Distance dependence of enhanced intersystem crossing in BODIPY–nitroxide dyads

Photogenerated organic triplet–doublet systems have attracted an increasing amount of attention in recent years due to their versatility and suitability for a range of technological applications in the emerging field of molecular spintronics. Such systems are typically generated by enhanced intersystem crossing (EISC) preceded by photoexcitation of an organic chromophore covalently linked to a stable radical. After formation of the chromophore triplet state by EISC, triplet state and stable radical may interact, whereby the nature of the interaction depends on the exchange interaction JTR between them. If JTR surpasses all other magnetic interactions in the system, molecular quartet states may be formed by spin mixing. For the design of new spintronic materials based on photogenerated triplet–doublet systems, it is crucial to gain further knowledge about the factors influencing the EISC process and the yield of the subsequent quartet state formation. Here we investigate a series of three BODIPY–nitroxide dyads characterised by different separation distances and different relative orientations of the two spin centres. Our combined results from optical spectroscopy, transient electron paramagnetic resonance, and quantum chemical calculations suggest that the chromophore triplet formation by EISC is mediated by dipolar interactions and depends primarily on the distance between the chromophore and radical electrons, while the yield of the subsequent quartet state formation by triplet–doublet spin mixing is influenced by the absolute magnitude of JTR.

: Overview of the procedure employed for the synthesis of the three BODIPY−eTEMPO dyads. S1

General methods
All reactions were performed under an argon atmosphere unless otherwise indicated. All reagents and solvents were purchased at the highest commercial quality and used without further purification unless otherwise noted. Dry solvents were obtained using a double column SolvTech purification system. Thin layer chromatography was performed with TLC silica on aluminium foil (Silica gel/UV254, Aldrich). In most cases, irradiation using a Bioblock VL-4C UV-lamp (6 W, 254 nm and/or 365 nm) was used as well as suitable TLC stains for visualisation. Preparative adsorption flash column chromatography was performed using silica gel (60 Å, 230-400 mesh, 40-63 µm, Sigma-Aldrich). 1 H NMR and 13 C NMR spectra were recorded on a Bruker Avance III HD 400 MHz spectrometer equipped with a BBFO probe at 298 K. The spectra were internally referenced to the residual proton solvent signal. For 1 H NMR and 13 C assignments, the chemical shifts are given in ppm. Coupling constants J are listed in Hz.
After cooling down to room temperature, the mixture was poured onto water (200 mL) and extracted with Et 2 O (2×200 mL). The organic phase was dried (Na 2 SO 4 ), filtered, and evaporated and the residue was purified by column chromatography (hexane/Et 2 O, 2:1 followed by hexane/EtOAc, 2:1). No starting material was recovered (as opposed to the reported procedure) and the main impurity was the homocoupling of the compound Bpin-eTEMPO. The pure product was isolated as a light brown solid (1.85 g, 74%). The characterisation was consistent with the reported data. [2] Compound par a par a par a-I-BODIPY were added and the reaction mixture was refluxed for 30 minutes and then evaporated to dryness under reduced pressure. The residue was dissolved in chloroform (100 mL). The organic layer was washed with saturated Na 2 CO 3 (100 mL) and water (2×100 mL), dried (Na 2 SO 4 ), filtered and evaporated.
The residue was purified by column chromatography on silica gel (20% chloroform/n-hexane) to yield S3 compound para-I-BODIPY (1.00 g, 46%) as an orange solid. 3-iodobenzoyl chloride (1 eq., 3.501 g, 13.14 mmol) was dissolved in dry CHCl 3 (140 mL), 2,4-dimethylpyrrole (2 eq., 2.5 g, 2.706 mL, 26.28 mmol) was added under argon and the reaction mixture was stirred at room temperature for 3 days. The reaction mixture was cooled with an ice bath and Et 3 N (5.39 eq., 7.17 g, 9.85 mL, 70.86 mmol) and boron trifluoride (8.10 eq., 15.11 g, 13.14 mL, 106.47 mmol) were added dropwise. The reaction was left to react overnight at room temperature. The solvent was evaporated under reduced pressure and the residue purified by column chromatography on silica gel (3:1 hexane/chloroform). A second purification by column chromatography was necessary to obtain the pure product as an orange solid (0.8 mg, 14%). The characterisation was consistent with the reported data. [ In a 50 mL round-bottom flask, meta-I-BODIPY (1 eq., 19.2 mg, 0.043 mmol), Bpin-eTEMPO (1.67 eq., 20 mg, 0.071 mmol) and sodium carbonate (3.14 eq., 14.2 mg, 0.13 mmol) were dissolved in 18 mL of water/THF/toluene (1:1:1). Pd(PPh 3 ) 4 (0.0609 eq., 3 mg, 0.0026 mmol) was added and the reaction mixture was stirred at 80 • C for 3 h (monitoring by UPLC). After reaction completion, a saturated solution of Na 2 CO 3 (15 mL) was added and the mixture was extracted with dichloromethane (3×50 mL). The combined organic extracts were dried over Na 2 SO 4 and filtered. The solvent was removed under reduced pressure and the residue was purified by PTLC (SiO 2 , DCM, R f ∼ 0.2) to yield BODIPY-m-eTEMPO Although structural elucidation is not possible by NMR due to the presence of the radical, the spectra are reported at the end of this document.
Although structural elucidation is not possible by NMR due to the presence of the radical, the spectra are reported at the end of this document.

