Axially-chiral boramidine for detailed (chir)optical studies

The inclusion of boron atoms into chiral π-conjugated systems is an effective strategy to unlock unique chiroptical properties. Herein, the preparation and characterization of a configurationally stable axially-chiral boramidine are reported, showcasing absorption in the UV domain, deep-blue fluorescence (Φ up to 94%), and ca. |10−3| gabs and glum values. Detailed photophysical studies and quantum-chemical calculations clearly elucidate the deactivation pathways of the emissive state to triplet excited states, involving increased spin–orbit coupling between the lowest singlet excited state and an upper triplet state.


Infrared Spectroscopy
IR spectra were recorded on a Perkin-Elmer 1650 FT-IR spectrometer using a diamond ATR Golden Gate sampling, and are reported in wave numbers (cm -1 ).
Melting points (M.p.) M.p. were measured in open capillary tubes and were uncorrected.

High-resolution Mass Spectrometry (HRMS)
Electrospray mass spectra were obtained on a Xevo-G2-TOF HRMS by the Department of Mass Spectroscopy of the University of Geneva.

Chiral Stationary Phase (CSP) HPLC resolutions
Semi-preparative Chiral stationary phase HPLC resolutions were performed on an Agilent LC 1100 instrument using CHIRALPAK® IC column.
(Chir)Optical Measurements All (chir)optical measurements were performed in 1cm optical quartz cells.

Steady-State Absorption
Spectra were recorded on a JASCO V-650 spectrophotometer at 20°C with parameters as follows: scan speed -100 nm/min, bandwidth -1 nm, and data interval -1 nm.All samples were measured in 1 cm optical path quartz cells.All solvents were of spectroscopic or HPLC grade and were used as received.atprecise concentrations ca. 2 10 -5 M.

Electronic Circular dichroism (ECD)
Spectra were recorded on either JASCO J-815 or JASCO J-1500 spectropolarimeters at 20°C with parameters as follows: scan speed -100 nm/min, bandwidth -1 nm, data interval -1 nm, and integration time -0.1 sec.Measurement were performed in analytical grade solvents in 1 cm optical path quartz cells at precise concentrations ca. 2 10 -5 M. All the spectra are the average of 10 accumulations each.

Steady-State Emission
Spectra were measured using a FluoroMax+ spectrofluorometric from Horiba scientific with parameters as follows: Entrance and Exit slit -1 nm bandpass, integration time -0.1 sec.All fluorescence spectra were corrected for the wavelength-dependent sensitivity of the detection.Fluorescence quantum yields Φ were measured in diluted solutions with an optical density lower than 0.1 relative to 9,10-diphenylanthracene (Φ r = 0.97 in cyclohexane) using the following equation: where A is the absorbance at the excitation wavelength (λ), n the refractive index and I the integrated emission intensity; "r" stands for reference.Excitations of reference and sample compounds were performed at the same wavelength 373 nm.

Circularly Polarized Luminescence (CPL)
Measurements were performed using a homemade spectrofluoropolarimeter 1 under 365 nm irradiation from commercial LED-sources with a 90° geometry between excitation and detection.The spectra were run in DCM and ACN in ~2•10 -5 M solutions using the following parameters: scan-speed -2 nm/s, integration time -2 or 4 s, photomultiplier tube driving voltage -from 400 to 650 V, emission bandwith ca. 10 nm.All the spectra are the average of 4 accumulations each.

Specific Optical Rotation
were measured using JASCO P-1030 polarimeter.Measurements were performed in []  20 analytical grade CH 2 Cl 2 solvent at precise concentrations in a Faraday cell Pb glass, temperature 20°C, and sodium lamp.

Time-Correlated Single Photon Counting (TCSPC) Experiments
The samples were bubbled with nitrogen or air at least 15 minutes before the measurement, and sealed.The TCSPC experiments were performed at the magic angle, using a 375 nm laser diode (PicoQuant LDH-P-C-375) as the excitation source, and recording the emission at 400 nm.Such emission was collected in an optical fiber, filtered using a spectrograph (Horiba Triax 190), and finally detected using a photomultiplier tube (PicoQuant PMA 192C).The instrument response function (IRF) had a full width at half maximum of about 200 ps.The fluorescence lifetimes were determined from iterative reconvolution of a monoexponential function with the IRF.

