethyl)-4'-methyl-2,2'-bipyridine (236 mg, 0.9 mmol) as the starting materials. The crude product was purified by recrystallization in the mixed solvents of DCM and

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CH3OH to afford TXOBPy as solid powders (268 mg, 80%). Anal. 1H NMR (400 MHz, CDCl3):

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2.46 (s, 3 H), 5.28 (s, 2 H), 7.06 (d, J = 2.8 Hz, 1 H), 7.12-7.10 (m, 2H), 7.65-7.35 (m, 4 H), 8.26 (s, 1 H), 8.48 (s, 1 H), 8.65-8.50 (m, 3 H), 8.71 (d, J = 5.2 Hz, 1 H);

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C NMR (400 MHz,

CDCl3): 21.21, 68.67, 109.27, 115.31, 118.89, 121.31, 122.09, 123.58, 125.00, 125.75, 126.31, 129.24, 129.74, 132.00, 132.21, 136.83, 139.59, 145.95, 148.29, 149.02, 149.59, 155.42, 156.77, 161.06, 179.01; IR (KBr, cm-1): 2922, 1638, 1592, 1254.

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2.2.5. Metal complexes Metal complexes were synthesized by using of its trifluoromethanesulfonate salt with TXOTPy. Because the same synthetic process was used for each complex together with similar H NMR and FTIR results, we described the process and analytic data of Tb3+- and Fe2+-

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1

TXOTPy complexes as follows.

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For the Tb3+-TXOTPy complex, to 20 mL methanol-chloroform solution of TXOTPy (137 mg 2.5 mmol), Tb(OTf)3 (75.8 mg 1.25mmol) in the same mixed solvents was slowly added. The

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solution was then stirred at 60 oC for 4 h. After cooled to room temperature, the precipitate was well washed with methanol and ethyl acetate and finally dried under vacuum. Anal.

1

H NMR

(400 MHz, DMSO-d6): 8.80 (s, 5 H), 8.44 (d, J = 8.0 Hz, 3 H), 8.17 (t, J = 7.2 Hz, 2 H), 8.05 (d, J = 8.0 Hz, 2 H), 7.86-7.78 (m, 4 H ), 7.67-7.51 (m, 4 H), 7.29 (d, J = 8.4 Hz, 1 H), 5.44 (s, 2 H); IR (KBr, cm-1): 3059, 1632, 1596, 1440, 1312, 1239, 1029, 637. Calcd for C73H46F9N6O13S5Tb:

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C, 51.41; H, 2.72; N, 4.93. Found: C, 52.29; H, 3.03 N, 5.27%. For the Fe2+-TXOTPy complex, Fe(OTf)2 (44 mg, 1.25 mmol) and TXOTPy (137 mg, 2.5

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mmol) were used as the starting materials. Anal. 1 H NMR (400 MHz, DMSO-d6): 9.70 (s, 2 H), 9.06 (d, J = 7.6 Hz, 2 H), 8.61 (d, J = 7.2 Hz, 2 H), 8.49 (t, J = 8.0 Hz, 2 H), 8.04 (t, J = 7.2 Hz, 2

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H), 7.96 (d, J = 7.6 Hz, 2 H), 7.86 (d, J = 8.0 Hz, 1 H), 7.79 (t, J = 6.8 Hz, 1 H), 7.60 (d, J = 12.4 Hz, 2 H), 7.36 (d, J = 8.8 Hz, 1 H), 7.28 (d, J =4.4 Hz, 2 H), 7.20 (t, J = 6.4 Hz, 2 H), 5.57 (s, 2 H); IR (KBr, cm-1): 3066, 1619, 1596,1437, 1161, 1268, 637. Calcd for C37H23F6FeN3O8S3: C, 49.18; H, 2.57; N, 4.65. Found: C, 48.95; H, 2.77 N, 4.91%.

2.3. General processes for solvent, concentration and counter ion effects on luminescence

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The solvent effects on the luminescent properties of TXs and their Ln-complexes were investigated by the following ways: (1) preparation of dilute (1 × 10-6 mol/L) methanol, butanol, DMF, acetonitrile and chloroform solutions; (2) addition of methanol to the TXOTPy chloroform

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solution at the ratios of 0.001:1, 0.01:1 and 0.1:1 (v/v); (3) addition of methanol to the LnTXOTPy acetonitrile solution at the ratios of 1:10, 1:2 and 1:1 (v/v).

