Triplet sensitization enables bidirectional isomerization of diazocine with 130 nm redshift in excitation wavelengths

Diazocines are bridged azobenzenes with phenyl rings connected by a CH2–CH2 group. Despite this rather small structural difference, diazocine exhibits improved properties over azobenzene as a photoswitch and most importantly, its Z configuration is more stable than the E isomer. Herein, we reveal yet another unique feature of this emerging class of photoswitches. In striking contrast to azobenzenes and other photochromes, diazocine can be selectively switched in E → Z direction and most intriguingly from its thermodynamically stable Z to metastable E isomer upon successive excitation of two different triplet sensitizers present in solution at the same time. This approach leads to extraordinary large redshift of excitation wavelengths to perform isomerization i.e. from 400 nm blue to 530 nm green light (Z → E) and from 530 nm green to 740 nm far-red one (E → Z), which falls in the near-infrared window in biological tissue. Therefore, this work opens up of potential avenues for utilizing diazocines for example in photopharmacology, smart materials, light energy harvesting/storage devices, and out-of-equilibrium systems.


Materials
All the solvents were purchased from Sigma-Aldrich (Merck KGaA, Germany) and used as provided.PdTPBP, PdOEP and PtOEP were purchased from Frontier Scientific Inc. (USA).The synthesis of PdTPNP 1 was previously published.Diazocine was synthesized according to a modified procedure (detailed and characterized below) published by Moormann et al. 18 Bis(methylthio)methane was purchased from Tokyo Chemical Industry (Japan).

Triplet energies of the sensitizers
Phosphorescence spectra and phosphorescence decays for quenching studies were measured with FLS1000 spectrofluorometer (Edinburgh Instruments Ltd, UK).Phosphorescence samples were prepared in a 1 cm 2 SOG9 cuvette (Starna Scientific Ltd, UK) sealed with a screw cap and a silicon/PTFE septum. 1 µM concentration of sensitizer was dissolved in DMSO to which 0.48 M concentration of Bis(methylthio)methane was added.The sample was then purged of oxygen by bubbling vigorously with nitrogen for 1 hour.The phosphorescence spectra (Fig. S1-S4) were measured by exciting the sensitizers at their Q band maxima with a Xe lamp.Their triplet energies were then determined by the maxima wavelengths.

Phosphorescence quenching experiments
Quenching samples were prepared as described above with 1 µM concentration of sensitizer.Diazocine was added as chloroform solution to the sample and purged for 1 hour after each addition before measuring the phosphorescence decays.The quenching with diazocine in Econformation was performed by illuminating the sample by 385 nm excitation with the xenon lamp of the spectrofluorometer for 15 min.The resulting phosphorescence decays were then tail-fitted to extract the phosphorescence lifetimes.All decays were monoexponential (χ < 1.2

Isomerization experiments
Photoisomerization studies were performed also in DMSO to which 0.48 M concentration of Bis(methylthio)methane was added.The concentration of diazocine was constant, 500 µM, for all isomerization studies.The samples were also prepared in screw cap cuvettes (vide supra) and purged vigorously with nitrogen for 1 h prior to measuring.
The photoisomerization measurements were performed on a Cary 60 spectrophotometer (Agilent Technologies Inc, USA) equipped with an Ocean Optics (USA) qpod 2e Peltier-thermostated cell holder at 20 .Excitation source was Prior Lumen 1600 (Prior Scientific Inc, USA) with multiple ℃ choices for narrow-band LEDs at different wavelengths.Direct excitation of Z-diazocine was performed with a 385 nm LED (230 mW/cm 2 ), excitation of PdOEP or PtOEP was performed with a 530 nm LED (140 mW mW/cm 2 ), excitation of PdTPBP was performed with 640 nm LED (255 mW/cm 2 ) and excitation of PdTPNP was performed with a 740 nm LED (255 mW/cm 2 ).The conversion between Z-and E-isomers was monitored by absorption at 500 nm.

Computational details
DFT and TD-DFT TD-DFT calculations were carried out on a ωB97X-D3/def2-TZVP 4,5 level of theory using ORCA 6 with Grimmes D3 dispersion correction 7 and counter poise corrections for basis set superposition errors (BSSE). 8Preliminary scans along the CNNC torsion were carried out for diazocine and azobenzene (AB), all other coordinates were allowed to relax freely.S 1 and T 1 energies were determined for vertical excitation starting from the respective S 0 geometry.Unfortunately, the CNNC torsional angles become ill-defined between 70° and 110° because one of the NNC angles is close to 180° (inversion mechanism).Vertical excitations of the E and Z isomers obtained with TD-DFT are shown in Table S1.To circumvent the problem of ill-defined CNNC torsion angles we choose to use a nudged elastic band approach (NEB) 9 to determine the minimum energy pathway (MEP) from the diazocine Z to the E isomer.After the minimum energy pathway was determined we calculated the vertical excitations from the obtained S 0 geometries.The S 0 MEP of the diazocine isomerization, as well as S 1 and T 1 directly above this path are shown in Figure S9.The corresponding MEP of azobenzene is shown in Figure S10.