BODIPY-biph
O O B + S10 R f ∼ 0.5) to yield compound BODIPY-biph (6.9 mg, 52%) as an orange solid. The characterisation was consistent with the reported data. [7] HRMS-ESI calculated for C 25   12.02 mg, 0.059 mmol), CsF (3 eq., 24.40 mg, 0.16 mmol), and Pd(dppf)Cl 2 ·CH 2 Cl 2 (0.05 eq., 2.19 mg, 2.7 µmol) were dissolved in an argon saturated dioxane/water mixture (2:1, 5 mL). The reaction mixture was heated to 100 • C in a closed reaction vessel for 3 days. After cooling down to room temperature, the reaction mixture was diluted in DCM (20 mL) and extracted with brine (15 mL) and water (15 mL For the calculation of the excitation energy transfer rate constants, the molar absorption coefficient of eTEMPO was determined in toluene solution at room temperature and compared to that of TEMPO.
The data are shown in Figure S2.  Table S1.

Determination of the singlet oxygen quantum yields
Singlet oxygen quantum yields were measured as detailed in the main part. Figure S5   The Förster radius R 0 (obtained in nm) can be calculated from [9] where Φ D F,0 and I D F are the fluorescence quantum yield and fluorescence intensity of the donor, ε A is the molar absorption coefficient (in M −1 cm −1 ) of the acceptor and n the refractive index of the medium. The S10  The energy transfer rate constant and FRET efficiency are then given as where τ D F,0 is the fluorescence lifetime of the donor in the absence of any quenchers and r DA is the centreto-centre distance (point dipole) between donor and acceptor. The results are summarised in Table S2 and the spectral overlap between chromophore fluorescence emission and radical absorption is visualised in Figure S6.

Calculation of the driving forces for electron transfer
The calculations of the driving forces −∆G 0 for charge separation (CS) and charge recombination (CR) were performed assuming the validity of the following equations [10] −∆G 0, CS = −∆G 0, IP + E 00 (S5) where the subscript IP stands for ion pair, E 00 is the energy of the first excited singlet state and E ox (D) and E red (A) are the oxidation potentials of the electron donor and reduction potentials of the electron acceptor, respectively. The terms C and S represent the coulomb and solvent correction terms, defined as C = − e 2 4πε 0 ε r r ee S = e 2 8πε 0 where r ee , ε r , ε 0 , ε r, ref and r i are the edge-to-edge distance between the reaction partners, the relative solvent permittivity, the vacuum permittivity, the relative permittivity of the solvent used to determine the redox potentials and the Van-der-Waals radii, respectively.