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Concentration effect on the luminescent properties was done by dissolving the TXs and their metal complexes in the methanol solutions at the concentrations from 1 × 10-6 to 1 × 10-4 mol/L.

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The counter ion effect was done by measuring the fluorescent emission spectra of the TXOTPy methanol solutions containing the ZnCl2, Zn(NO3)2, Zn(Ac)2 and Zn(OTf)2 salts in the molar

2.4. Characterization 1

H and

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ratio of 2:1.

C-NMR spectra were recorded on a Varian Gemini (400 MHz) spectrometer with

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CDCl3 or DMSO-d6 as a solvent, and tetramethylsilane as an internal standard. Fourier transform

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infrared spectra (FITR) were measured by using a Nicolet NEXUS 470 spectrometer at 25°C. UV-vis

absorption

spectra

were

measured

by

using

Shimadzu

UV-2550

UV-vis

spectrophotometer. Fluorescence emission spectra were recorded by using Shimadzu RF-5300PC spectrophotometer with the excited wavelength for the TXs and their complexes at 286 and 380 nm, respectively. The fluorescence QE was estimated by using quinine sulfate in the 0.1 mol/L H2SO4

solution

as

the

standard

and

using

the

following

equation,

ΦX

=

ΦST(GradX/GradST)(ηX/ηST)2, where the subscripts ST and X denote standard and test

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respectively, Φ is the fluorescence quantum yield, Grad the gradient from the plot of integrated fluorescence intensity vs absorbance, and η the refractive index of the solvent.[35,36]

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Time-resolved single photon fluorescence measurements were performed using an Edinburgh Instruments FLS-920 spectrometer with microsecond pulsed lamp as the light source. The fluorescence lifetime was estimated from the decay of the main transitions with the use of the

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Edinburgh Instruments software package based on the following equation of ln It = ln I0 − t/τ, where I0 and It are the fluorescence intensity at time zero and t, respectively, and τ is the

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experimental lifetime of the excited state of the TXs. For the TXs and their Fe/Zn/Ni-complexes, the excited (λex) and collected (λem) wavelengths were 286 nm and 424 nm, respectively. For the Tb-TXOBPy and Tb-TXOTPy complexes, the λex and λem were 340 and 547 nm; and for the Eu-

3. Results and discussion

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TXOBPy and Eu-TXOTPy, the λex and λem were 340 and 619 nm, respectively.

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3.1. Absorbance of TXs and their metal-organic complexes

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Fig. 2 shows absorption spectra for the TX derivatives in the dilute methanol solutions. Each curve is mainly composed of two absorption bands. Similar to those reported in the literatures [11,25], the maximum absorption peak of TXOH appears at approximately 256 nm, which slightly shifts to approximately 266 nm (10 nm red shift) for TXOMe and TXOBenMe. This absorption band has been designated to the π–π* electron transition of the TX aromatic rings. When the polypyridyl substituents of BPy and TPy are combined with the TX rings to produce TXOBPy and TXOTPy, the maximum peak shifts to 268−274 nm (12−18 nm red shift). Such a

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peak red shift can be attributed to the following two reasons: (i) formation of a larger πconjugated units in TXOMe, TXOBenMe, TXOBPy and TXOTPy, and (ii) there is a π−π* electron transfer of the BPy and TPy substituents (at approximately 270 nm) in TXOTPy and

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TXOBPy. Figure 2

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The other absorption band appears at approximately 366 nm, which is attributed to the n–π electron transfer of the TX aromatic rings [10]. No obvious peak shift is observed for all TXs,

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indicating that formation of the present TXs has little influence on their n–π electron transfer, though several research groups have reported that this peak could red shift to the visible light region when a large conjugation structure of TXs was formed with anthracene or carbazone [8,11-13]. Here, the TX aromatic rings are connected with Me, BenMe, BPy or TPy substituents

the literatures [8,11-13].

