Energy Transfer Processes in Tb ( III )-Dibenzoylmethanate Complexes with Phosphine Oxide Ligands

Este trabalho relata a síntese, a caracterização e as propriedades luminescentes dos complexos de fórmulas [Tb(DBM)3L], [Tb(DBM)2(NO3)L2)] e [Tb(DBM)(NO3)2(HMPA)2)] (DBM = dibenzoilmetanato; L: TPPO = óxido de trifenilfosfina ou HMPA = óxido de hexametilfosforamida). Os compostos foram caracterizados por análise elementar (CHN), titulação complexométrica com EDTA e espectroscopia no infravermelho com transformada de Fourier (FTIR), e as propriedades de fotoluminescência foram avaliadas. As energias dos estados tripletos do ligante DBM foram determinadas experimentalmente a partir dos espectros de fosforescência resolvidos no tempo dos compostos análogos do íon Gd. As energias aumentam em função do número de ânions nitratos que substituem o ligante DBM nos complexos. Ademais, os espectros de luminescência e os tempos de vida dos níveis emissores revelaram que a eficiência de transferência de energia ligante-metal segue a mesma tendência. Ao contrário dos complexos tris-DBM, o bise o mono-DBM apresentaram elevada intensidade de luminescência, sendo candidatos promissores para camadas emissoras de luz em dispositivos moleculares conversores de luz (LCMD).


Introduction
2][3][4] In lanthanide compounds, the coordinated ligands play various roles that permit obtaining complexes with high luminescence quantum yields.They can (i) relax the Laport's parity selection rule; [5][6][7] (ii) protect the emitting ion from solvent molecules, that can act as quenchers and (iii) help overcome the low absorption coefficients of intraconfigurational-4f electronic transitions (e ca.][9] It is easy to find Ln 3+ -b-diketonate complexes exhibiting quantum yields above 70% in the literature, 2 in which the efficiency of the b-diketonate-to-Ln 3+ intramolecular energy transfer process is largely dependent on the energy difference between the donor state of the ligand and the acceptor state of the Ln 3+ ion (DE). 10,11][12] Furthermore, b-diketonate ligands generally have their donor states localized on the chelating ring, shortening the donor-acceptor distance, R L , and thus enhancing the luminescence quantum yield. 1,125][16][17][18][19][20] The main emitting level of the Tb 3+ ion, 5 D 4 , is approximately 3250 cm -1 above the main emitting level of the Eu 3+ ion, 5 D 0 .Therefore, a single b-diketonate ligand is often not optimum to sensitize both ions, since the triplet states are not resonant for both of them. 17nergy levels of b-diketonate ligands remain essentially the same in tris and tetrakis-complexes such that the ligand-to-metal intra-molecular energy transfers in these complexes are very similar.][24][25] In this work, our group extends the investigation to the mono-, bis-and tris-DBM complexes, [Ln(DBM) 3 L], [Ln(DBM) 2 (NO 3 )L 2 )] and [Ln(DBM)(NO 3 ) 2 L 2 )] (DBM = dibenzoylmethanate; L: TPPO = triphenylphosphine oxide or HMPA = hexamethylphosphine oxide; Ln = Tb 3+ and Gd 3+ ), and the study of the dependence of Tb 3+ luminescence properties as a function of the number of coordinated b-diketonate ligands.The corresponding Gd 3+ complexes were used to mimic the Tb 3+ complexes and determine the excited state energies of the coordinated ligand.

Reagents and syntheses
Terbium oxide (Tb 4 O 7 ), dibenzoylmethane, phosphine oxide ligands (TPPO and HMPA) as well as the solvents (ethanol and acetone) were purchased from Aldrich Co., and used without any previous treatment.Terbium chloride and nitrate were synthesized as described in the literature. 26The terbium and gadolinium complexes were synthesized in the same way; and the preparation of terbium complexes are given as representative.
Syntheses of the tris-diketonate complexes Tb(DBM) 3 L (L = TPPO or HMPA) To an ethanol solution containing 1.00 g (4.46 mmol) of DBM, and 0.41g (1.49 mmol) of TPPO (or 0.29 g of HMPA), a solution of 0.56 g (1.49 mmol) of terbium chloride in 30 mL of ethanol was added dropwise, under stirring.The pH value of the resulting solution was then adjusted to approximately 6.0 using NaOH (0.01 mol L -1 ) in a 50/50 water/ethanol mixture.The resulting yellow solid was filtered, washed with ethanol and dried under vacuum.1.00 g (4.46 mmol) of DBM and 1.24 g (4.46 mmol) of TPPO (or 0.87 g of HMPA) were dissolved in 30 mL of ethanol and added dropwise under stirring to 30 mL of an ethanol solution containing 0.56 g (1.49 mmol) of terbium chloride.The pH value of the resulting mixture was adjusted to 1.0 with concentrated HNO 3 solution , and then to 6.0 using NaOH (0.01 mol L -1 ) in a 50/50 water/ethanol mixture.The yellow solid was filtered, washed with ethanol and dried under vacuum.
[Gd(DBM) 2 (HMPA)  Mono-dibenzoylmethanate complex was obtained only with HMPA as neutral ligand.The synthetic route for this complex was similar to that used for [Tb(DBM) 2 (HMPA)(NO 3 )], but using the 1:2:1 molar ratio of DBM:HMPA:Tb 3+ .An ethanol solution (ca.20 mL) of terbium nitrate (1.66 g, 4.44 mmol) was added to 20 mL of an ethanol solution containing a mixture of 1.00 g (4.46 mmol) of DBM and 1.61 g (8.92 mmol) of HMPA.After ca.24 h, the yellow single crystals were filtered, washed with ethanol and dried in a desiccator under vacuum.