S11
The edge-to-edge distance for electron transfer, as well as the Van-der-Waals radii for donor (eTEMPO) and acceptor (BODIPY) were estimated from DFT models. For the calculation of the Van-der-Waals radii an ellipsoidal model was assumed; the radius was calculated according to r VdW = 3 √ a · b · c, where a, b, and c are the dimensions in the three different directions. The following values were obtained: r ee = 8.7 Å for BODIPY−p−eTEMPO, r ee = 7.6 Å for BODIPY−m−eTEMPO, r ee = 13.0 Å for BODIPY−xy−eTEMPO, r D = 3.1 Å, and r A = 4.8 Å.
The oxidation potential of eTEMPO in o-dichlorobenzene was measured to be 0.91 V vs. SCE, while for the reduction potential of BODIPY a value of −1.19 V vs. SCE was obtained (see Figure S4).
The solvent o-dichlorobenzene has a relative dielectric constants of 9.9, while toluene has a relative dielectric constant of ε r = 2.4 at room temperature [11]. E 00 is calculated from the crossing point of the absorption and fluorescence spectra and amounts to 2.436 eV (509 nm) for all investigated BODIPY structures. The calculated driving forces for electron transfer −∆G 0 for charge separation and charge recombination between BODIPY and eTEMPO are summarised in Table S3. The negative value of −∆G 0 suggests that charge separation does not occur spontaneously. The actual rate constants for electron transfer will further depend on the corresponding electronic matrix elements |H AB | 2 and the reorganisation energies λ. According to the classical Marcus theory for non-adiabatic electron transfer [12,13] where λ = λ inner + λ outer (S11) and the inner and outer sphere contributions to the reorganisation energy are given as The outer sphere solvent reorganisation energy can be calculated using the van-der-Waals radii listed above. The inner sphere reorganisation energy for the BODIPY/eTEMPO couple was calculated with ORCA [14] at the DFT/B3LYP level of theory to amount to 0.2044 eV using the procedure outlined, S12 for instance, in reference 15 (see also Subsection 3.3 below). For all neutral or cationic species the def2-TZVP basis set was used, while the ma-def2-TZVP basis set was used for anionic species. Note, that the shifts from the use of two different basis sets cancel each other out.
The value of H AB is difficult to estimate reliably, but could be responsible for a further reduction in the electron transfer rate constant. While the short distance between electron donor and acceptor could favour a large value of H AB , the saturated carbon atoms of the eTEMPO radical will exponentially reduce the coupling between the nitroxide group and the BODIPY chromophore. In addition, an inspection of the HOMO and LUMO orbitals of the BODIPY chromophore reveals that a nodal plane runs through the axis connecting BODIPY to the eTEMPO substituents.

Dark state EPR spectra
Continuous wave EPR spectra were measured for all investigated BODIPY−eTEMPO dyads in toluene solution at room temperature to confirm the presence of the nitroxide radical. The spectra were acquired using a modulation amplitude of 1 G and a microwave power of 1 mW (20 dB). After baseline correction, the spectra were frequency-corrected to 9.75 GHz and field-corrected using a carbon fibre standard with g = 2.002644 [16]. For an accurate determination of the g and A tensors of the eTEMPO radical, a simultaneous fit of a field-and frequency-corrected room temperature cw EPR spectrum and a pulse Q-band EPR spectrum recorded at 80 K was performed. The data are shown in Figure S8 together with the best numerical fit using A( 14 N) = [16,97] MHz and g R = [2.0103, 2.0070, 2.0025].

