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7Фталоцианины
Phthalocyanines
Макрогэтэроцмклы
Статья
Paper
http://macroheterocycles.isuct.ru
DOI: 10.6060/mhc2012.121193r
The First Example of Near-Infrared 4f Luminescence of SandwichType Lanthanide Phthalocyaninates
Sergey S. Smola,3 Olga V. Snurnikova,a Evgeniy N. Fadeyev,a Anna A. Sinelshchikova,b Yulia G. Gorbunova,bc Lyudmila A. Lapkina,c Asian Yu. Tsivadze,bc and Nataliya V. Rusakovaa@
aA.V. Bogatsky Physico-Chemical Institute of National Academy of Sciences of Ukraine, 65080 Odessa, Ukraine hA.N. Frumkin Institute of Physical Chemistry and Electrochemistry of Russian Academy of Sciences, 119071 Moscow, Russia
cN.S. Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, 119991 Moscow, Russia @Corresponding author E-mail: lanthachem@ukr.net
A detailed analysis ofphotophysical characteristics of mono-, double- and triple-decker complexes of ErI11, Yb111 and Lu111 with 2,3,9,10,16,17,24,25-tetrakis(15-crown-5)phthalocyanine (L) was performed. The values of the quantum yields and the rate constants of molecular fluorescence and intersystem crossing of the compounds were estimated. It was discovered that the solutions of the sandwich ErI11 and Yb111 double- and triple-decker phthalocyaninates possess 4f luminescence in near-infrared range (NIR) at 1540 nm and 980 nm, respectively. It was demonstrated that Yb111 mono-phthalocyaninate also exhibits 4f luminescence. The proposed mechanism of excitation energy transfer is discussed.
Keywords: Phthalocyanine, lanthanides, luminescence, 4f luminescence, energy transfer, erbium(III), ytterbium(III).
Первый пример 4f люминесценции сэндвичевых фталоцианинатов лантанидов в ближней ИК-области
С. С. Смола,а О. В. Снурникова,а Е. Н. Фадеев,a А. А. Синельщикова,ь Ю. Г. Горбунова,Ьс Л. А. Лапкина,с А. Ю. Цивадзе,Ьс Н. В. Русаковаа@
Физико-химический институт им. А.В. Богатского Национальной академии наук Украины, 65080 Одесса, Украина ЬФГБУН Институт физической химии и электрохимии им. А.Н. Фрумкина Российской академии наук, 119071 Москва, Россия
°ФГБУНИнститут общей и неорганической химии им. Н.С. Курнакова Российской академии наук, 119991 Москва, Россия.
@E-mail: lanthachem@ukr.net
Проведен детальный анализ фотофизических характеристик моно-, двух- и трехпалубных тетра(15-краун-5) фталоцианинатов Er111, Yb111 и Lu111. Оценены значения квантовых выходов и вероятностей молекулярной флуоресценции и интеркомбинационной конверсии соединений. Для сэндвичевых фталоцианинатов Er111 и Yb111 впервые обнаружена 4f люминесценция в ближнем ИК-диапазоне в области 1540 нм и 980 нм, соответственно. Установлено, что 4f люминесценция ионов Yb111 реализуется также в случае монофталоцианината. Предложены варианты механизма переноса энергии возбуждения.
Keywords: Фталоцианин, лантаниды, 4f люминесценция, перенос энергии, эрбий(Ш), иттербий(Ш).
