Two-Photon Circularly Polarized Luminescence of Chiral Eu Complexes

We report the synthesis of chiral lanthanide complexes with extended π conjugation for efficient circularly polarized luminescence (CPL) via two-photon excitation (2PE). The pyridine bis-oxazoline (PyBox) core provides the chiral Ln3+ environment, while the extension of the conjugated backbone through the pyridine 4-position with a phenylacetylene unit increases the two-photon absorption cross section. This work presents an important step toward the development of chiral systems displaying enhanced nonlinear optical properties, with potential applications in imaging and sensing, as well as in photodynamic therapy due to the selective excitation of molecules within a specific focal volume.

NH4Cl solution.After back-extraction of the aqueous phases with DCM, the combined organic extracts were dried (Na2SO4) and concentrated at reduced pressure.The resulting crude material was purified by column chromatography with an Isolera system.The product was purified further by triturating with Et2O to give (R,R)-i Pr-PyBox-Ph as a white solid (74 mg, 65% mL, 2.5 mmol) and THF (5 mL), to give (S,S)-Ph-PyBox-Ph as a white solid (41 mg, 36%

Instrumentation UV-Vis/CD and Polarimetry measurements
UV-Vis spectra were recorded using a Jasco-V650 spectrophotometer in the spectral range of 200 to 450 nm.All samples were measured in 1 mM ACN solutions at room temperature with 0.01 cm cell length.The same solutions were used to record CD spectra using a J1500 spectropolarimeter in 0.01 cm optical glass cells and averaging 16 accumulations.The optical rotation was measured with a Jasco DIP360 digital polarimeter in a 10 cm polarimetric cell, using the Sodium lamp's D line.(Eq S1) In Eq S1, A is the absorbance at the excitation wavelength, n is the refractive index and F the luminescence integrated intensity.The indices r and x stand for reference and sample respectively.The absorption and excitation wavelengths for both [Eu( i Pr-PyBox-Ph)2] 3+ and [Eu(Ph-PyBox-Ph)3] 3+ were 360 nm.

Lifetime measurements
The observed lifetimes (τobs) were measured using a Perkin Elmer LS55 spectrometer using FL Winlab (3.1) software, with the following parameters: Gate time 0.1 ms, excitation slit width 10 nm, emission slit width 5 nm, minimum delay 0.1 ms, maximum delay 3 ms, delay step 0.1 ms and recorded in triplicates.The decay profiles were fitted using a mono-exponential function.

1PE-CPL
CPL was measured with a home-built (modular) spectrometer. 8The excitation source was a broad band (200 -1000 nm) laser-driven light source EQ 99 (Elliot Scientific).Sample PL emission was collected perpendicularly to the excitation direction.The emission was fed through a photoelastic modulator (PEM) (Hinds Series II/FS42AA) and through a linear sheet polarizer (Comar).The light was then focused into a second scanning monochromator (Acton SP2155) and subsequently on to a photomultiplier tube (PMT) (Hamamatsu H10723 series).
The detection of the CPL signal was achieved using the field modulation lock-in technique.The electronic signal from the PMT was fed into a lock-in amplifier (LIA, Hinds Instruments Signaloc Model 2100).The reference signal for the lock-in detection was provided by the PEM control unit.The monochromators, PEM control unit and LIA were interfaced to a desktop PC and controlled by a custom-written Labview2014 graphic user interface.The LIA provided two signals, an AC signal corresponding to (IL-IR) and a DC signal corresponding to (IL + IR) after background subtraction.The emission dissymmetry factor was therefore readily obtained from the experimental data, as 2 AC/DC.Spectral calibration of the scanning monochromator was performed using a Hg-Ar calibration lamp (Ocean Optics).A correction factor for the wavelength dependence of the detection system was constructed using a calibrated lamp).The measured raw data was subsequently corrected using this correction factor.The validation of the CPL detection systems was achieved using light emitting diodes (LEDs) at various emission wavelengths.The LED was mounted in the thermally stabilized sample holder (25 ⁰C, Thor labs CVH100) and the light from the LED was fed through a broad band polarizing filter and /4 plate (Ocean Optics) to generate circularly polarized light.Prior to all measurements, the /4 plate and a 650 nm LED were used to set the phase of the lock-in amplifier correctly.The emission spectra were recorded with 0.5 nm step size and the slits of the detection monochromator were set to a slit width corresponding to a spectral resolution of 0.25 nm.CPL spectra (as well as total emission spectra) were obtained through an averaging procedure of several scans.

2PE-CPL
Two photon excitation CPL spectroscopy was achieved by coupling (beam routing using mirrors, Thor Labs BB1-E03) a tunable femtosecond pulsed laser (680 -1300 nm, Coherent Discovery TPC, 100 fs, 80 MHz) to the pre-existing CPL spectrometer detailed above. 9Initial proof of concept two photon spectroscopy has been achieved by perpendicularly mounting an Ocean Optics HR2000Pro (2048-pixel linear CCD Sony ILX5 chip, 200 µm slit, H3 grating, 350 -850 nm spectral region) spectrometer as a 'third arm' to the Discovery TPC laser.The laser beam was focused onto the center of the 1 cm path thermally stabilized sample holder (25 ⁰C, Thor labs CVH100) by a dedicated ultrafast laser lens (Edmund Optics 11711, 50 mm focal length).The spectrometer has also been equipped with a perpendicularly mounted 365 nm LED (nichia, 500 mW) and been operated using a modified version of the above-mentioned custom time resolved detection and accumulation algorithm written in Labview2013 program.In order to eliminate unwanted artefacts associated with stray light from 2PE each spectrometer has been equipped with a rotating filter wheel (Thor Labs, CFW6) housing an LP420 (Comar Optics, for 365 nm UVLED excitation) and SP650 and SP700 (Edmund Optics, 8472 and 8474 for MP excitation) filters.

Cross section determination
The 2PE cross sections (σ 2 ) of the complexes were measured according to established procedures and using Eq S2.

(Eq S2)
Where x is the sample, r is the reference,  is the total emission quantum yield of the compound, C is the concentration, n the refractive index and F is the integrated PL spectrum.

Results and Discussion
Additional spectra   The red lines represent the black line smoothed using an adjacent-averaging method over 26 points, to account for the difference spectral resolution achieved in the present work and in Ref [13].
Figure S4.The ECD spectra for each enantiomer and absorption spectrum traced in the background of the ligands i Pr-PyBox-Ph (left) and Ph-PyBox-Ph (right).Spectra were recorded in 1 mM acetonitrile solutions at room temperature with 0.01 cm cell length.Red solid line: R,R enantiomer, blue solid line: S,S enantiomer.