Magnetic Circularly Polarized Luminescence Research Articles

Unusual Magnetic Field Responsive Circularly Polarized Luminescence Probes with Highly Emissive Chiral Europium(III) Complexes

AbstractChirality is ubiquitous within biological systems where many of the roles and functions are still undetermined. Given this, there is a clear need to design and develop sensitive chiral optical probes that can function within a biological setting. Here we report the design and synthesis of magnetically responsive Circularly Polarized Luminescence (CPL) complexes displaying exceptional photophysical properties (quantum yield up to 31 % and |glum| up to 0.240) by introducing chiral substituents onto the macrocyclic scaffolds. Magnetic CPL responses are observed in these chiral EuIII complexes, promoting an exciting development to the field of magneto‐optics. The |glum| of the 5D0 → 7F1 transition increases by 20 % from 0.222 (0 T) to 0.266 (1.4 T) displaying a linear relationship between the Δglum and the magnetic field strength. These EuIII complexes with magnetic CPL responses, provides potential development to be used in CPL imaging applications due to improved sensitivity and resolution.

Unusual Magnetic Field Responsive Circularly Polarized Luminescence Probes with Highly Emissive Chiral Europium(III) Complexes.

Chirality is ubiquitous within biological systems where many of the roles and functions are still undetermined. Given this, there is a clear need to design and develop sensitive chiral optical probes that can function within a biological setting. Here we report the design and synthesis of magnetically responsive Circularly Polarized Luminescence (CPL) complexes displaying exceptional photophysical properties (quantum yield up to 31 % and |glum| up to 0.240) by introducing chiral substituents onto the macrocyclic scaffolds. Magnetic CPL responses are observed in these chiral EuIII complexes, promoting an exciting development to the field of magneto‐optics. The |glum| of the 5D0 → 7F1 transition increases by 20 % from 0.222 (0 T) to 0.266 (1.4 T) displaying a linear relationship between the Δglum and the magnetic field strength. These EuIII complexes with magnetic CPL responses, provides potential development to be used in CPL imaging applications due to improved sensitivity and resolution.

Generation of Circularly Polarized Luminescence by Symmetry Breaking

Circularly polarized luminescence (CPL) has attracted significant attention in the fields of chiral photonic science and optoelectronic materials science. In a CPL-emitting system, a chiral luminophore derived from chiral molecules is usually essential. In this review, three non-classical CPL (NC-CPL) systems that do not use enantiomerically pure molecules are reported: (i) supramolecular organic luminophores composed of achiral organic molecules that can emit CPL without the use of any chiral auxiliaries, (ii) achiral or racemic luminophores that can emit magnetic CPL (MCPL) by applying an external magnetic field of 1.6 T, and (iii) circular dichroism-silent organic luminophores that can emit CPL in the photoexcited state as a cryptochiral CPL system.

Non-classically Controlled Sign in a 1.6 Tesla Magnetic Circularly Polarized Luminescence of Three Pyrenes in a Chloroform and a PMMA Film

Molecular pyrene, 1-hydroxylpyrene, and 2-hydroxypyrene in chloroform and embedded in poly(methyl methacrylate) film reveal mirror-image magnetic circularly polarized luminescence (MCPL) when south (S)-up or north (N)-up direction of the longitudinal external magnetic field of 1.6 T to unpolarized incident light (Faraday geometry) is employed. The |gMCPL| magnitude is on the order of 10−3 T−1 in 374–416 nm. The sign of MCPL is controlled by the S-up and N-up geometry and by the position (1- or 2-) of the hydroxy group.

Characterization of tetrakis(thiadiazole)porphyrazine metal complexes by magnetic circular dichroism and magnetic circularly polarized luminescence.

The magnetic circular dichroism (MCD) spectra of metal complexes of tetrakis(thiadiazole)porphyrazines ([TTDPzM] with M = 2HI , ZnII , MgII (H2 O), and CdII ) have been recorded in dimethyl formamide solution. Together with the UV-Vis spectra, the MCD spectra provide useful information about the structure and electronic properties of the complexes. The experimental UV-Vis and MCD spectra compare pretty well with DFT calculations of two sorts, based either on the sum-over-states (SOS) approach or on the complex polarization propagator approach. They further corroborate the findings and interpretation of MCD spectra of porphyrazines based on the model of Michl for peripheral molecular orbitals. Magnetic circularly polarized luminescence (MCPL) spectra, quite uncommon in the literature, have been recorded for [TTDPzM] (M = 2HI , ZnII , MgII (H2 O)).