Apparatus
The elemental analyses of carbon, hydrogen and nitrogen in the tris-, bis-and mono-diketonate complexes were performed using a Perkin-Elmer model 2400 microanalyzer, whereas the Tb 3+ ion contents were determined by complexometric titration with EDTA. 27nfrared absorption spectra were recorded in the range of 400 up to 4000 cm -1 in KBr pellets using a Shimadzu FTIR spectrophotometer model IRPRESTIGE-21.
Steady state excitation and emission spectra of the solid complexes were recorded at liquid nitrogen temperature, using a Fluorolog-3 spectrofluorometer (Horiba) equipped with 0.22 m excitation and emission double grating monochromators, a 450 W Xenon lamp as the excitation source, and an R928P PMT photomultiplier as detector.All spectra were recorded using detector mode correction.The second-order diffraction of the source radiation was eliminated by using a cut-off filter.Time-resolved luminescence spectra of the Gd 3+ -complexes and the luminescence decay curves of the Tb 3+ -complexes were recorded at 77 K using the same equipment, but operating in phosphorescence mode with a pulsed Xenon lamp as the excitation source.A time delay of 0.100 ms was applied.The luminescence instruments were fully controlled by the FluorEssence program.All luminescence data were obtained from samples contained in a 2 mm diameter quartz tube.

Characterization of the complexes
The elemental analysis (C, H and N) and the complexometric titration data indicated that complexes presenting the formulas [Ln(DBM) 3 L], [Ln(DBM) 2 (NO 3 )L 2 ] and [Ln(DBM)(NO 3 ) 2 (HMPA) 2 ] (Ln = Gd 3+ and Tb 3+ ; DBM = dibenzoylmethanate; L: TPPO = triphenylphosphine oxide or HMPA = hexamethylphosphine oxide) were obtained.The tris-and bis-diketonate complexes of either HMPA or TPPO were easily obtained.However, it was not possible to obtain the mono-diketonate with TPPO, even after many attempts.This is probably due to the lower donor capacity and steric hindrance of TPPO when compared to HMPA, which allows coordination of more than one DBM ligand to the lanthanide ion.
The coordination modes of the dibenzoylmethanate, phosphine oxide and nitrate ligands were investigated based on their characteristic FTIR absorption bands (Figure S1 in the Supplementary Information (SI) section).The FTIR spectra exhibited strong bands at around 1600 cm -1 that might be assigned to ν(C=O) coupled with ν(C=C) of the DBM ligand.These bands are shifted to a lower wavenumber in comparison with the free ligand, indicating that DBM is coordinated to the metal ion in chelating mode. 28The spectra also show strong bands around 1160 cm -1 , which might be assigned to ν(P=O) of the phosphine oxide ligands (TPPO and HMPA).These bands are also shifted to lower wavenumbers in comparison with the respective free ligands.Two absorption bands were also observed around 1180 and 1036 cm -1 which may be assigned to ν a (NO 2 ) and ν s (NO 2 ) modes, indicating that the NO 3 -is chelated and has C 2v symmetry. 28,29Furthermore, the two characteristic bands assigned to ν 1 + ν 4 combination modes (1820 and 1767 cm -1 ) are separated about 55 cm -1 , reinforcing the implication that nitrate is coordinated as a bi-dentate ligand. 28