Introduction
The investigation of near-infrared 4/ luminescence of lanthanide-based systems is one of the most promising research areas owing to their attraction for multiple applications in high technology, first of all in biomedicine.[1-3] Such factors as low background signal ofbio-objects inthis spectral range (high when measured in visible region of spectrum), possibility of excitation of luminescence in a wide range of wavelengths, including soft visible light irradiation, high values of luminescence lifetimes are factors, responsible for possible applications.^5 The values of resonance energy levels of the lanthanide ions emitting in NIR spectral range are at about 10800-11500 cm-1, 6490 cm-1 and 10000-10200 cm-1, for Nd111, Er111 and Yb111, respectively,[6] that allows to study 4/ luminesence of lanthanide containing complexes with organic ligands with low triplet levels. Among such organic ligands the macrocyclic tetrapyrroles are most attractive. Indeed, a large number of studies are devoted to luminescence of lanthanide complexes with various porphyrins, which triplet levels are about 14000-15000 cm-1. [7-9] The lanthanide ion in investigated complexes can be located directly at porphyrin N4 coordination cavity[10-12] or bound by chelating peripheral substituents of a porphyrin.[13] Although there are many reports on the synthesis and physico-chemical properties of lanthanide complexes with phthalocyanines -analogues of porphyrins,[14-18] but number of works devoted to investigation of 4/ luminescence in these compounds are limited.[19-21]
Higher thermal stability of phthalocyanines compared to porphyrins, ability of these molecules to self-assembling due
Figure 1. Investigated complexes (a) [LnLOAcX], Ln = Lu, X = 344
to stacking interactions between aromatic rings makes them attractive for spectroscopic investigations and optical applications. From the other side sandwich lanthanide-based phtha-locyaninato complexes possess a wide set of unique spectral, electrochemical, semiconductor and catalytic properties.117-181 Such peculiarities of sandwich phthalocyaninates combined with unique luminescent properties of lanthanide ions stimulate the appearance of new fields of investigations and application of sandwich lanthanide phthalocyaninates.
Herein we describe the photophysical characteristics of mono, double- and triple-decker complexes of Er111, Yb111 and Lu111 with tetra(15-crown-5)phthalocyanine, including N1R 4f luminescence.
Experimental
The monophthalocyaninates [LuLOAcPhen] (Phen -1,10-phenanthroline), [LnLOAcDBU] (Ln = Er, Yb, DBU -1,8-diazabicyclo[5.4.0]undec-7-ene), as well as sandwich type complexes LnL2 and Ln^L (Ln = Er, Yb, Lu) were investigated in this study (Figure 1). Tetra(15-crown-5)phthalocyanine (L), Er111, Yb111, Lu111 complexes were synthesized and characterized using previously described methods.[15,22-24]
Spectroscopic measurements were performed in absolute CHCl3 in concentration of 10-5 M using quartz cells with optical path of 1 cm. Ln2L3 complexes were dissolved in CHCl3 with addition of 2 vol.% of CH3OH to prevent the oxidation of phthalocyaninates. [25] Electronic absorption spectra in the UV- and visible range were recorded on Specord M40 and Perkin-Elmer Lambda 9 UV/V1S/ N1R spectrophotometers.
Excitation, molecular and 4f luminescence spectra were registered on a Fluorolog 3-22 (Horiba Jobin Yvon) spectrofluorimeter.
l; Ln = Yb, Er X = DBU; (b) LnL2 and (c) Ln2L3, Ln = Lu, Yb, Er. Макрогетер0цикJlbl /Macroheterocycles 2012 5(4-5) 343-349
The Хе-lamp (450 W), detectors R928P (Hamamatsu, Japan) was used for visible range and InGaAs photoresistor DSS-IGA020L (Electro-Optical Systems, Inc, USA), cooled by liquid nitrogen was applied for NIR range. The excitation and emission spectra were corrected taking into account distribution of Xe lamp emission and sensitivity of photoelectric multiplier.
The relative quantum yields were determined as described in [26]. Znn tetraphenylporphyrinate (ZnTPP, ф = 0.03[27]) in ethanol and YbnI tris-(2-thenoyltrifluoroacetonate) in toluene (Yb(TTA)3, ф = 0.0035[28]) were used as a reference in determination of fluorescence and 4f luminescence quantum yields, respectively.