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through an ether linkage that is unable to form so large conjugation structure as that reported in

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Among the present five TXs, TXOBPy and TXOTPy contain bipyridyl and terpyridyl substituents, both of them can be used as a coordinative site to bind with various transition metal

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ions, resulting in interesting optical properties [3,27]. During experiments, five transition metal ions were selected to form metal-organic complexes with TXOBPy and TXOTPy. Fig. 3 shows absorption spectra for the M-TXOTPy complexes (M is Zn2+, Ni2+, Fe2+, Eu3+ and Tb3+ ions) at the molar ratio of 1 : 2 in the methanol solutions. These curves display a strong and broad absorption band between 230 and 320 nm; with the maximum peak at 264−270 nm that designated to the π–π* electron transition of the TX aromatic rings.

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Figure 3 Compared with the absorption spectra of the pure TXs in Fig. 2, no significant difference is

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observed for the Ln-TXOBPy and Ln-TXOTPy complexes (Ln = Eu3+ and Tb3+). On the other hand, the absorption spectra for the complexes of M-TXOBPy and M-TXOTPy (M = Zn2+, Ni2+ and Fe2+) display a relatively large increase at 295−310 nm (π–π* electron transfer of the

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ligands). Particularly, for the Fe-complexes, another absorption band appears at 520−550 nm, corresponding to the metal−ligand charge transfer (MLCT) process [37]. Because of such a

respectively, others are colorless.

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MLCT process, the Fe-TXOBPy and Fe-TXOTPy complexes are purple and pale red,

Figure 4

Based the elemental analysis of the solid samples of the metal-complexes, we found that, under

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the present experimental conditions, the chemical composition was MTXOTPy(OTf)2 (M = Zn2+, Ni2+ and Fe2+) and that was M(TXOTPy)2(OTf)3 (M = Eu3+ and Tb3+). Fig. 4 shows a schematic drawing for the possible chemical structure of the M-TXOTPy complexes produced in the

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present work, wherein each metal ion of Zn2+, Ni2+ and Fe2+ was coordinated with one TXOTPy

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ligand, and each lanthanide ion was coordinated with two ligands. Further, similar spectral feature was recorded when the solid powders of the metal-complexes were dissolved in the methanol solutions (figures not shown).

3.2. Luminescent properties of TXs

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Similar to most organic dyes, the luminescent behaviors of the TXs are concentration dependent. Further, due to their unique structural features and smaller energy gap between the first singlet and triplet excited states, they are also solvent dependent. These luminescent

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properties are interesting for understanding the TX photophysical processes and for potential applications as the light-harvesting units, emitters and photoinitiators.

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Figure 5

Fig. 5 shows the fluorescent emission spectra for the five TXs in the methanol solutions,

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excited at 286 nm, which reveals the following features. First, one broad emission band is recorded at approximately 421−424 nm for TXOH, TXOMe, TXOBenMe and TXOBPy, designated to the electron transfer from the excited S1 state to the ground one of the TX aromatic rings [12,20]. Second, two emission bands are recorded at about 321−355 and 423−437 nm for

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TXOTPy; the former one attributed to the emission from the excited S1 state of the TPy substituent [29], and the latter one to that from the excited S1 state of the TX aromatic ring. The luminescent behaviors of all TXs investigated displayed strong solvent dependent effect.

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As an example, Fig. 6A shows fluorescence spectra of TXOTPy in the solvents of methanol,

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butanol, chloroform, acetonitrile and DMF, excited at 286 nm. It can be observed that, except for that in the methanol and butanol solutions, the emission spectra for TXOTPy in the other solvents displayed one broad band at approximately 344−360 nm. This emission has been attributed to the electron transfer from the excited S1 sate of the TPy substituent to its ground state,29 that is, no fluorescence emission was recorded from the TX aromatic ring. On the other hand, in the methanol and butanol solutions, the fluorescence spectra are composed of two bands at approximately 364 and 422 nm, respectively. The former one is designated to the electron

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transfer from the excited S1 state of TPy, and the latter one to that from the excited S1 state of TX aromatic rings.

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Photos inserted in Fig. 6A are TXOTPy in the solvents of chloroform, acetonitrile, DMF and methanol, under radiation with a commonly used UV lamp at 365 nm. These images indicated bright blue luminescence for TXOTPy, the others are mixtures from both TPy and TX

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substituents.

