Straightforward Access to Multifunctional π‐Conjugated P‐Heterocycles Featuring an Internal Ylidic Bond

Abstract We report the straightforward one‐pot synthesis of novel 5‐ or 6‐membered P‐heterocycles featuring an internal ylidic bond: P‐containing acenaphthylenes and phenanthrenes. The stability of the compounds tolerates post‐functionalization through direct arylation to introduce electron‐rich/poor substituents and the synthetic strategy is also compatible with the preparation of more elaborate polyaromatic scaffolds such as acenes and helicenes. Using a joint experimental (X‐ray analysis, optical and redox properties) and theoretical approach, we perform a full structure–property relationships study on these new platforms. In particular, we show that molecular engineering allows not only tuning their absorption/emission across the entire visible range but also endowing them with chiroptical or non‐linear optical properties, making them valuable dyes for a large panel of photonic or opto‐electronic applications.


X-ray crystallography
Single crystals suitable for X-Ray crystal analysis were obtained by slow diffusion of vapors of pentane into a dichloromethane solution of the derivatives at rt. Single crystal data collection were performed at 150 K with an D8 Venture Bruker-AXS diffractometer with Mo-K radiation ( = 0.71073 Å). The structure was solved by dualspace algorithm using the SHELXT program, and then refined with full-matrix least-squares methods based on F 2 (SHELXL). All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions and treated as riding on their parent atom with constrained thermal parameters.

Nonlinear optical experiments
All solvent used for photophysical measurements were of spectrometric grade. The absolute quantum yields were measured with a C9920-03 Hamamatsu system. Emission spectra by two photon (2PE) absorption were recorded using a femtosecond laser chain (Ti-Sapphire Chameleon ultra II Coherent + pulse picker + SHG module when needed, pulse duration: 100-130 fs; pulse frequency: 5 MHz) and an Ocean optics QEPro CCD detector with integrating times ranging from 1 to 20s. The excitation beam crossed a lens before arriving on the sample and a short-pass 750 nm filter after the sample to remove the excitation signal and prevent damages on the CCD detector. The power of the beam was measured with a PMD100 console and a S142C integrating sphere sensor from Thorlabs. For emission intensity vs excitation power measurements, a /2 waveplate and a polarizer were used to vary the laser power. The 2PE was recorded in solution, perpendicularly to the beam using an optical fiber connected to a CCD detector.

Analytical chiral HPLC separation
• 16a is dissolved in dichloromethane, injected on the chiral column, and detected with an UV detector at 254 nm and a circular dichroism detector at 254 nm. The flow-rate is 1 mL/min.

Methods
We have performed the DFT and TD-DFT calculations with Gaussian 16. [1] Default Gaussian 16 thresholds and algorithms were used but for an improved optimization threshold (10 -5 au on average residual forces), a stricter self-consistent field convergence criterion (10 -10 a.u.) and the use of the ultrafine DFT integration grid. Firstly, the S0 geometries have been optimized with DFT and the vibrational frequencies have been analytically determined, using the M06-2X meta-GGA hybrid exchange-correlation functional. [2] These calculations were performed with the 6-31G(d) atomic basis set and account for solvent effects through the linear-response PCM approach considering DCM as solvent. [3] Secondly, starting from the optimal ground-state geometries, we have used TD-DFT with the same functional and basis set to optimize the S1 geometry and compute the vibrational frequencies.
All optimized structures correspond to true minima of the potential energy surface. Thirdly, the vertical transition energies were determined with TD-DFT and the same functional, but a larger basis set, namely 6-311+G(2d,p), in gas-phase as well as in solution using the cLR 2 variant of the PCM, [4] in its non-equilibrium limit. The selection of M06-2X for computing the lowest energy excited state is justified by numerous benchmarks on organic derivatives demonstrating that this functional offers one of the best compromise with high-correlations and rather low average deviations with respect to trustworthy experiments. [5] We use Le Bahers' dCT model [6] on the so-called relaxed TD-M06-2X densities to quantify CT. The full UV/Vis and ECD spectra have been modelled with LR-PCM-B3LYP/6-311+G(2d,p), using a convoluting Gaussian of HWHM of 0.2 eV. The selection of B3LYP rather than M06-2X in this context is explained by the fact that the shape of ECD spectra depends on numerous excited states and that B3LYP globally provides a good balance for this task. [7] Ref. 7 concludes: "Overall for ECD and OR, 'traditional' GHs seem to provide the best choice". The TPA cross-sections have been obtained using QR-TD-DFT, with the CAM-B3LYP [8] range-separated hybrid functional and the 6-31G atomic basis set, with the Dalton code, [9] under its 2016 version. Default Dalton parameters were used to obtain the cross sections. Using a range-separate hybrid is typically necessary for modelling nonlinear optical properties, and CAM-B3LYP is a typical choice for TPA. [7] Eventually, NICS(0) and NICS(1) values have been computed at the usual reference B3LYP/6-311+G(d,p) [10] level on the PCM-M06-2X/6-31G(d) geometries.

Structures of 5a and 7a
Top view side view HOMO LUMO Density differences plots of 5a, 7a, and 9 5a 7a 9 Figure S40: Density difference plots of 5a, 7a, and 9 obtained at the PCM-M06-2X/6-311+G(2d,p) level of theory for the lowest excited-transition (S0-S1). The blue and red lobes correspond to regions of decrease and increase of electron density upon excitation (absorption), respectively. Contour threshold: 15 x 10 -4 .   Table S3: TD-DFT data determined for selected dyes. We report the vertical absorption and emission, together with the associated oscillator strength. The transition energies are obtained at the cLR 2 (neq)-PCM(DCM)-TD-M06-2X/6-311+G(2d,p) level, the oscillator strength ate the correspond LR(neq)values. In the rightmost column, we provide the CT distance (for absorption) as given by Le Bahers' model.   Table S4: One and two-photon excitation obtained with (QR-)TD-DFT (CAM-B3LYP/6-31G) for dye 12.

Two-photon spectra of 12
We report the (one-photon) transition energy, the corresponding oscillator strength as well as the TPA cross section.

S0-S1
S0-S2 Figure S46: Density difference plots (CAM-B3LYP/6-31G) of 12: two lowest excited-state. The blue and red lobes correspond to regions of decrease and increase of electron density upon excitation (absorption), respectively. Contour threshold: 10 x 10 -4 . Note that for the lowest one, the topology is totally similar to the one reported in the main text with the M06-2X functional and a much larger basis set.