Circularly Polarized Luminescence: A Review of Experimental and Theoretical Aspects

We review the present status of experiments and calculations for circularly polarized luminescence (CPL) of simple organic molecules and of stimuli‐responsive organic molecules. Together with the historical report of the main instrumental approaches, a few crucial points about experiments are tackled, with the aim of defining measurement protocols, in view of the wide availability of commercial apparatuses in the near future. The calculations aimed at interpreting the CPL spectra, mostly based on time‐dependent Density Functional Theory (TD‐DFT) calculations, which started around 2010, are reviewed, limiting the discussion to small to mid‐sized molecules. Some applications of CPL spectra of organic molecules‐based systems are presented, with a focus especially on two fields: material science and biology. Chirality 28:696–707, 2016. © 2016 Wiley Periodicals, Inc.     

Circularly polarized luminescence of Sm (III) and Eu (III) complexes with chiral ligand (R/S)‐BINAPO

Luminescent lanthanide (III) ions have been exploited for circularly polarized luminescence (CPL) for decades. However, very few of these studies have involved chiral samarium (III) complexes. Complexes are prepared by mixing axial chiral ligands (R/S))‐2,2’‐bis(diphenylphosphoryl)‐1,1′‐binaphthyl (BINAPO) with europium and samarium Tris (trifluoromethane sulfonate) (Eu (OTf)₃ and Sm (OTf)₃). Luminescence‐based titration shows that the complex formed is Ln((R/S)‐BINAPO)₂(OTf)₃, where Ln = Eu or Sm. The CPL spectra are reported for Eu((R/S)‐BINAPO)₂(OTf)₃ and Sm((R/S)‐BINAPO)₂(OTf)₃. The sign of the dissymmetry factors, g_em, was dependent upon the chirality of the BINAPO ligand, and the magnitudes were relatively large. Of all of the complexes in this study, Sm((S)‐BINAPO)₂(OTf)₃ has the largest g_em = 0.272, which is one of the largest recorded for a chiral Sm³⁺ complex. A theoretical three‐dimensional structural model of the complex that is consistent with the experimental observations is developed and refined. This report also shows that (R/S)‐BINAPO are the only reported ligands where g_em (Sm³⁺) > g_em (Eu³⁺).     

Exploring potentialities and limitations of stapled o‐oligo(phenyleneethynylene)s (o‐OPEs) as efficient circularly polarized luminescence emitters

In this paper, we have studied the chiroptical properties of a family of o‐oligo(phenyleneethynylene) (o‐OPE) derivatives with different steric hindrance. Experimental results show high dissymmetry factors (g_abs and g_lum up to 1.1 × 10^?2) and very similar electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) for all the derivatives that make this basic o‐OPE scaffold a robust pure organic emitter. Vibrational circular dichroism spectra are used to characterize conformational properties in solution. Density functional theory and time‐dependent density functional theory calculations support experimental results also proving that ECD and CPL are almost exclusively linked to helical moiety and not to size or conformation of substituents. As chiroptical properties of these emitters are independent of substituents, this OPE scaffold can be used as basic skeleton for the design of sensing probes with high CPL efficiencies.     

Synthesis and luminescence properties of lanthanide complexes with a new tripodal ligand featuring N‐thenylsalicylamide arms

To explore the relationship between the structure of the ligands and the luminescent properties of the lanthanide complexes, luminescent lanthanide complexes of a new tripodal ligand, featuring N‐thenylsalicylamide arms, were synthesized and characterized by elemental analysis, IR and TGA measurements. Photophysical properties of the complexes were studied by means of UV ? visible absorption and steady‐state luminescence spectroscopy. The results of UV ? vis spectra indicate that metal binding does not disturb the electronic structure of the ligand. Excited‐state luminescence lifetimes and quantum yields of the complexes were determined. The photoluminescence analysis suggested that there is an efficient ligand ? Ln(III) energy transfer for the Tb(III) complex, and the ligand is an efficient 'antenna' for Tb(III). From a more general perspective, the results demonstrated the potential application of the lanthanide complex as luminescent materials in material chemistry. Copyright © 2012 John Wiley & Sons, Ltd.     