An Enantiopure Propeller-Like Trityl-Brominated Radical: Bringing Together a High Racemization Barrier and an Efficient Circularly Polarized Luminescent Magnetic Emitter.

A new persistent organic free radical has been synthetized with Br atoms occupying the ortho- and para-positions of a trityl core. After the isolation of its two propeller-like atropisomers, Plus (P) and minus (M), their absolute configurations were assigned by a combination of theoretical and experimental data. Remarkably, no hints of racemization were observed up to 60 °C for more than two hours, due to the higher steric hindrance imposed by the bulky Br atoms. Therefore, when compared to its chlorinated homologue (t1/2 =18 s at 60 °C), an outstanding stability against racemization was achieved. A circularly polarized luminescence (CPL) response of both enantiomers was detected. This free radical shows a satisfactory luminescent dissymmetry factor (|glum (592 nm)|≈0.7×10-3 ) despite its pure organic nature and low luminescence quantum yield (LQY). Improved organic magnetic CPL emitters derived from the reported structure can be envisaged thanks to the wide possibilities that Br atoms at para-positions offer for further functionalization.

Excimer-origin CPL vs. monomer-origin magnetic CPL in photo-excited chiral binaphthyl-ester-pyrenes: critical role of ester direction.

Two chiral binaphthyl (BNp) derivatives bearing oppositely oriented ester linkers to two pyrene (Py) moieties [(R)/(S)-1 and (R)/(S)-2] enabled Py-origin circularly polarized luminescence (CPL), magnetic CPL (MCPL), and circular dichroism (CD). (R)-1 that exhibited (-)-sign CD showed (+)-sign Py-excimer CPL but did not exhibit MCPL. Conversely, (R)-2, with (-)-sign CD, did not show excimer-origin CPL, but exhibited clear Py-monomer MCPL.

Effect of the J–J Interaction of Excited States of the Rare-Earth Ion Pr3+ on Magnetically Polarized Luminescence of Praseodymium-Yttrium Aluminum Garnet

The spectra of luminescence and magnetic circular polarization of luminescence of praseodymium–yttrium aluminum garnet Pr3+ : Y3Al5O12 (PrYAG) are studied in the visible spectral region at temperature T = 300 K. An analysis of spectral dependences of magnetooptical and optical spectra makes it possible to identify optical 4 f–4 f-transitions between Stark sublevels of multiplets 3P0, 3P1, 3Н5, and 3Н6 in PrYAG. It was shown that an important role in the spectrum of the degree of magnetic circular polarization of luminescence of this paramagnetic garnet is played by the effect of quantum-mechanical J–J mixing of states of Stark singlets 3Н5 and 3Н6 of non-Kramer rare-earth ion Pr3+ in the “green” luminescence band related to forbidden 4 f → 4 f transition 3P0 → 3Н5 in the visible spectral region. To interpret the spectra of magnetic circular polarization of luminescence, the energy of experimentally determined Stark sublevels of multiplets under study, their irreducible representations and wave functions determined by numerical simulation of the energy spectrum of the rare-earth ion Pr3+ in the garnet structure are used.

Влияние J-J-взаимодействия возбужденных состояний редкоземельного иона Pr-=SUP=-3+-=/SUP=- на магнитополяризованную люминесценцию празеодим-иттриевого граната-алюминия

AbstractThe spectra of luminescence and magnetic circular polarization of luminescence of praseodymium–yttrium aluminum garnet Pr^3+ : Y_3Al_5O_12 (PrYAG) are studied in the visible spectral region at temperature T = 300 K. An analysis of spectral dependences of magnetooptical and optical spectra makes it possible to identify optical 4 f –4 f -transitions between Stark sublevels of multiplets ^3 P _0, ^3 P _1, ^3 Н _5, and ^3 Н _6 in PrYAG. It was shown that an important role in the spectrum of the degree of magnetic circular polarization of luminescence of this paramagnetic garnet is played by the effect of quantum-mechanical J – J mixing of states of Stark singlets ^3 Н _5 and ^3 Н _6 of non-Kramer rare-earth ion Pr^3+ in the “green” luminescence band related to forbidden 4 f → 4 f transition ^3 P _0 → ^3 Н _5 in the visible spectral region. To interpret the spectra of magnetic circular polarization of luminescence, the energy of experimentally determined Stark sublevels of multiplets under study, their irreducible representations and wave functions determined by numerical simulation of the energy spectrum of the rare-earth ion Pr^3+ in the garnet structure are used.