CASSCF/CASPT2
The state-averaged (SA) complete active space self-consistent field (CASSCF) and second order perturbation theory CAS (CASPT2) calculations were carried out with the OpenMolcas program package (version 22.06) 10 .For both azobenzene and diazocine, we adopted most of the computational setup from a previous work 11 : The active space consists of 14 electrons in the 12 relevant n,  and  * orbitals; different atoms were treated with different basis sets (Dunning's augcc-pVTZ basis set for the two nitrogen atoms and cc-pVTZ for all carbon atoms 12 and Pople's STO-3G for the H atoms 13 ; Cholesky decomposition was enabled with the 'medium' keyword. The structures for diazocine were also taken from 11 .For azobenzene a simple scan around the central N=N bond in 15 steps resulted in 13 structures which were optimized using D4-RI-B3LYP/def2-SVP 4,14-16 with Turbomole v7.4 17 .The following procedure was then applied to all structures: SA-CASSCF energy calculations for the first three electronic singlet states followed by the corresponding CASPT2 energy calculation; both CASSCF and CASPT2 energy calculations for the first triplet state; calculation of the spin-orbit coupling (SOC) on the CASPT2 level.
Table S2 lists all S 0 , T 1 , S 1 and S 2 energies on the CASPT2 level relative to the lowest energy in the electronic ground state for both diazocine and azobenzene.The SOC calculation turned out to be not necessary as all relative energies are virtually the same compared to the "pure" CASPT2 energies.All energies in eV.The essential S 0 → S 1 and S 0 → T 1 energy differences are compiled in Table S2.There, we also list the intermediate CASSCF and final CASPT2+SOC energies obtained during our calculations for the sake of completeness.
Table S3 Essential S 0 → S 1 and S 0 → T 1 energy differences All energies in eV.7 mmol), and the mixture was stirred for 18 h under reflux.The reaction mixture was filtered through celite and the solvent was removed under reduced pressure.The crude product was dissolved in CH 2 Cl 2 and filtered through celite, dried over MgSO 4 , and the solvent was removed under reduced pressure.The crude product was dissolved in 500 mL methanolic NaOH solution, CuCl 2 was added, and air was bubbled through the solution while stirring at room temperature until completion of the reaction.The reaction mixture was neutralized with 1 M HCl solution.After addition of saturated sodium bicarbonate solution, the aqueous layer was extracted with CH 2 Cl 2 three times.The combined organic layers were dried over MgSO 4 and the solvent was removed under reduced pressure.The crude product was purified by column chromatography (cyclohexane/EtOAc 3:1) on silica gel to afford the product as a yellow solid.

Figure S1 .
Figure S1.Normalized phosphorescence spectrum of PdTPBP.The wavelength of the maximum is labeled.

Figure S2 .
Figure S2.Normalized phosphorescence spectrum of PdOEP.The wavelength of the maximum is labeled.

Figure S3 .Figure S4 .
Figure S3.Normalized phosphorescence spectra of PdTPNP.The wavelength of the maximum is labeled.

Figure S12 .
Figure S12.Comparison of E-to-Z isomerization of Diazocine in the dark with and without porphyrin sensitizers.

Figure S13 .
Figure S13.MEP of the diazocine isomerization at the ωB97X-D3/def2-TZVP level of time dependent density functional theory (TDDFT) with vertical excitations to S 1 and T 1 .Relative energies are in eV.The reaction coordinate R is defined by the nudged band (NEB) approach.The stationary points (Z and E isomers and the transition state) at the S 0 energy hypersurface are fully optimized.

Figure S14 .
Figure S14.MEP of the azobenzene isomerization at the ωB97X-D3/def2-TZVP level of time dependent density functional theory (TDDFT) with vertical excitations to S 1 and T 1 .Relative energies are in eV.The reaction coordinate R is defined by the nudged band (NEB) approach.The stationary points (Z and E isomers and the transition state) at the S 0 energy hypersurface are fully optimized.
Table S1 S 0 →S 1 and S 0 →T 1 Energy gaps obtained on a ωB97X-D3/def2-TZVP level of theory.All energies in eV.