Relaxation process and phase transition of lanthanide liquid crystalline complexes by photoacoustic

Available online at www.sciencedirect.com)OURNALOPScienceDirectRARE EARTHSEL SEVIERJOURNAL OF RARE EARTHS, Vol. 26, No. 3, Jun. 2008,p. 320ww.e-joumal.comRelaxation process and phase transition of lanthanide liquid crystallinecomplexes by photoacoustic spectroscopyL Junjia (李俊嘉), YANG Yuctao (杨跃涛), LIU Xiaojun (刘晓峻), ZHANG Shuyi (张淑仪), ZHANG Zhongning (张仲宁(Key Laboratory of Modem Acoustie, Ministry of Education, Instinte of Acoustics. Navjing Universit,. Nanjing 210093, China)Received 24 April 2007; revised 7 November 2007Abstract: Lanthanide containing liquid crystals exhibiting smectic A phase close to room temperature were obtained. Photoacoustic (PA)spectroscopy was used to study the spectral properties and phase transitions of liquid crystalline metal complexes. It was found that PA in-tensity of the ligand had a relationship with the probability of nonradiative transitions, which increased in the order of Eu(ta)LrLa(ta)JL>Tb(ta)zI2. This is coincident2.3 Relaxation processeswith the PA spectra. As the probability of radiative transi-The excited state manifold diagram of lanthanide termarycomplexes is shown in Fig.3. The lowest triplet state To of(细)the complexes is from the phosphorescence spectrum of(1)^m=613 mGd(ta)L2, and the singlet excited state S; and energy levelsofLn'+ are based on their PA assignments and referencet".It is generally accepted that the intramolecular energytransfer from the ligand to lanthanide ion occurs from thelowest triplet state energy level of the ligand to the reso-nance energy level of lanthanide ion, which atributes the(2) λ_ =550 nmexcellent fluorescence property of lanthanide chelate. Theenergy transfer efficiency of lanthanide complexes depends(3) A._=546 nm”mainly on the two energy transfer processes; one is from thelowest triplet state of organic ligand to the resonant level of250350450s50lanthanide ion through the resonant exchange interaction,Walelength/mand the other is the inverse energy transfer by the thermalde- excitation mechanism. The thermal de excitation rate(b)K(T) is given al2),(1)。 Radiativeλ_=370 nmS、20T D,iD。510(2).-. "xa5055Walelengt/nmFu2Er“1s2Fig.2 Excitation spectra (a) and emission spectra (b) of Eu(ta)Lz Fig.3 E中国煤化I(ta)sLs, radiative (solid(1), La(ta)l2z (2) and Tl(ta)zLz (3):MYHCNMHGu JJ et al, Relaxation process and phase transition of lanthanide liquid crstalline complexes by photoacoustic spectroscopy 323K(T)= Aexp( -AEr:un*/RT)(420where, -△Erut* is the energy difference between the ligand1:triplet state and the resonant level of lanthanide ion.10PFor La(ta)}L2, La'+ with 4f electronic configuration hasno excited states below the triplet state of the ligand; the en-ergy absorbed by the ligand cannot transfer to La+, but canrelax through its own lower energy levels, which results inthe fluorescence of the ligand. For Er(ta)3Lz, the energyH1levels of Er'+ are often intermixed and provide paths for ef-ficient quenching of the excited states of the ligand. Nofluorescence emission of ligand can be found for Er(ta)]Lr283 290300 310 320 330 340 350 358in the region detected. As for Eut(ta)3I2 complex, the energyTemperatureKdifference between the lowest triplet state and the fluores-Fig.4 PA amplitude P and shifts of phase angle Sp of Eu(ta)Lr ascence levels of Eu'+ is suitable for efficient energy transfer.a function of temperature at heating a rate of 0.3 K/minIn the case of Tb(ta)L2, the energy difference between thelowest triplet state (21 ,300 cm ) and the resonance level 'D4of Tb+ (20,500 cm~l) is quite small, and the thermalde-excitation can remarkably weaken the fluorescence ofTb+I13.trrir2.4 PA study of phase transitionsWe measured the PA spectra of Ln(ta)3Lz at differenttemperatures, and found that both the position of the PAmsinsignal and the PA band of the complexes hardly change. Itindicates that one wavelength can be chosen to measure thechange of PA amplitude and phase, and to investigate the283290 300 310 320 330 340 350 358phase transition of Ln(ta)}Ls. The samples at 370 nm can beTemperature/Kmeasured in the region of ligand absorption with the modu-Fig.5 PA amplitade P and shifts of phase angle Op of Er(ta)Lr aslated frequency of 28 Hz.All the above liquid crystals are in the mesophase close toroom temperature and transform to the isotropic liquid pbasenB1Y。at elevated temperatures. The dependence of the PA ampli-2√2T。