Previous work has revealed that the luminescent emissions of TXs are closely dependent on the

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solvent polarity [18-20]. An early work was done by Dalton and Montgomery, they found that the fluorescent QE of TXs was over 10 times higher in the polar solvents like acetonitrile than that in the nonpolar ones like hexane [20]. The stronger the solvent polarity the higher the fluorescent QE. Particularly, much higher QE could be recorded for the TX in the alcohol

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solvents like methanol and 2,2,2-trifluoroethanol, where the QE was nearly 100 times stronger than that in the acetonitrile [20].

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Figure 6

Such a solvent effect on the QE of TXs has been attributed to the reason that the energy of the

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(3n, π*) state becomes greater than that of the (1π, π*) state in the more polar solvent and better hydrogen bonder, thus resulting in the intersystem crossing rate of the TXs decreasing with the increase of the solvent polarity and hydrogen bonding [20]. More recently, Gilch, Marian and their coworkers investigated in detail the TX fluorescent emissions in the methanol and 2,2,2trifluoroethanol solutions by both experimental and theoretical methods [21,22]. Based on their results, they suggested that the solvent polarity dependence can be attributed to the reversible

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intersystem crossing between the primarily excited (1π, π*) and a triplet (3n, π*) state, because the latter one is energetically slightly (∼0.02 eV) above the former one.

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Here, effect of methanol on the TXs’ fluorescence emission was further investigated by measuring the fluorescent emission spectra of TXOTPy in the chloroform solution mixed with small amount of methanol. As shown in Fig. 6B, one broad emission band with the maximum

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peak at 350 nm was recorded for TXOTPy in the pure chloroform solution. When 0.1% (v/v) methanol was added in, two emission peaks were recorded at approximately 360 and 422 nm.

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Apparently, these peaks can be designated to the electrons transfer from the excited TPy and TX aromatic rings, respectively. With more and more methanol added (from 0.1 to 10%), the relative emission intensity of the TX aromatic ring was largely increased, suggesting that the energy absorbed by TXOTPy was mainly deactivated through the emission from the excited state (1π,

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π*) of TX aromatic ring to the ground one.

Photos inserted in Fig. 6B are TXOTPy in the chloroform solutions mixed with small amount of methanol from 0, 0.1, 1 to 10% (v/v), under radiation with a commonly used UV lamp at 365

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nm. The images clearly indicated that, when the chloroform solution contains 10% methanol, bright blue luminescent light can be observed owing to the emission from the TX aromatic rings.

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These results provide further evidences that the protic solvent like alcohol takes an important role at the fluorescent emissions for the TX aromatic rings due to the decrease of intersystem crossing rate [18-20]. It can be also suggested that the luminescence of TXs may be tunable in the mixed organic solvents; the feature may be used for designing various emitters that will be discussed with their Ln-complexes below. Figure 7

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In addition to the solvent dependence property, similar to most organic dyes, the luminescence of TXs was also concentration dependent [38,39]. Fig. 7 shows the emission spectra for TXOTPy in the methanol solution with the concentrations increased from 1 × 10-6 to 1 × 10-4

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mol/L, excited at 286 nm. Each curve is composed of two broad emission bands designated the electrons transferring from the excited S1 states of TPy and TX substituents. The emission intensity increases with the TXOTPy concentration increasing from 1 × 10-6 to 8 × 10-6 mol/L,

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while decreasing when it is further increased due to the concentration quenching. Similar

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phenomena are also observed for the other TXs of TXOH, TXOMe, TXOBenMe and TXOBPy.

3.3. Luminescent properties of M-TXOBPy and M-TXOTPy complexes Energy absorbed by the organic dyes can be transferred to other species co-existed such as the

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inorganic ions through an inter- or intramolecular energy transfer process, resulting in luminescent emissions from inorganic ions that can be used as sensors and luminors [40,41]. Here, TXOBPy and TXOTPy can coordinate with many transition metal ions such as Zn2+, Fe2+,

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Ni2+, Tb3+ and Eu3+, among which the Tb- and Eu-complexes can give off strong and unique

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lanthanide fluorescent emissions.