Investigation of J – J “mixing” mechanism influence on optical and magnetooptical properties of praseodymium yttrium-aluminum garnet PrYAG

The spectra of the luminescence and magnetic circular polarization of luminescence (MCPL) in the praseodymium yttrium garnet aluminate Pr3+:YAG (PrYAG) have been studied within the visible spectral range at temperature T = 300 K. Analysis of the spectral dependences of the magnetooptical and optical spectra has made it possible to identify the optical 4f → 4f transitions occurring between the Stark sublevels of the 3P0, 3P1 and 3H5, 3H6 multiplets in PrYAG. It has been shown that for this paramagnetic garnet in the MCPL degree spectrum for the “green” luminescence band associated with forbidden 4f→ 4f transitions 3P0 → 3H5 within the visible spectral range, a significant role is being played by the effect of quantum mechanical J - J “mixing” of the energy states of Stark singlets of the 3H5 and 3H6 multiplets of the non-Kramers rare-earth (RE) Pr3+ ion. A parameterized Hamiltonian defined to operate within the entire 4 f(2) ground electronic configuration of Pr3+ was used to model the experimental Stark levels, their irreducible representations (irreps.) and wavefunctions. Nine independent CF parameters, Bqk, were optimized using standard least-squares fitting between calculated and experimental levels. The final fitting standard deviation between 61 calculated-to-experimental levels is 17.86 cm–1.

Magnetic Circular Polarization of Exciton Photoluminescence

Experimental and theoretical studies of circular polarization of photoluminescence of excitons (MCPL) in semiconductors placed in an external magnetic field are reviewed. The advantage of the MCPL method is its relative simplicity. In particular, it does not require spectral resolution of the Zeeman sublevels of an exciton and may be applied to a wide class of objects having broad photoluminescence spectral lines or bands: in bulk semiconductors with excitons localized on the defects of the crystal lattice and composition fluctuations, in structures with quantum wells and quantum dots of types I and II, in two-dimensional transition metals dichalcogenides and quantum microcavities. The basic mechanisms of the magnetic circular polarization of luminescence are considered. It is shown that either known mechanisms should be modified or additional mechanisms of the MCPL should be developed to describe the polarized photoluminescence in newly invented nanosystems.

Магнитная циркулярная поляризация фотолюминесценции экситонов

AbstractExperimental and theoretical studies of circular polarization of photoluminescence of excitons (MCPL) in semiconductors placed in an external magnetic field are reviewed. The advantage of the MCPL method is its relative simplicity. In particular, it does not require spectral resolution of the Zeeman sublevels of an exciton and may be applied to a wide class of objects having broad photoluminescence spectral lines or bands: in bulk semiconductors with excitons localized on the defects of the crystal lattice and composition fluctuations, in structures with quantum wells and quantum dots of types I and II, in two-dimensional transition metals dichalcogenides and quantum microcavities. The basic mechanisms of the magnetic circular polarization of luminescence are considered. It is shown that either known mechanisms should be modified or additional mechanisms of the MCPL should be developed to describe the polarized photoluminescence in newly invented nanosystems.

Mid-Gap States and Normal vs Inverted Bonding in Luminescent Cu+- and Ag+-Doped CdSe Nanocrystals

Mid-gap luminescence in copper (Cu+)-doped semiconductor nanocrystals (NCs) involves recombination of delocalized conduction-band electrons with copper-localized holes. Silver (Ag+)-doped semiconductor NCs show similar mid-gap luminescence at slightly (∼0.3 eV) higher energy, suggesting a similar luminescence mechanism, but this suggestion appears inconsistent with the large difference between Ag+ and Cu+ ionization energies (∼1.5 eV), which should make hole trapping by Ag+ highly unfavorable. Here, Ag+-doped CdSe NCs (Ag+:CdSe) are studied using time-resolved variable-temperature photoluminescence (PL) spectroscopy, magnetic circularly polarized luminescence (MCPL) spectroscopy, and time-dependent density functional theory (TD-DFT) to address this apparent paradox. In addition to confirming that Ag+:CdSe and Cu+:CdSe NCs display similar broad PL with large Stokes shifts, we demonstrate that both also show very similar temperature-dependent PL lifetimes and magneto-luminescence. Electronic-structure calculations further predict that both dopants generate similar localized mid-gap states. Despite these strong similarities, we conclude that these materials possess significantly different electronic structures. Specifically, whereas photogenerated holes in Cu+:CdSe NCs localize primarily in Cu(3d) orbitals, formally oxidizing Cu+ to Cu2+, in Ag+:CdSe NCs they localize primarily in 4p orbitals of the four neighboring Se2- ligands, and Ag+ is not oxidized. This difference reflects a shift from "normal" to "inverted" bonding going from Cu+ to Ag+. The spectroscopic similarities are explained by the fact that, in both materials, photogenerated holes are localized primarily within covalent [MSe4] dopant clusters (M = Ag+, Cu+). These findings reconcile the similar spectroscopies of Ag+- and Cu+-doped semiconductor NCs with the vastly different ionization potentials of their Ag+ and Cu+ dopants.