Circularly polarized luminescence of helically assembled pyrene π‐stacks on RNA and DNA duplexes

In this report, we describe the circularly polarized luminescence (CPL) of the RNA duplexes having one to four 2′‐O‐pyrene modified uridines ( Upy ) and the DNA duplexes having two, four, and six pyrene modified non‐nucleosidic linkers ( Py ). Both the pyrene π‐stack arrays formed on the RNA and DNA double helical structures exhibited pyrene excimer fluorescence. In the pyrene‐modified RNA systems, the RNA duplex having four Upy s gives CPL emission with g_lum value of <0.01 at 480 nm. The structure of pyrene stacks on the RNA duplex may be rigidly regulated with increase in the Upy domains, which resulted in the CPL emission. In the DNA systems, the pyrene‐modified duplexes containing two and four Pys exhibited CPL emission with g_lum values of <0.001 at 505 nm. The pyrene π‐stack arrays presented here show CPL emission. However, the g_lum values are relatively small when compared with our previous system consisting of the pyrene‐zipper arrays on RNA.     

Chiroptical Spectra of Tetrakis (+)‐3‐Heptafluorobutylrylcamphorate Ln(III) Complexes with an Encapsulated Alkali Metal Ion: Solution Structures as Revealed by Chiroptical Spectra

The preparation of tetrakis((+)‐hfbc) lanthanide(III) complexes with an encapsulated alkali metal and ammonium ions M[Ln((+)‐hfbc)₄] (hereafter abbreviated as M‐Ln : (+)‐hfbc, (+)‐heptafluorobutyrylcamphorate; M, ammonium or benzyl ammonium ions as well as alkali metal ions) was reported and discussed. The electronic circular dichroism (CD) spectra in the intraligand π?π* transition of M–Ln were examined in view of the solvent effect. Here, the concentration, alkali metal, and ammonium ion dependences are compared with the solid CD, ⁵D₀ ← ⁷F₀(Eu(III)) excitation spectra, circularly polarized luminescence, and vibrational circular dichroism. It has been revealed that the dodecahedral eight coordinate DD‐8‐M‐Ln complexes in crystals are equilibrated between the diastereoselectively formed square antiprism eight coordinate SAPR‐8‐M‐Ln and [Ln((+)‐hfbc)₃] in EtOH and CH₃CN solutions or between the SAPR‐8‐M‐Ln and DD‐D_2d(mmmm)‐8‐M‐Ln complexes in CHCl₃ solution. The observed CD couplets are found to reflect the exciton CD couplets which are useful to determine the four‐bladed SAPR‐(llll) absolute configuration around the lanthanide(III) ion. Chirality 24:1055–1062, 2012. © 2012 Wiley Periodicals, Inc.     

Visible‐near‐infrared luminescent lanthanide ternary complexes based on beta‐diketonate using visible‐light excitation

We used the synthesized dinaphthylmethane (Hdnm) ligand whose absorption extends to the visible‐light wavelength, to prepare a family of ternary lanthanide complexes, named as [Ln(dnm)₃phen] (Ln = Sm, Nd, Yb, Er, Tm, Pr). The properties of these complexes were investigated by Fourier transform infrared (FT‐IR) spectroscopy, diffuse reflectance (DR) spectroscopy, thermogravimetric analyses, and excitation and emission spectroscopy. Generally, excitation with visible light is much more advantageous than UV excitation. Importantly, upon excitation with visible light (401–460 nm), the complexes show characteristic visible (Sm³⁺) as well as near‐infrared (Sm³⁺, Nd³⁺, Yb³⁺, Er³⁺, Tm³⁺, Pr³⁺) luminescence of the corresponding lanthanide ions, attributed to the energy transfer from the ligands to the lanthanide ions, an antenna effect. Now, using these near‐infrared luminescent lanthanide complexes, the luminescent spectral region from 800 to 1650 nm, can be covered completely, which is of particular interest for biomedical imaging applications, laser systems, and optical amplification applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and luminescence properties of lanthanide complexes with a new tripodal ligands featuring salicylamide arms