Fluorescence lifetime measurements were performed with the same Fluorolog 3-22 instrument using TCSPC technique. A pulsed NanoLED-370 (Horiba Jobin Yvon, Xex = 370 nm, repetition rate 500 KHz, pulse duration 1.3 ns) was used as a light source, and R928P (Hamamatsu, Japan) as a detector in photon counting mode, resolution of a time-to-amplitude converter - 0.112 ns per channel. The instrumental response function was measured with the use of Ludox® (Sigma-Aldrich) colloidal solution at monochromator wavelength set to 370 nm. Signals were acquired using an IBH Datastation HUB photon counting module and data analysis was performed using the commercially available DAS 6 decay analysis software package from Horiba Jobin Yvon. Goodness of fit was assessed by minimizing the reduced chi squared function, each trace contained 10000 points and the reported lifetime values are the result from at least three independent measurements.
Results and Discussion
Electronic Absorption Spectra
The UV-visible absorption spectrum of free-base tetra(15-crown-5)phthalocyanine consists of a Soret band with maximum at 348 nm and two g-bands at 662 and 702 nm (Table 1). During the formation of lanthanide phthalocyaninates a typical changes were observed in UV-visible spectrum, namely, conversion of two g-bands into one due to increasing of molecule symmetry.
The electronic absorption spectra of mono-phthalocyaninates solutions consist of typical set of absorption bands in UV and visible region; the most intensive are narrow g-band with maxima at 679 or 685 nm with vibrational satellites and Soret band in the range of 350-370 nm (Table 1).
The position of the absorption bands in the spectra of mono-phthalocyaninates solutions does not depend on radius
of the metal ion, however the influence of the nature of axial ligands in Ln111 coordination sphere is observed. Thus, the UV-visible absorption spectra of Er111 and Yb111 mono-phthalocyaninates [LnL OAc DBU] solutions completely coincide while there is a shift of all of the absorption bands in spectrum of chloroform solution of Lu111 complex that contains according to NMR data axially coordinated phenanthroline molecule.
In contrast to mono-phthalocyaninates the position of the absorption bands in the spectra of sandwich compounds solutions depends on radius of the lanthanide ion as far as the value of the radius defines distance between decks and as a results degree of n-n interaction. The intensive g-band at 665-669 nm and Soret band around 370 nm (Table 1) are observed in the UV-visible absorption spectra of solution of double-decker complexes LnL^ in chloroform. Less intensive absorption band is observed also in the range of 476-480 nm, that is typical for electroneutral radical forms of the complexes [L2-]Ln3+[L"]. This band corresponds to electron transfer from LUMO-1 to semi-occupied LUMO orbital. Main absorption bands undergo a small bathochromic shifts passing from Lu111 to Er111.
The electronic absorption spectra of triple-decker complexes Ln^ reveals two g-bands (intensive g1 at 638-640 nm and less intensive g2 at 707-718 nm), as well as Soret band at 362 nm (Table 1). The absorption band gj undergoes hypsochromic shift relatively to g-band of monophthalocyaninate and has additional less intensive g2 band. The splitting of g-bands is increased according to decrease of lanthanide ionic radius passing from Er111 to Lu111. Such spectral transformations are determined by exciton interactions between three L2- ligands in contrast to monophthalocyaninate.[15,29] Position of gx absorption band also undergoes a small bathochromic shift passing from Lu111 to Er111.
Molecular Fluorescence
It was found that molecular fluorescence of lutetium phthalocyaninates is observed in the visible region under excitation at the absorption maximum at 298 K. The fluorescence band maxima (XF), lifetimes (tf), the rate constants (kF), the fluorescence quantum yields (фД as well as the rate constants (kST) and quantum yields (фет) of intersystem crossing of tetra-(15-crown-5)-phthalocyanine and its lutetium complexes are presented in Table 2.
Table 1. UV-visible spectral data of investigated compounds in CHCl3 solution.
Compound
X , nm
h2l 701 662 601 422 348
[ErLOAcDBU] 679 612 359 290
[YbLOAcDBU] 679 612 359 290
[LuLOAcPhen] 685 612 357
ErL2 669 605 480 368 291
YbL2 667 605 479 369 292
LuL2 665 603 476 367 290
Er2L3* 707 640 362 293
Yb2L3* 715 638 362 294
Lu2L3* 718 638 362 294
*Measurements were performed in CHCl3 with 2% of CH3OH in order to avoid the oxidation
Table 2. Photophysical characteristics of tetra(15-crown-5)phthalocyanine and its complexes with lutetium.