Identification of Lanthanide(III) Luminophores in Magnetic Circularly Polarized Luminescence Using Raman Optical Activity Instrumentation.

Luminescence of lanthanide(III) ions sensitively reflects atomic environment. However, the signal may be weak and covered by Raman scattering. In the present study magnetic circularly polarized luminescence (MCPL) is explored as a more sensitive tool to recognize the lanthanide signal and assign underlying electronic transitions. MCPL spectra of the Na3[Ln(DPA)3] (Ln = Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, and Er) complexes were recorded on a Raman optical activity (ROA) instrument. The ROA spectrometer equipped with the 532 nm laser excitation sensitively detects differences in scattering of left- and right-circularly polarized light caused by the magnetic field. Weak bands sometimes invisible in unpolarized measurement could be detected as MCPL. Observed transitions were assigned with the aid of the ligand-field theory. MCPL also reflects the environment: chloride and nitrate salts (LnCl3 and Ln(NO3)3) provide a different signal than the complex; for NdIII the signal responds to distribution of chloride and nitrate ions around the metal. The MCPL technique thus appears useful for identification and assignment of lanthanide transitions and increases the potential of fluorescent probes for applications in analytical chemistry and imaging.

Magnetic optical activity in Bi-doped Mg–Al–Si glass

We report the results of the experiments on the magnetic optical activity in Bi-doped Mg–Al–Si oxide glass in the temperature range of 1.48–200 K and high magnetic fields—up to 6.5 T. Temperature dependences of the magnetic circular polarization of luminescence and magnetic circular dichroism indicate the spin multiplicity of the ground and excited states. The investigated system has at least three different defect centers. Experimental data are explained in the assumption of an even-electron system with axial and rhombic zero-field splitting components.

Singlet-Triplet Splittings in the Luminescent Excited States of Colloidal Cu(+):CdSe, Cu(+):InP, and CuInS2 Nanocrystals: Charge-Transfer Configurations and Self-Trapped Excitons.

The electronic and magnetic properties of the luminescent excited states of colloidal Cu(+):CdSe, Cu(+):InP, and CuInS2 nanocrystals were investigated using variable-temperature photoluminescence (PL) and magnetic circularly polarized luminescence (MCPL) spectroscopies. The nanocrystal electronic structures were also investigated by absorption and magnetic circular dichroism (MCD) spectroscopies. By every spectroscopic measure, the luminescent excited states of all three materials are essentially indistinguishable. All three materials show very similar broad PL line widths and large Stokes shifts. All three materials also show similar temperature dependence of their PL lifetimes and MCPL polarization ratios. Analysis shows that this temperature dependence reflects Boltzmann population distributions between luminescent singlet and triplet excited states with average singlet-triplet splittings of ∼1 meV in each material. These similarities lead to the conclusion that the PL mechanism in CuInS2 NCs is fundamentally different from that of bulk CuInS2 and instead is the same as that in Cu(+)-doped NCs, which are known to luminesce via charge-transfer recombination of conduction-band electrons with copper-localized holes. The luminescence of CuInS2 nanocrystals is explained well by invoking exciton self-trapping, in which delocalized photogenerated holes contract in response to strong vibronic coupling at lattice copper sites to form a luminescent excited state that is essentially identical to that of the Cu(+)-doped semiconductor nanocrystals.

Magnetic circular polarization of luminescence in bismuth-doped silica glass

Bismuth-doped-silica-based optical fibers as a laser gain medium covers the spectral regions inaccessible to conventional rare-earth-doped fibers. Unfortunately, the nature of the luminescent centers in these fibers is not well understood, which complicates the development of efficient devices. In this Letter the magnetic-field-induced circular polarization of near-infrared (NIR) photoluminescence (PL) in Bi-doped pure silica glass was studied in the spectral range of 660–1600 nm, covering three excited state levels. The results of variable temperature and magnetic field measurements allow concluding that the NIR PL originates from an isolated non-Kramers doublet of the even-electron system. This result significantly narrows the list of possible origins of NIR PL in Bi-doped silica glasses.