A series of luminescent lanthanide complexes with a new tripodal ligand featuring salicylamide arms, 2,2′,2″‐nitrilotris(2‐furfurylaminoformylphenoxy)triethylamine (L), were synthesized and characterized by elemental analysis, IR and molar conductivity measurements. Photophysical properties of the complexes were studied by means of UV–vis absorption and steady‐state luminescence spectroscopy. Excited‐state luminescence lifetimes and quantum yield of the complexes were determined. Luminescence studies demonstrated that the tripodal ligand featuring salicylamide arms exhibits a good antennae effect with respect to the Tb(III) and Dy(III) ion due to efficient intersystem crossing and ligand to metal energy transfer. From a more general perspective, this work offers interesting perspectives for the development of efficient luminescent stains and enlarges the arsenal for developing novel luminescent lanthanide complexes of salicylamide derivatives. Copyright © 2009 John Wiley & Sons, Ltd.     

Circularly polarized luminescence of Eu(III) complexes with chiral 1,1′-bi-2-naphtol-derived bisphosphate ligands

The interest for lanthanide circularly polarized luminescence (CPL) has been quickly growing for 10 years. However, very few of these studies have involved correlation between the dissymmetry factor (g_lum) and the chemical modifications in a series of chiral ligands. Four polymeric compounds of Eu(III) were prepared by using a series of binaphtyl derivatives for which the size of the π system as well as the number of stereogenic elements (i.e., the binaphtyl moiety) are modulated. The resulting {[Eu(hfac)₃((S)/(R)-L^x)]}_n (x = 1 and 3) and {[Eu(hfac)₃((S,S,S)/(R,R,R)-L^x)]}_n (x = 2 and 4) have been characterized by powder X-ray diffraction by comparison with the X-ray structures on single crystal of the Dy(III) analogs. In solution, the structure of the complexes is deeply modified and becomes monomeric. The nature of the ligand induces change in the shape of the CPL spectra in CH₂Cl₂ solution. Furthermore, a large |g_lum| = 0.12 of the magnetic-dipole transition for the [Eu(hfac)₃((S,S,S)/(R,R,R)-L²)] complex involving the ligand with three stereogenic elements and an extended ?? system has been measured. This report also shows CPL measurements in solid state for the series of {[Eu(hfac)₃((S)/(R)-L^x)]}_n (x = 1 and 3) and {[Eu(hfac)₃((S,S,S)/(R,R,R)-L^x)]}_n (x = 2 and 4) polymers.     

On the determination of empirical absolute chiral structure: chiroptical spectrum correlations for D3 lanthanide (III) complexes

Circularly polarized luminescence (CPL) from selected transitions of Eu(III) in resolved single crystals of Na3[Eu(ODA)3].2NaClO4.6H2O are compared to CPL results obtained from solutions containing perturbed racemic mixtures of Eu(2,6-pyridine-dicarboxylate)3 (3-) and enantiomerically pure d-f helicate LambdaLambda-(-)EuCr(L8)3] in order to determine an empirical relationship between helicity and CPL spectra. Comparison of the CPL results, even for the magnetic dipole allowed transitions of Eu(III) where one measures large chiral discrimination, shows that the signs and magnitudes do not correlate with the overall helicity of the Eu(III) site. It is concluded that the symmetry of the Eu(III) site in LambdaLambda-(-)EuCr(L8)3 is not close enough to D3 to allow for the confirmation of the presumed spectra:structure correlation.     