Luminescent properties of the Tb 3+ -DBM complexes
According to the intra-molecular energy transfer mechanism suggested by the experimental data and theoretical models, 1,12,14 the excited T 1 states of the ligands play a critical role in defining the Ln 3+ b-diketonate complexes.In order to estimate these energy levels in the Tb 3+ complexes, phosphorescence spectra of equivalent Gd 3+ complexes that do not present intraconfigurational-4f transitions in the visible region were recorded. 1igures 1a and 1b show the steady state emission spectra of the [Gd(DBM) 3 L], [Gd(DBM) 2 (NO 3 )L 2 ] and [Gd(DBM)(NO 3 ) 2 (HMPA) 2 ] complexes with excitation at 370 nm.These spectra are characterized by one very low intensity broad band in the spectral range of 420-455 nm and the strongest bands are in the spectral range of 460-700 nm, which are assigned to the S 1 → S 0 and to the T 1 → S 0 transitions of the DBM ligand, respectively.In order to determine the exact position of 0-0 phonon transitions, the time-resolved luminescence spectra (Figures 2a and 2b) were recorded using a time delay of 0.100 ms.As can be seen, these spectra present only those broad bands that may be attributed to the triplet to singlet transitions.The T 1 state energies determined as the shortest wavelength phosphorescence bands for the complexes are [Gd(DBM) 3 (TPPO)] (20325 cm -1 ), [Gd(DBM) 3 (HMPA)] (20660 cm -1 ), [Gd(DBM) 2 (TPPO) 2 (NO 3 )] (21186 cm -1 ), [Gd(DBM) 2 (HMPA) 2 (NO 3 )] (21142 cm -1 ) and [Gd(DBM)(NO 3 ) 2 (HMPA) 2 ] (21231 cm -1 ).
A significant increase in the T 1 state energies is observed when changing the inner coordination sphere (around the lanthanide ion) from tris-to either bis-or mono-DBM complexes.This is in agreement with results previously observed for Gd(III)-TTA-phosphine oxide complexes 22,26 and suggests that the inter-ligand interactions play an important role in the energy level structures of Ln 3+ diketonate complexes.
In order to obtain evidence for the relationship between the T 1 state position and the luminescent intensity of the Tb 3+ ion in the tris-, bis-and mono-dibenzoylmethanate complexes, the luminescence decay curves were measured (Figures S2 and S3 in the SI section).The decay curves for bis-and mono-dibenzoylmethanate complexes were adjusted with a single exponential function and the lifetime values (t) of the 5 D 4 emitting level were found to be: 0.4948 ms for [Tb(DBM) 2 (NO 3 )(TPPO) 2 ], 0.6661 ms for [Tb(DBM) 2 (NO 3 )(HMPA) 2 ], and 0.6847 ms for [Tb(DBM)(NO 3 ) 2 (HMPA) 2 ].The luminescence decay curves for tris-DBM complexes were better adjusted by a bi-exponential function (t 1 = 0.8946 ms, t 2 = 0.0214 ms) for [Tb(DBM) 3 (TPPO)], and (t 1 = 1.0499 ms, t 2 = 0.0508 ms)  for [Tb(DBM) 3 (HMPA)], and indicate that both the DBM ligand and Tb 3+ ion are acting as emitting species with lifetimes of t 1 and t 2 , respectively.The values of t for the mono-and bis-dibenzoylmethanate complexes are higher than the values of t 2 for the tris-dibenzoylmethanate complexes.The results suggest that the emitting 5 D 4 level of the Tb 3+ ion is efficiently deactivated in the tris-dibenzoylmethanate complexes.This is consistent with the energy level diagram presented in Figure 5, that shows T 1 states for [Tb(DBM) 3 (TPPO)] and [Tb(DBM) 3 (HMPA)], below and a little above of the 5 D 4 level, respectively.

Conclusions
In this work, three series of Tb 3+ -diketonate complexes containing DBM, and phosphine oxide ligands were successfully synthesized and characterized.These compounds with general formulas [Tb(DBM) 3 L], [Tb(DBM) 2 (NO 3 ) L] and [Tb(DBM)(NO 3 ) 2 L] (L = TPPO or HMPA) exhibit different luminescence properties under excitation at DBM transitions.This is in contrast with Tb 3+ tris-DBM compounds, and most of the Tb 3+ -diketonate complexes reported in the literature, which display only very weak luminescence intensities.Bis-and mono-dibenzoylmethanate forms are characterized by strong green luminescence arising from the Tb 3+ ion.Notably, the luminescent sensitizer activity of the DBM ligands for the Tb 3+ center in these complexes increases when the number of the DBM ligand in the first coordination sphere decreases.In mono-and bis-DBM complexes, both stronger DBM-metal interactions and conformational changes of DBM ligand due to the replacement of other coordinated DBM by nitrate ion probably play the main role in increasing the energies of remaining DBM ligand triplet states, thus intensifying the antenna effect.This behavior emphasizes the importance of the inter-ligand interactions on the T 1 state energy, and consequently on the efficiency of T 1 → 5 D 4 energy transfer process.Finally, the experimental results reveal that mono-and bis-DBM complexes of the Tb 3+ ion are promising candidates for light converting molecular devices (LCMD).