Compound
L, nm
F'
(intensity ratio)
Í
kF, 106 s-1 kST, 106 s-1
h2l 708, 738, 784 (4.7 : 1.4 : 1.0) 7.64 0.740 96.85 34.03 0.260
[LuLOAcPhen] 704, 734, 775 (5.9 : 1.6 : 1.0) 6.07 0.064 10.54 154.15 0.936
LuL2 704, 737, 784 (9.2 : 1.3 : 1.0) 6.13 0.010 1.63 161.37 0.989
LU2L3 702, 736, 785 (4.9 : 1.4 : 1.0) 5.81 0.001 0.17 169.83 0.986
The fluorescence spectrum of free-base ligand consists of the intense band with a maximum at 708 nm and shoulders at 738 nm and 784 nm. In the spectra of Lu111 complexes solution (Figure 2) these bands undergo insignificant changes, but the general view of the fluorescence spectrum is remain the same for all LuIII-phthalocyaninates. The spectral distribution of the radiation energy remains constant under excitation in both cases: at Soret band (350-360 nm) and 0-band (~600 nm).
650
700
750 800
wavelength (nm)
850
Figure 2. Molecular fluorescence spectra of lutetium III complexes [LuLOAcPhen] (1), LuL2 (2) and Lu2L3 (3) (Xex = 350-360 nm, c = 10"5 M, CHCl3).
The kinetics of fluorescence decay for all Lu111 complexes are described by mono-exponential curve fitting indicating the presence of one type of compounds in solution. The lifetimes are in the range of 5.8 - 6.0 ns (Table 2), these values are typical for fluorescence of tetrapyrrolic ligands. The quantum yields of fluorescence and intersystem crossing S1—>T1 were calculated based on the obtained data and their rate constants were estimated. Notwithstanding, the processes of internal conversion S1—S0 formally should not be neglected. The formation of metallocomplexes lead to quenching of molecular fluorescence (Table 2), moreover fluorescence quantum yield decreases with each subsequent addition of the phthalocyanine ligand. At the same time, the probability of intersystem crossing increases as compared to the ligand. Quenching of sandwich compounds fluorescence compared to monophthalocyaninates may be caused mainly by two factors. At first, probability of S1—So transition (kF) decreases with the increase of number of tetrapyrrolic
fragments in the complex. Second, due to the relatively small Stokes shift and overlapping of the absorption and fluorescence spectra, reabsorption of photons emitted by the phthalocyanine molecules occurs,[30] especially in the case of triple-decker complexes, where photons emitted by the central ligand can be absorbed by two peripheral macrocycles. Efficiency of intersystem crossing process of studied phthalocyanine complexes is high and comparable to Zn11 porphyrinates.[31,32] Thus, intersystem crossing is the main route of S1-state deactivation in Lu111 phthalocyaninates. Phosphorescence spectra and the position of the triplet level of studied compounds could not be observed, but because there is quite effective intersystem crossing, the energy difference between the singlet and the triplet state is probably not less than 5000 cm-1.[33] It should be noted, that the photogeneration of singlet oxygen is typical for lanthanide phthalocyaninates. p°,34] Thus, it can be assumed that in the studied compounds the quenching of ligand phosphorescence occurs through the deactivation of the triplet state by singlet oxygen molecules, but in this research, such studies were not carried out.
In contrast to Lu111 phthalocyaninates the complexes with Er111 and Yb111 may exhibit not only the molecular fluorescence in the visible region, but infrared 4/luminescence can also occur. Therefore, for all complexes of ErIII and YbIII with tetra(15-crown-5)phthalocyanine intrinsic 4/ luminescence was studied.