Experimental and Calculated CPL Spectra and Related Spectroscopic Data of Camphor and Other Simple Chiral Bicyclic Ketones

UV, circular dichroism (CD), fluorescence and circularly polarized luminescence (CPL) spectra were recorded for a set of four related [2.2.1] bicyclic compounds ((1S,4S)‐and (1R,4R)‐1,7,7‐trimethylbicyclo[2.2.1]heptan‐2‐one, namely (1S)‐ and (1R)‐camphor ( 1 ), (1S,4R)‐4,7,7‐trimethylbicyclo[2.2.1]hept‐5‐en‐2‐one, (1S)‐dehydro‐epicamphor ( 2 ), (1S,4S)‐1,7,7‐trimethylbicyclo[2.2.1]heptane‐2,5‐dione, (1S)‐5‐oxocamphor ( 3 ), (1S,4R)‐ and (1R,4S)‐1,7,7‐trimethylbicyclo[2.2.1]heptane‐2,3‐dione, (1S)‐ and (1R)‐camphorquinone ( 4 )) and a set of three related [2.2.2] bicyclic compounds (1S,4S)‐bicyclo[2.2.2]octan‐2,5‐dione (saturated diketone ( 5 )), (1R,4R)‐bicyclo[2.2.2]oct‐7‐en‐2,5‐dione (unsaturated diketone ( 6 )), ((1S,4S)‐bicyclo[2.2.2]oct‐7‐en‐5(S)‐ol‐2‐one (which we refer to as unsaturated hydroxy‐ketone ( 7 )). For the latter three compounds also mid‐IR vibrational circular dichroism (VCD) spectra were recorded and are presented. Time‐Dependent Density Functional (TD‐DFT) calculations provide a satisfactory interpretation of both absorption and emission chiroptical spectra and permit insight into ground and excited state electronic properties. We discuss the applicability of the octant rule or of other approximated models to rationalize the observed sign of the CPL. Chirality 25:589–599, 2013. © 2013 Wiley Periodicals, Inc.     

Over 104-fold amplified upconversion luminescence of lanthanide nanocrystals through optical oscillator-like system

Recently, lanthanide (Ln) luminescent nanocrystals have attracted increasing attention in various fields such as biomedical imaging, lasers, and anticounterfeiting. However, due to the forbidden 4f–4f transition of lanthanide ions, the absorption cross-section and luminescence brightness of lanthanide nanocrystals are limited. To address the challenge, we constructed an optical oscillator-like system to repeatedly simulate lanthanide nanocrystals to enhance the absorption efficiency of lanthanide ions on excitation photons. In this optical system, the upconversion luminescence (UCL) of Tm³⁺ emission of ~450 nm excited by a 980 nm laser can be amplified by a factor beyond 10⁴. The corresponding downshifting luminescence of Tm³⁺ at 1460 nm was enhanced by three orders of magnitude. We also demonstrated that the significant luminescence enhancement in the designed optical oscillator-like system was general for various lanthanide nanocrystals including NaYF₄:Yb³⁺/Ln³⁺, NaErF₄@NaYF₄ and NaYF₄:Yb³⁺/Ln³⁺@NaYF₄:Yb³⁺@NaYF₄ (Ln = Er, Tm, Ho) regardless of the wavelengths of excitation sources (808 and 980 nm). The mechanism study revealed that both elevated laser power in the optical system and multiple excitations on lanthanide nanocrystals were the main reason for the luminescence amplification. Our findings may benefit the future development of low-threshold upconversion and downshifting luminescence of lanthanide nanocrystals and expand their applications.     