Magnetooptics of the luminescent transitions in Tb3+:Gd3Ga5O12

The spectra of the luminescence and magnetic circular polarization of luminescence in terbium–gadolinium gallium garnet Tb3+:Gd3Ga5O12 (Tb3+:GGG) were studied within the visible spectral range at temperatures T=90 and 300K in an external magnetic field of 0.45T. The Zeeman effect in the luminescence “green” band associated with 4f→4f transition 5D4→7F5 of Tb3+:GGG was also studied at T=90K in an external field of 0.55T. Measurement of the Zeeman effect in Tb3+:GGG carried out for some doublet lines of the luminescence band 5D4→7F5 at T=90K shows that a magnetooptical effect of the intensity change of the emitted light is observed on these lines, in contrast to pure Zeeman splitting of the emission lines measured in the luminescence band 5D4→7F6. For the systems we have studied, the maximal value of the magnetooptical effect of the intensity change of the luminescence line at low temperatures has been achieved in paramagnetic garnet Tb0.2Y2.8Al5O12 at comparatively low magnetic fields.

Some interesting features of the Tb3+ magnetooptics in the paramagnetic garnets

The spectra of the absorption, luminescence, magnetic circular dichroism (MCD) and magnetic circular polarization of luminescence (MCPL) in the terbium–yttrium gallium garnet Tb3+:Y3Ga5O12 (Tb:YGG) have been studied within the visible and near ultraviolet (UV) spectral range for temperatures T=85 and 300K. The MCD spectrum observed within the UV absorption band for Tb:YGG is associated with spin- and parity-allowed electric-dipole 4f→5d transitions occurring between levels of the ground 7F6 multiplet and the 7D state of the excited 4f(7)5d configuration of the Tb3+ ion. Analysis of the spectral and the temperature dependences of the magnetooptical and optical spectra has made it possible to identify magneto-optically-active 4f→4f transitions occurring between Stark sublevels of the 5D4 and 7F5 multiplets in Tb3+:YGG. Quantum mechanical “mixing” of the three lowest energy Stark singlets in the excited 5D4 multiplet by an external magnetic field H leads to the change of the circularly polarized luminescence line intensities. The Zeeman effect in the UV absorption band 7F6→5L10 of Tb3+:YGG at T=85K was also studied. The magnetic field dependence of the Zeeman splitting of some absorption lines is found to exhibit unusual behavior: as the magnetic field increases, the band splitting decreases rather than increases. A parameterized Hamiltonian defined to operate within the entire 4f(8) ground electronic configuration of Tb3+ was used to model the experimental Stark levels, their irreducible representations (irreps.) and wave functions. The crystal-field parameters were determined using a Monte-Carlo method in which nine independent crystal-field parameters, were given random initial values and optimized using standard least-squares fitting between calculated and experimental levels. The final fitting standard deviation between 101 calculated and experimental Stark levels is 16.7cm−1.

Magnetooptics of non-Kramers Eu3+ ions in garnets: analysis complemented by crystal-field splitting modeling calculations

Spectra of absorption, luminescence, magnetic circular dichroism (MCD), and magnetic circular polarization of luminescence (MCPL) in Gd3Ga5O12:Eu3+ and Eu3Ga5O12 garnets were studied within the visible spectral range at 300 K. Analysis of the spectral and temperature dependences of the magnetooptical and optical spectra made it possible to identify the magneto-dipole (MD) and electro-dipole (ED) 4f→4f transitions occurring between Stark sublevels of the 7FJ (J=1, 2) and 5D0 multiplets in Eu3+-containing garnet structures. Quantum mechanical “mixing” had significant influence on quasi-degenerate states of the non-Kramers rare-earth Eu3+ ion for Eu3Ga5O12 in MCD due to forbidden MD transition 7F1→5D0 and for Gd3Ga5O12:Eu3+ in MCPL due to MD 4f→4f transition 5D0→7F1 and forced ED-transition 5D0→7F2. A parameterized Hamiltonian defined to operate within the entire 4f(6) ground electronic configuration of Eu3+ ion was used to model the experimental Stark levels, including their irreducible representations and wavefunctions. The crystal-field parameters were determined through a Monte-Carlo method in which nine independent crystal-field parameters, Bkq, were given random starting values and optimized using standard least-squares fitting between calculated and experimental levels. The final fitting standard deviation between 57 calculated-to-experimental levels was 0.73 meV.