Figure 8

Fig. 8A shows fluorescence emission spectra for the Zn/Fe/Ni-TXOBPy complexes in the methanol solution, excited at 286 nm. It can be seen that, in each case, the luminescent spectrum is composed of one strong emission band at approximately 421 nm, together with a weak one at 306 nm. As having been pointed out that these two bands are attributed to the electrons transfer

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from the excited S1 state of TX and TPy substituents, respectively. The photos inserted in Fig. 8A are the Zn/Fe/Ni-TXOTPy complexes in the methanol solution, under radiation with a commonly used UV lamp at 365 nm. These images display strong blue light owing to the TX

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emission, particularly for the Zn/Ni-TXOTPy complexes. A rather weak light is observed for the Fe-TXOTPy complex, which can be attributed to its small QE value that will be described

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below.

Effect of the counter ions on the TXOTPy luminescent behaviors was investigated by

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measuring the emission spectra of their methanol solutions containing the ZnCl2, Zn(NO3)2, Zn(Ac)2 and Zn(OTf)2 salts in the molar ratio of 2:1. No obvious difference was recorded (figure not shown), which suggested that the counter ions had no influence on the excited state of the ligands.

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Fig. 8B shows fluorescent emission spectra for Eu/Tb-TXOTPy complexes in the acetonitrile solution, excited at 286 nm. Each curve displays several sharp emission peaks at the wavelength between 450 and 750 nm. The photos inserted displayed bright red and green light, attributed to

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the luminescent emissions from Eu3+ and Tb3+ ions. In details, for Eu-TXOTPy complex, the sharp peaks at approximately 580, 591-596, 619, 652 and 695-710 nm designated to the

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fluorescent emissions from the excited level of 5D0 of Eu(III) ions (5D0 → 7F0,1,2,3,4), respectively [42]. For Tb-TXOTPy complex, four sharp peaks were recorded appeared at 489, 545, 585 and ~620 nm, corresponded to the electron transitions of 5D4 → 7Fn (n = 3,4,5,6) of the Tb(III) ions [43]. In addition, two very weak broad bands can be recorded at approximately 360 nm and 425 nm (not shown in Fig. 8B), which can be attributed to the emissions from the TXOTPy ligands. It was found that emissions from either TPy or TX aromatic rings were very weak in the Ln-

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complexes, which suggested that the excited energy of the ligands has been transferred to the excited energy level of the central Ln3+ ions, resulting in strong lanthanide emissions.

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Figure 9 Fig. 9 shows the fluorescent spectra for the M-TXOTPy (M = Fe2+, Tb3+ and Eu3+) complexes in their solid powders, together with that of the TXOTPy one as a control experiment, excited at

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286 nm. The broad emission band of the TXOTPy ligand appeared in the range of 430−500 nm and centered at approximately 470 nm, that is, nearly 40 nm red shift as compared with that in

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the methanol solution. Such kind of red shift has been well observed in the aggregates of the organic dyes and attributed to the enhancement of molecular interactions in the condensed or solid states [44,45]. The emission spectrum for the FeTXOTPy(OTf)2 was similar to that of the ligand. For the Ln(TXOTPy)2(OTf)3 complexes, a broad emission centered at approximately 470

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nm together with several sharp peaks were recorded; apparently, the broad band was attributed to the emission from the TXOTPy ligand and the sharp peaks to that from the central Ln3+ ions [42,43].

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A comparison of the luminescent emission curves in Fig. 9 with those in Fig. 8B revealed that

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the relative emission intensity for the TXOTPy ligand was much stronger in the solid powders than that in the methanol solutions. The phenomenon indicated that the energy transfer was not as efficient in the solid powders as that in the solutions. Possible reason may be due to an increase of molecular interaction in the solid state, which will be further discussed together with the energy transfer mechanism of the Ln-complexes below. Based on the literatures [46,47], it has been known that, after adsorption of energy, the TXOBPy or TXOTPy ligand is generally excited to produce its excited singlet state (S1*). Fig. 10

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shows an energy transfer mechanism in the present Ln-TXOTPy complexes. After a nonradiative process to the lowest excited state, the excited electron can be back to its ground state (S0) by the ligand fluorescence emission at approximately 364 and 422 nm (corresponding to the

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TPy and TX substituents). The excited energy at the S1-TX state can transfer through a nonradiative intersystem crossing process to produce its excited triplet state (T1-TX). Such kind of energy transfer process is generally occurred in the TXs and their Zn2+/Fe2+/Zn2+-complexes. For

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the Ln-complexes, the energy at the T1-TX state can be transferred to the excited level of 5D4 (Tb3+) or 5D0 (Eu3+) ions following with typical Ln3+ ions’ fluorescent emissions of 5D4 → 7Fn

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(Tb3+) or 5D0 → 7Fn (Eu3+).