4/Luminescence
It was found that in the solution of ytterbium complexes molecular fluorescence is not completely quenched. Fluorescence quantum yields are 0.0380, 0.0012 and 0.0009 for [YbL OAc DBU], YbL2 and Yb2L3, respectively. For effective excitation energy transfer to the emitting levels of lanthanide ions, the spectral overlapping between the energy levels of the donor, in this case, tetra(15-crown-5) phthalocyanine, and the acceptor - lanthanide must exist. It is known[35 36] that the values of the triplet levels of phthalocyanines and resonance 2F5/2-level of YbIn ion are close enough. The 4/ luminescence of YbIn was registered in the range of 920-1060 nm in all studied YbIn complexes under excitation in the region of Soret band absorption as well as in the region of the intense 0-band (Figure 3). Therewith the excitation spectra in the region of 300 - 600 nm are almost identical to absorption spectra of the corresponding complexes.
The most intensive 4/ luminescence is observed in the spectrum of solution of triple-decker complex Yb2L3.
V ns
ST
1.0
3
ni —'
■iH
GO
g 0.5.1 u
•s
5/2 7/2
975
900 950 1000 1050
wavelength (nm)
Figure 3. 4/Luminescence spectrum (Xex = 350-360 nm) of Yb2L3 complex (c = 10-5 M, CHCl3)
The quantum yield values are less than 0.06 10-4 for mono-and diphthalocyaninate, within 0.59 10-4 for Yb2L3, that is one order less compared to YbIn-monoporphyrinate.[37'381 The band corresponding to the transition 2F5/2 ^ 2F7/2 in the luminescence spectrum of triple-decker complex is splitted by the crystal field on five components. Peaks with maxima at 920-980 nm correspond to the transitions between the lower Stark components of the 2F5/2 and 2F7/2 levels, and bands in the region of 1000-1060 nm caused by the transitions from the lower Stark components of the 2F5/2. Peak with a maximum at 1053 nm corresponds to the Stark component of the ground state, the splitting of which is about 760 cm-1.
It can be proposed that, the observed 4/luminescence of Yb111 in phthalocyaninates may be due to one of the following factors. The Forster-type of energy transfer is forbidden by the selection rule |AJ| = 2, 4, 6 for 2S+1L.-levels for the resonance 2F5/2-level of Yb 111 ion,[39,40] moreover, the spectral overlapping of the energy levels of the donor and acceptor is minimal. Therefore, this mechanism of energy transfer (from both singlet and triplet levels of phthalocyanine) is impossible, unlike Dexter migration, because |AJ| = 1. Dexter energy transfer is effective only at distances less than 10 A and may be observed in the studied systems.
It is also necessary to take into account that among of the lanthanide ions Yb3+ is one of the easily reduced to Yb2+; the redox potential of Yb3+/Yb2+ is -1.05 V.[41-42] It was shown that the ligand cannot reduce ion YbIII in the ground state of double- and triple-decker YbIII phthalocyaninates. However, in the excited state this process can occur as it has been described for complexes of YbIII with organic ligands containing conjugated aromatic moieties.[43,44] In this case, the observed fluorescence quenching may be due to a rapid transition from Sn state to the charge transfer state, that corresponds to the transition of n-electron from the phthalocyanine orbitals to the lanthanide orbital with the reduction of the Ybffl to divalent state. In addition, the intensity of the 4/luminescence under excitation in both S^ and Sn state is almost the same. This means that the complex goes from the charge-transfer state to the 2F5/2, but it is difficult to estimate the participation of S^ and T states in this process.
Thus, the 4/ luminescence of Yb111 phthalocyaninates may be caused also by photoinduced electron transfer.