Simulations of the transient EPR spectra
To determine the magnetic parameters of the triplet excited state of the BODIPY chromophore, a trEPR spectrum of 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-BODIPY was measured in frozen toluene solution at 80 K. The data and a numerical simulation are shown in Figure S9. The best fit, as shown in the value of E T = −550 MHz was found to lead to the best fit for all coupled systems. Since all spectra are well within the strong coupling regime, J TR cannot be determined experimentally as its magnitude has no effect any more on the spectral shape. In the simulations, J TR was therefore set to a fairly high value (+10 cm −1 ) and kept fixed during the fitting procedure. The sign was adapted based on the exchange coupling calculations, which suggest antiferromagnetic coupling for all compounds. EasySpin uses the conventionĤ J = +JŜ 1Ŝ2 , meaning that a positive sign corresponds to antiferromagnetic coupling.
The only truly variable parameters were thus the linewidths and populations. The final populations obtained in the coupled triplet-radical basis (in EasySpin: Sys.initState = 'coupled') are listed in Table S4.
When translating these populations into the doublet-quartet basis we obtain the populations listed in Table S5.
For reference, a direct comparison of the trEPR spectra of the three BODIPY-eTEMPO compounds is shown in Figure S10 together with an illustration of the spectral differences between the quartet spectra measured for the dyads and the triplet reference spectrum of 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-BODIPY. S14

Structures and transition dipole moments
A visualisation of the optimised structures with an indication of the distances between BODIPY and eTEMPO is shown in Figure S11. For the determination of the transition dipole moments of BODIPY and eTEMPO, TD-DFT calculations were carried out at the CAM-B3LYP/def2-TZVP level of theory using the RIJCOSX approximation for the Coulomb-and exchange integrals [19,24,25]. The orientation of the relevant transition dipole moment within the structures is also shown in Figure S11. Figure S11: Transition dipole moment orientations and distances between chromophore and radical (centre of the BODIPY core to centre of N−O bond) in the BODIPY−eTEMPO dyads. A calculation of κ 2 based on these structures yields values of 0.08, 0.33, and 0.10 for the para, meta, and xy-linked compounds, respectively. Figure S12 shows the HOMO and LUMO orbitals of the BODIPY chromophore, demonstrating that a nodal plane runs through the axis connecting BODIPY and eTEMPO. The inner sphere reorganisation energy λ in was calculated using

HOMO and LUMO orbitals of BODIPY
where λ Acc and λ Don describe the contributions from the acceptor (i.e. the bare BODIPY chromophore) and the donor (eTEMPO radical). R 0 , R − and R + refer to the equilibrium structures of the corresponding neutral, anionic and cationic species. Similarly, E 0 , E − and E + refer to the SCF energies of the corresponding neutral, anionic and cationic species at a certain geometry. The structures were optimised and the SCF energies were calculated at the B3LYP/def2-TZVP level of theory for all neutral and cationic species, while for the anionic BODIPY chromophore the B3LYP/ma-def2-TZVP level of theory was used [26].

Exchange coupling calculations
The orbitals, that were later used as starting orbitals in the CASSCF procedure, were computed using TD-DFT at the CAM-B3LYP/def2-TZVP level of theory with the RIJCOSX approximation for the Coulomband exchange integrals [19,24]. TD-DFT requires only a short computing time and therefore provides a quick overview of the orbitals that are crucial for the excited state mechanism and thus define the active space in the CASSCF procedure. As expected, the active orbitals in all investigated molecules turned out to be the chromophore HOMO and LUMO orbitals as well as the radical SOMO.
The excited state exchange interactions were calculated using QD-NEVPT2/def2-TZVP on an optimised CASSCF(3,3) chromophore triplet state wavefunction with starting orbitals obtained as described above [27]. The calculations were again accelerated by the RIJCOSX approximation. For an easier interpretation of the calculated wavefunctions, the active orbitals, which are shown in Figure S13, were localised by the Foster-Boys method [28].
The excited state exchange interaction between the chromophore triplet state and the radical doublet state can be calculated by: or approximately by: where D 1 is the trip-doublet state and Q 0 is the trip-quartet state. J 12 and J 23 are the exchange interactions between the HOMO and SOMO and between the LUMO and SOMO electrons, respectively.