Figure 10

Further, different from other harvesting units, the energy gap of the TXs between the excited

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S1-TX and T1-TX states is very close (~0.3 eV), thus, this intersystem crossing rate should be very fast and reversible. That is, it is possible for a quick energy transferring from the ligand S1-TX state to T1-TX state, followed by transferring to the excited energy levels of the Ln3+ ions. As

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having been pointed out that, the energy level of the T1-TX state may become higher than that of the S1-TX state in the high polar solvents like methanol, resulting in a low energy transfer

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efficiency from the S1-TX to T1-TX state. In this case, there should be a low fluorescent emission from the Ln3+ ions, reversely, a higher fluorescent emission from the TX aromatic rings. Figure 11

To confirm such a hypothesis and clarify the effect of solvent polarity on the luminescence from the central Eu3+/Tb3+ ions, we measured fluorescent emission spectra for the Ln-TXOTPy complexes in the acetonitrile solutions mixed with methanol. As shown in Fig. 11, the ligand

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emission between 350 and 460 nm was very weak in the acetonitrile solution, while with increase of the methanol from 0, 10%, 20% to 50%, the emissions from the TX and TPy substituents gradually enhanced, especially for the Eu-TXOTPy complex (Fig. 11A). At the

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same time, emissions from the central Ln3+ ions gradually decreased.

The photos in Fig. 11 provided visible and intuitive images of solvent effect on the Ln-

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TXOTPy complexes. For the Eu-complex, the luminescence changed from red to dark red, violet and blue when the methanol was gradually added. For the Tb-complex, the luminescence

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changed from green to cyan and then blue.

Based on the energy transfer mechanism in Fig. 10 and unique features of TXs, we can explain these observations and our hypothesis as follows. The excited T1-TX state of Ln-TXOTPy is more stable in the acetonitrile than that in the methanol, resulting in a higher possibility for the energy

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transferring from T1-TX to the Ln3+ ions. Thus, strong Ln3+ ions’ fluorescent emission is recorded for Ln-TXOTPy complexes in acetonitrile. With the addition of methanol in the Ln-TXOTPy acetonitrile solution, the T1-TX state becomes unstable, leading to lower energy transfer from the

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S*1-TX to T1-TX, thus, the luminescent emissions from Ln3+ ions decreased together with an

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increase of the luminescence from TX aromatic rings. This hypothesis may be further confirmed from the luminescent emissions of the solid powders of the Ln(TXOTPy)2(OTf)3 complexes. As the luminescent emission spectra in Figs 8 and 9, the ligand TXOTPy emission is 40 nm red shifted in the solid powders, which suggest that the excited S1-TX is lower in this case than that in the methanol solution. So, the energy level of the T1-TX state may become closer or higher than that of the S1-TX state, as a consequence, there is a

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low energy transfer efficiency from the S1-TX to T1-TX state resulting in a stronger emission from the TXOTPy ligand and weaker one from the Ln3+ ions.

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Finally, because the excited energy of the T1-TX is closer to the excited level of Tb3+ (5D4) than that to Eu3+ (5D0) ions, stronger fluorescent emissions are observed from the Tb-complexes (5D4

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→ 7Fn) than those from the Eu-complexes (5D0 → 7Fn).

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3.4. Quantum efficiency and time-resolved fluorescence decay

Table 1 lists the fluorescent QE and fluorescent emission lifetime of TXs, Zn/Ni/Fe-TXOTPy and Zn/Ni/Fe-TXOBPy complexes in the methanol solutions, based on their fluorescence spectra and equation of ΦX = ΦST(GradX/GradST)(ηX/ηST)2 [35,36]. These data indicated that the QE of

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the present five TXs was in the range of 0.05 to 0.11 in the methanol solutions, which was well in agreement with that reported in the literatures [1,2,8]. Because of weak fluorescent emissions from the TX aromatic rings in other solvents, we did not calculate their QE values. After

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formation of M-TXOBPy and M-TXOTPy complexes, similar QE was observed except for FeTXOBPy and Fe-TXOTPy; in both cases, very small QE (~0.004) was recorded. Such a