UV light‐tunable fluorescent inks and polymer hydrogel films based on carbon nanodots and lanthanide for enhancing anti‐counterfeiting

Novel water‐soluble green fluorescent carbon nanodots (CNs) using methacrylic acid and m‐phenylenediamine as precursors were first synthesized using a one‐pot hydrothermal method. Red fluorescent lanthanide complexes were prepared using lanthanide ion Eu³⁺ and pyridine‐2,6‐dicarboxylic acid. The optical properties of CNs were characterized using ultraviolet visible (UV) spectra and fluorescence spectra, microscopic morphology was characterized using transmission electron microscopy (TEM) and dynamic light scattering (DLS), and the elemental composition was characterized using Fourier transform‐infrared spectroscopy (FT‐IR) and X‐ray photoelectron spectra (XPS). The fluorescence spectra of the lanthanide complexes were also measured. A simple strategy was developed to prepare UV light‐tunable fluorescent inks and polymer hydrogels films based on CNs and lanthanide complexes. The fluorescent inks and polymer hydrogels films could be repeatedly switched between green and red fluorescence. The change of color depended on luminescence of the CNs and the lanthanide complexes under 254 and 365 nm UV light, respectively. The UV light‐tunable fluorescent inks and polymer hydrogels films could enhance its anti‐counterfeiting function for data and information.     

Extraterrestrial Handedness: A Reply

Recent investigations of stable isotope ratios of amino acids from the Murchison meteorite have shown them to be of unambiguous extraterrestrial origin, and examinations of their enantiomeric compositions, where terrestrial contamination can be excluded, have found a consistent excess of L-enantiomers. One explanation for this observation has been the asymmetric photolysis of racemic extraterrestrial amino acids by circularly polarized light (CPL) in the synchrotron radiation from orbiting electrons around the pulsar remnants of supernovae. Mason (1997) has attempted to discredit this mechanism on the grounds that circular dichroism (CD) bands for optically active molecules alternate in sign and sum to zero over the entire spectrum, and hence enantioselective photochemical reactions cannot be induced by broad band CPL. We submit arguments disputing this conclusion and present reasons for expecting that broad band CPL synchrotron radiation would be quite capable of inducing asymmetric photolysis, particularly in aliphatic amino acids.     

Synthesis and luminescence properties of two novel lanthanide (III) complexes of acetophenonylcarboxymethyl sulphoxide

A novel ligand containing multiple coordinating groups (sulfinyl, carboxyl and carbonyl groups), acetophenonylcarboxymethyl sulphoxide, was synthesized. Its corresponding two lanthanide (III) binary complexes were synthesized and characterized by element analysis, molar conductivity, FT‐IR, TG‐DTA and UV spectroscopy. Results showed that the composition of these complexes was REL₃L^‐ (ClO₄)₂·3H₂O (RE = Eu (III), Tb (III); L = C₆H₅COCH₂SOCH₂COOH; L^‐ = C₆H₅COCH₂SOCH₂COO^‐). FT‐IR results indicated that acetophenonylcarboxymethyl sulphoxide was bonded with an RE (III) ion by an oxygen atom of the sulfinyl and carboxyl groups and not by an oxygen atom of the carbonyl group due to high steric hinderance. Fluorescent spectra showed that the Tb (III) complex had excellent luminescence as a result of a transfer of energy from the ligand to the excitation state energy level (⁵D₄) of Tb (III). The Eu (III) complex displayed weak luminescence, attributed to low energy transfer efficiency between the triplet state energy level of its ligand and the excited state (⁵D₀) of Eu (III). As a result, the Tb (III) complex displayed a good antenna effect for luminescence. The fluorescence decay curves of Eu (III) and Tb (III) complexes were also measured. Copyright © 2012 John Wiley & Sons, Ltd.     

Fluorescence enhancement of rare earth Tb(III) by Tm(III) in benzyl benzoylmethyl sulphoxide complexes

A series of rare earth complexes [(Tb_xTm_y)L₅(ClO₄)₂](ClO₄)·3H₂O (x:y = 1.000:0.000, 0.999:0.001, 0.995:0.005, 0.990:0.010, 0.950:0.050, 0.900:0.100, 0.800:0.200, 0.700:0.300; L = C₆H₅CH₂SOCH₂COC₆H₅) (Tb(III) luminescence ion; Tm(III) doped inert ion) were synthesized and characterized by elemental analysis, infrared spectra (IR) and ¹H‐NMR. The photophysical properties of these complexes were studied in detail using ultraviolet absorption spectra, fluorescent spectra and lifetimes. The fluorescence spectra of complexes indicated that the fluorescence emission intensity was significantly enhanced by Tm(III). The complexes showed the best luminescence properties when the mole ratio Tb(III):Tm(III) was 0.990:0.010. The fluorescence intensity could be increased to 390%. Additionally, phosphorescence spectra and the luminescence mechanisms are discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