It is well known that the NIR-luminescence of Er111 complexes is generally observed in non-aqueous media or deuterated solvents.[45,46] Low-lying emissive level of Er111 4/13/2 is extremely sensitive to quenching effects of the OH-, NH and C-H bonds oscillations. However, the position of the energy levels of phthalocyanine ligands is favorable for the implementation of the intramolecular energy transfer to ErIII ion to a greater extent than other lanthanides. The absorption spectrum of Er111 ion is characterized by a large number of bands in the UV and visible region. The most efficient spectral overlap is observed for levels 4F9/2 (15500 lioo cm-1), 4I9/2 (12500 cm-1) and 4I11/2 (10200 cm-1). For the latter, the selection rule of Forster-type of energy transfer (|AJ| = 2) is valid, and for the emissive level 4I13/2 it is the conditions of exchange mechanism (|AJ| = 1).
It has been shown that for ErIII mono-phthalocyaninate solution 4/ luminescence is absent in conditions of our experiments. This compound exhibits molecular fluorescence in solution with quantum yield equal 0.0148. As for the double- and triple-decker erbium complexes, they are also characterized by molecular fluorescence, but it is signally quenched; its intensity is only 14-15 % of mono-phthalocyaninate.
In contact to mono-phthalocyaninate, 4/luminescence of Erffl in sandwich complexes with tetra(15-crown-5)-phthalocyanine is observed under excitation in the ligand absorption region (Figure 4). Its absence in the case of ErIII monophthalocyaninate solutions is probably may be explained by quenching by the solvent molecules oscillations, because of the higher availability of the central ion, in contrast to the sandwich complexes, where the central complexing ion is "closed" by phthalocyanine ligands.
4/ Luminescence spectra profile and intensity of the double- and triple-decker phthalocyaninates solutions are almost identical. Similar to ytterbium complexes in erbium-containing sandwich complexes luminescence spectra are splitted into a number of components with maxima at 1487 nm, the most intensive at 1538 nm, 1548 nm, 1564 nm and 1576 nm.
The luminescence observed in this spectral region is of great interest for the development of signal amplifiers in
13/2 15/2
1.0 n
1450
1500 1550
wavelength (nm)
1600
Figure 4. 4/Luminescence spectrum Er2L3 (Xex = 350-360 nm, c = 10-5 M, CHCl3).
telecommunication systems, since the lowest attenuation of the radiation is observed in the third transmission window of optical fibers at 1550 nm. Herewith, the value of full-width half-maximum (FWHM) of the band has a great importance for optical amplification. The FWHM values of 4/ luminescence band in the solutions of studied complexes are 16 nm and 32 nm for ErL2 and Er2L3, respectively. This is comparable with the data obtained for the ErIII compounds in rigid inorganic matrix, for example, it is 25 nm for ErIII containing phosphosilicate glasses, 11 nm - in SiO2 and 55 nm - in Al2O3.[47]
Conclusions
Thus, the features of spectral (absorption and luminescent) properties of mono-, double- and triple-decker complexes of erbium(III), ytterbium(III) and lutetium(III) with tetra(15-crown-5)phthalocyaninates were determined. The basic photophysical parameters of fluorescence and intersystem crossing of lutetium-containing compounds were evaluated; their values are comparable with porphyrinates of d-metals. The 4/ luminescence of sandwich ErIn- and YbIn-phthalocyaninates in the near-infrared region was discovered for the first time. All complexes of YbIn with tetra(15-crown-5)phthalocyanine exhibit near-infrared 4/ luminescence (980 nm). The mechanism of this process was proposed. This luminescence can occur probably due to Dexter energy transfer or it can be caused by photoinduced electron transfer with the reduction of YbIII to YbII in the electronically excited state of the complex. The intensive 4/ luminescence in the NIR-region with maximum at 1535-1540 nm is also observed for the solutions of double- and tripledecker complexes of erbium(III). The found parameters of 4/ luminescence are comparable to the erbium-containing inorganic compounds.
Acknowledgements. This work was performed in the frame of the Ukrainian-Russian project of the State Foundation for Fundamental Research of Ukraine (#F40.3/007) and the Russian Foundation for Basic Research (#11-03-90443_ Ukr_a).
The authors gratefully remember professor Yu.V. Korovin, who has initiated this joint research.
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Received 02.11.2012 Accepted 16.12.2012