Transient Absorption (TA) Spectroscopy
Before experiments, the samples were bubbled for at least 15 minutes with nitrogen or air.All TA experiments were performed at the magic angle, keeping the samples under nitrogen or airflow during the entire duration of the measurement.The ps-s pump-probe setup is described in detail. 2 In brief, excitation was achieved using the output of a passively Q-switched, frequency tripled Nd:YAG laser (Teem Photonics, Powerchip NanoUV) producing pulses at 355 nm with a 500 Hz repetition rate, approximately 20 μJ energy, and 300 ps duration.

Data treatment:
The pixel-to-wavelength conversion was done using a standard filter of Holmium oxide, which shows narrow bands in the UV-Vis spectral region.The global analysis was performed using a homemade written script in Matlab®.
Geometry optimizations of the ground and S 1 state, TD-DFT calculations, and SOC calculations.
Geometry optimizations were carried out in vacuum using the Gaussian 16, rev.A.03 software 3 with the hybrid B3LYP functional and the 6-311G(d,p)++ basis set.Time-dependent density functional theory (TD-DFT) calculations were performed at the same level of theory, using the lowest 30 singlets and 30 triplets.With these results, the energy diagrams and the theoretical electronic absorption spectra were constructed using a gaussian line broadening function with a half width at half maximum of 0.333 eV; obtaining a good agreement with the experimental absorption spectra, as shown in Figures S13 and S14.
For the relaxed potential energy surface scan in the ground state, we used Gaussian 16 with the keywords OPT=modredundant, D and S. The dihedral angle of the optimized ground state structure used as an input is shown in detail in Figure S12.
For calculating the charge density difference isosurfaces (CDD), we used the output from the TD-DFT calculations for singlet and triplet states and ran the calculation in Multiwfn software, 4 with a higher grid.The resulting *.cube files were then plotted in GaussView 6 to obtain the figures shown.
The S 1 excited state optimizations were carried out at the same level of theory using the optimized S 0 geometry as an initial guess.The resulting optimized geometries for S o and S 1 were used to calculate the spin-orbit coupling (SOC) matrix elements, for which we used the ORCA 5.0 software 5 with the method of Zero-Order-Regular Approximation (ZORA) 6 to account for relativistic effects, at the B3LYP/6-311++G(d,p) level in a tight TD-DFT calculation with 30 states.We used a homemade script in Python to parse the SOC results.

VIII. Crystallographic Data
All data were collected on an XtaLAB Synergy-S diffractometer equipped with a hypix arc 150 detector, using Cu Kα radiation.Data reduction was carried out in the crysalis Pro Software. 9Structure solution was made using dual space methods in the shelxt program 10 Refinements were carried out in ShexlL 11 within the Olex2 12 software.The resonant scattering is weak for this sample (Friedif 12) as reflected by the large error of the refined Flack parameter (-0.1(4)).However, all indicators confirm the absolute structure.The Hooft parameter is 0.05 (6) with probability of the absolute structure being true P3(true) equal to 1 and the post refinement Parsons-Flack x determined using 2246 quotients is −0.03 (7)  Details for the refinement for the structure can be found in the table below, with a representation of the asymmetric unit with displacement ellipsoids drawn at 50 percent probability.

Figure S11 .
Figure S11.Experimental fluorescence decay measured with 1 in ACN under nitrogen flow (red), instrument response function (blue) and best single-exponential fit (black).

Figure S12 .
Figure S12.Transient absorption spectra measured after 355 nm excitation of 1 in ACN (left) and toluene (right), under nitrogen (upper) and air flow (bottom).

Figure S13 .
Figure S13.Transient absorption spectra measured after 355 nm excitation of 3 in ACN (left) and toluene (right), under nitrogen (upper) and air flow (bottom).

Figure S16 . 3 .
Figure S16.Comparison between the experimental (ACN) and calculated (vacuum) electronic absorption spectra of 3. The calculated oscillator strength is also shown.

Figure S17 .
Figure S17.Magnitude of the spin-orbit coupling (SOC) matrix element between the first 10 singlets and tripletstates of 1, calculated from the optimized S 1 state.

Figure S18 .
Figure S18.Magnitude of the spin-orbit coupling (SOC) matrix element between the first 10 singlets and triplet states of 3, calculated from the optimized S 1 state.

Figure S20 .
Figure S20.Ground-state energy of 3 as a function of the dihedral angle D. The dashed line indicates the value of kT at room temperature

Figure S21 .
Figure S21.Graphic representation of the SOC matrix element vs dihedral angle for the S 1 -T 3 states of 3. The most stable S 0 structure is highlighted in green.