k12aq(B-o)(5tude and phase of the liquid crystals Eu(ta)JLr2 an，(r-1)(b+1)e°1-(r+1)6-1)e"'+2(b-r)eBEr(ta)L2 measured on heating (0.3 K/min) as a function of(g + l)(b+1)e°'-(g -1)(6- I)e-°1temperature are shown in Figs.4 and 5. For Eu(ta)zL2, twoThe liquid crystals investigated here are optically opaquephase transitions can be observed from the PA signal; oneand thermally thick under our experimental conditions. Weoccurs around 295 K and the other around 335 K as showncan seteloO,et~Oand| r| <1. For the complex with thein Fig.4. It is clearly observed that as the temperatureconjugated π electron ligand, B-10 cm-', the acoustic signalcrosses the transition point, the PA amplitude shows athen becomes,minimum and the PA phase a maximum, and both the am-YP1ougu,plitude and phase signals show a discontinuity. Similar ob-P=47i1rk.-expl(ot-)(6servations have also been made in the Er(ta)}Lz; the phasetransitions are located at about 297 K and about 321 K.The phase shift of the PA signal relative to modulated lightThese phenomena are recorded during the heating process.is equal to -π/2. The amplitude of the PA signal may be ex-On cooling, the evolution is similar.The theory of the PA effect was developed byP(T )=K[C(T )K(T )-|/2(TRosencwaig and Gersholl4. Basically, it is a one-dimen-where, C(I) and K(T) are the specific heat and thermalsional heat flow model valid in most cases. For the cormplexconductivity of the sample at temperature T, respectively. ToPA signal P= qexp( ip) with ampliude q and phase φ (with中国煤化工. asured the temperaturerespect to the incident radiation), the following equationdepeatz window of the cellholds,coateYHC N M H Gack used as reference.324JOURNAL OF RARE EARTHS, VoL 26, No.3, Jun. 2008The normalized PA signals in Figs.4, 5 are then directly pro-portional to [C(T )K,(T )~2 of the samples at different tem-peratures.0.5The amplitude of Eu(ta)zLr changes more intensivelythan that of Er(ta)L2. Since Er+ ion relaxes dominantly0.4through nonradiative relaxation processes, ?≈1, the changeof PA amplitude for Ertta)3Lrz reflects the thermal property0.3change during phases transitions. However, for Eu(ta)Lr0.complex, which is strongly luminescent, Eq.(5) will includethe probability of nonradiative transition y of the complex.0.1The emission quantum yield of Eu(ta)Lzis 29% at 290 K(Aex= 370 nm); however, the fluorescence of Eu(ta)zL is285 290300 310 320 330 340 345obviously quenched as the temperature increases, indicatingTemperature/Kthat Y increases with the temperature. The PA amplitude ofFig.6 Fluorescence decay time of the D。level of Eu(ta)L2 as aEu(ta)Lz is then relevant to both the thermal property andfunction of temperature. The luminescence was monitored atthe luminescence efficiency of the complex.613 nm (Do- +F2 line) and the excitation wavelength wasPA phase is a time delay that occurs during the process370 nmfrom light absorbed by the sample to the acoustic signal be-ing detected by the microphone. The phase data contain theopaque and thermally thick sample by Korpiun and Til-contribution from a number of sources: the geometry of thegner'"T in the case of a reversible first order transition. Thephotoacoustic cell, the response of the detecting system, thecalculation takes into account the latent heat involved in theoptical absorption coefficient, the nonradiative decay paths,phase transition. The heat produced by the absorbed light isetc. In an actual photoacoustic measurement, many of theused as the latent heat required for the transition and there-non-sample-related parameters may be maintained constant,fore does not contribute to the PA signal. Korpiun and Til-and the PA phase for lanthanide complexes can be expressedgner predict a decrease of the PA amplitude and an increaseasl16)of the PA phase as the phase transition occurs. Further, the4=tan '(worp)+tan (wr)- tan“(-1(20p)1)(8)same signal changes should occur near the phase transitionwhere, o=2xf fis the modulated frequency), qp=1Iβa, (B istemperature in warming as well as in cooling experiments.the optical absorption coefficient of the sample, and a is theThe PA experimental results agree with the theoretical pre-thermal diffusivity), and r is the relaxation time. For thedictions for the transitions from glassy state to smectic Acomplexes with π-π' electron system, B-1x10 cm-' andphase and smectic A phase to liquid phase.p→0, the PA phase has the form,4= n/4+tan '(wr)(9)It can be seen from the fluorescence spectra that energyReferences:can transfer efficiently from the ligand to Eu'+, which makes[1] Binnemans K, Gorller-Walrand C. Lanthanide-containing liq-the lifetime of ligand shorter. Eu'*+ is a luminescent ion anduid crystals and surfactants. Chem. 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