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weakened QE can be attributed to a deactivation process of the LMCT process in the FeTXOBPy and Fe-TXOTPy complexes. Because of the small QE values, the Fe-TXOTPy complex produced very weak luminescent emissions as shown in the photo of Fig. 8A. Table 1 Further, for the Ln-TXOBPy and Ln-TXOTPy complexes, very weak luminescent emission from the TX aromatic rings was recorded in the acetonitrile solutions, as shown in Fig. 8B, thus

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their QE was also very small and not listed in Table 1. Such a weak emission from TX rings can be attributed to the fact that the excited energy of the TX rings is transferred to the central Ln3+

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ions through its excited triplet state as discussed above. The fluorescent lifetime for the TXs and their Zn/Fe/Ni-complexes is in the range of 0.21 to

recorded after the formation of their metal complexes.

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Table 2

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1.12 ns, again in agreement with reported in the literatures [1,8]. No significant difference was

Finally, for the Ln-TXOBPy and Ln-TXOTPy complexes, we measured their fluorescent emission decay of the central Ln3+ ions, with the emission lifetime estimated to be 0.3−0.8 ms (Table 2). Because the absorbed energy is transferred to the Ln3+ ions through an intersystem crossing transfer process, the emission lifetime of Ln3+ ions is generally much longer than that of

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4. Conclusions

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commonly used organic dyes [41,42].

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We have demonstrated synthesis and photophysical properties of new TX-polypyridyl photoinitiators and their metal complexes in various solutions. Because of the very smaller energy gap between the singlet S1 and triplet T1 states of the TXs, their fluorescent emissions are closely dependent on the solvent polarity. It was further revealed that the TX aromatic rings can act as efficient antenna to transfer the excited energy absorbed by polypyridyl and TX substituents to the central Eu3+ and Tb3+ ions, resulting in typical sharp emissions of the lanthanide ions. Also because of the unique structural features of the TXs, luminescent emissions

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from their Ln-complexes are also closely dependent on the solvent polarity, thus we can design and tune the luminescent light of the Ln-TX complexes. Thus, the present TX-polypyridyl

and efficient light antenna for the Ln3+ ions.

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photoinitiators can act as potential candidates for the various light emitters, luminescent sensors

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Acknowledgement. This work was supported by National Science Foundation of China

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(21373058).

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Figure and Table legends Fig. 1. Thioxanthone derivatives synthesized in the present work.

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Fig. 2. UV-vis absorption spectra of (a) TXOH, (b) TXOMe, (c) TXOBenMe, (d) TXOTPy and (e) TXOBPy in the dilute methanol solutions.

Fig. 3. UV-vis absorption spectra for the M-TXOTPy complexes in the methanol solutions,

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molar ratio of Mn+/TXOTPy = 1:2. (a) M = Eu3+, (b) M = Tb3+, (c) M = Fe2+, (d) M = Ni2+, (e) M

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= Zn2+.

Fig. 4. Schematic drawing for the possible structure of the M-TXOTPy complexes produced in the present work.

Fig. 5. Fluorescent emission spectra of (a) TXOH, (b) TXOMe, (c) TXOBenMe, (d) TXOTPy

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and (e) TXOBPy in the methanol solutions, excited at 286 nm.

Fig. 6. (A) Fluorescent emission spectra of TXOTPy in the solvents of (⋅⋅⋅⋅) acetonitrile, (-⋅-⋅) DMF, (---) chloroform and (─) methanol, excited at 286 nm. (Inserted) photos of TXOTPy in the

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solvents of (left to right) acetonitrile, DMF, chloroform and methanol under excitation of UV

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lamp at 365 nm. (B) Fluorescent emission spectra of TXOTPy in the chloroform solutions mixed with (-⋅-⋅) 0, (⋅⋅⋅⋅) 0.1, (---) 1 and (─) 10% methanol, excited at 286 nm. (Inserted) photos of TXOTPy in the mixed chloroform-methanol solutions under excitation of UV lamp at 365 nm. Fig. 7. Fluorescent emission spectra for the TXOTPy in the methanol solutions at various concentrations, excited at 286 nm. (a) 1 × 10-6, (b) 2 × 10-6, (c) 4 × 10-6, (d) 8 × 10-6, (e) 50 × 10-6 and (f) 100 × 10-6 mol/L.