Importance of hydrogen-bonding sites in the chiral recognition mechanism between racemic D3 terbium(III) complexes and amino acids

The perturbation of the racemic equilibrium of luminescent D3 terbium(III) complexes with chelidamic acid (CDA), a hydroxylated derivative of 2,6-pyridine-dicarboxylic acid (DPA), by added chiral biomolecules such as L-amino acids has been studied using circularly polarized luminescence and 13C NMR spectroscopy. It is shown in this work that the chiral-induced equilibrium shift of [Tb(CDA)3](6-) by L-amino acids (i.e. L-proline or L-arginine) was largely influenced by the hydrogen-bonding networks formed between the ligand interface of racemic [Tb(CDA)3](6-) and these added chiral agents. The capping of potential hydrogen-bonding sites by acetylation in L-proline led to a approximately 100-fold drop in the induced optical activity of the [Tb(CDA)3](6-):N-acetyl-L-proline system. This result suggested that the hydrogen-bonding networks serve as the basis for further noncovalent discriminatory interactions between racemic [Tb(CDA)3](6-) and added L-amino acids.     

Heterobinuclear Zn‐Ln (Ln = La,Nd, Eu,Gd, Tb,Er and Yb) complexes based on asymmetric Schiff‐base ligand: synthesis,characterization and photophysical properties

With a novel asymmetric Schiff‐base zinc complex ZnL (H₂L = N‐(3‐methoxysalicylidene)‐N′‐(5‐bromo‐3‐methoxysalicylidene)phenylene‐1,2‐diamine), obtained from phenylene‐1,2‐diamine, 3‐methoxysalicylaldehyde and 5‐bromo‐3‐methoxysalicylaldehyde, as the precursor, a series of heterobinuclear Zn‐Ln complexes [ZnLnL(NO₃)₃(CH₃CN)] (Ln = La, 1; Ln = Nd, 2; Ln = Eu, 3; Ln = Gd, 4; Ln = Tb, 5; Ln = Er, 6; Ln = Yb, 7) were synthesized by the further reaction with Ln(NO₃)₃·6H₂O, and characterized by Fourier transform‐infrared, fast atom bombardment mass spectroscopy and elemental analysis. Photophysical studies of these complexes show that the strong and characteristic near‐infrared luminescence of Nd³⁺, Yb³⁺and Er³⁺ with emissive lifetimes in the microsecond range has been sensitized from the excited state of the asymmetric Schiff‐base ligand due to effective intramolecular energy transfer; the other complexes do not show characteristic emission due to the energy gap between the chromophore and lanthanide ions. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and luminescence properties of 2‐(benzylcarbamoyl)phenyl derivatives and their europium complexes

Six novel 2‐(benzylcarbamoyl)phenyl derivatives were synthesized and characterized by ¹H‐NMR, mass spectrometry, infrared spectra and elemental analysis. Their europium complexes were prepared and characterized by elemental analysis, EDTA titrimetric analysis, IR and UV spectra as well as molar conductivity measurements. The luminescence properties of these complexes were investigated and results show that 2‐(benzylcarbamoyl)phenyl derivatives possess high selectivity and good coordination with the europium ion. Complex Eu‐2‐(benzylcarbamoyl)phenyl‐2‐phenylacetate showed green luminescence that was emitted by the ligand of 2‐(benzylcarbamoyl)phenyl‐2‐phenylacetate, while other complexes showed the characteristic red luminescence of europium ion and also possessed high luminescence intensity. Copyright © 2012 John Wiley & Sons, Ltd.