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Fig. 8. Fluorescent emission spectra for (A) Zn-TXOTPyP (─), Fe-TXOTPyP and Ni-TXOTPyP complexes in the methanol solutions and (B) Eu-TXOTPy (---) and Tb-TXOTPyP (─) complexes in the acetonitrile solutions, excited at 286 nm. Photos inserted (left to right) in A:

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Zn/Fe/Ni-TXOTPy and (B) Tb/Eu-TXOTPy under excitation of UV lamp at 365 nm.

Fig. 9. Fluorescent emission spectra for the solid powders of TXOTPy (····), FeTXOTPy(OTf)2 (-

SC

--), Tb(TXOTPy)2(OTf)3 (─) and Eu(TXOTPy)2(OTf)3 (-·-·-), excited at 286 nm. Fig. 10. Energy transfer mechanism in the Ln-TXOTPy complexes.

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Fig. 11. Fluorescent emission spectra for (A) Eu-TXOTPy and (B) Tb-TXOTPy in the mixed solutions of acetonitrile and methanol at different ratios. From bottom to top (CH3CN/CH3OH, v/v): 1:0, 1:0.1, 1:0.5, 1:1, 0:1. Photos inserted in (A) Eu-TXOTPy and (B) Tb-TXOTPy in the

TE D

mixed acetonitrile-methanol solutions under excitation of UV lamp at 365 nm.

Table 1. Fluorescent QE and emission lifetime (ns) of TXs, Zn/Fe/Zn-TXOBPy and Zn/Fe/Ni-

EP

TXOTPy complexes in the methanol solutions.

Table 2. Emission lifetime (ms) of Eu/Tb-TXOBPy and Eu/Tb-TXOTPy complexes in the

AC C

acetonitrile solutions.

32

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Luminescent properties of newly synthesized thioxanthone-polypyridyl

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derivatives and their metal-organic complexes

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Dong-Mei Ma et al

Table 1

Φ

τ (ns)

Materials

Φ

τ (ns)

TXOH

0.058

0.79

Zn-TXOTPy

0.071

1.07

TXOMe

0.077

1.1

Ni-TXOTPy

0.039

0.59

TXOBenMe

0.077

1.05

Fe-TXOTPy

0.006

0.87

1.13

Zn-TXOBPy

0.085

1.1

1.17

Ni-TXOBPy

0.049

0.98

Fe-TXOBPy

0.004

0.798

EP 0.11

AC C

TXOTPy

TE D

Materials

TXOBPy

0.086

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SC

Table 2

Tb-TXOTPy

Eu-TXOTPy

Tb-TXOBPy

Eu-TXOBPy

τ (ms)

0.47

0.54

0.50

0.38

AC C

EP

TE D

Materials

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Luminescent properties of newly synthesized thioxanthone-polypyridyl

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derivatives and their metal-organic complexes

O

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Dong-Mei Ma et al.

O

S

OH

TXOH

S

O

Me

N

N

TXOBPy

O

O

OMe

TE D

S

TXOMe O

TXOBenMe

AC C

TXOTPy

O

EP

S

S

Me

Figure 1

O N N N

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Absorbance (arb.unit)

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(e)

200

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(d)

300

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Wavelength (nm)

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Figure 2

400

(c) (b) (a) 450

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(e)

(d) (c)

(b)

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(a)

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Figure 3

600

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Figure 4

(d)

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Fluorescence Intensity (arb. unit)

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(e)

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Figure 5

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Fluorescence Intensity (arb. unit)

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Figure 6

475

500

350

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a b c d e f

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450

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Figure 7

500

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0

350

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400

450

500

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Intensity (arb. unit)

(A)

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Intensity (arb. unit)

(B)

AC C

450

500

550

600

Wavelength (nm)

Figure 8

650

700

400

500

TE D

300

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Intensity (arb. unit)

SC

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AC C

EP

Wavelength (nm)

Figure 9

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700

AC C

EP

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Figure 10

350

400

450

500

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600

SC

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(A)

650

700

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Fluorescence Intensity (arb. unit)

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750

(B)

AC C

EP

TE D

Fluorescence Intensity (arb. unit)

Wavelength (nm)

350

400

450

500

550

Wavelength (nm)

Figure 11

600

650

700