Intramolecular Charge Transfer and Spin–Orbit Coupled Intersystem Crossing in Hypervalent Phosphorus(V) and Antimony(V) Porphyrin Black Dyes

Porphyrin dyes with strong push–pull type intramolecular charge transfer (ICT) character and broad absorption across the visible spectrum are reported. This combination of properties has been achieved by functionalizing the periphery of hypervalent and highly electron-deficient phosphorus(V) and antimony(V) centered porphyrins with electron-rich triphenylamine (TPA) groups. As a result of the large difference in electronegativity between the porphyrin ring and the peripheral groups, their absorption profiles show several strong charge transfer transitions, which in addition to the porphyrin-centered π → π* transitions, make them panchromatic black dyes with high absorption coefficients between 200 and 800 nm. Time-resolved optical and electron paramagnetic resonance (EPR) studies show that the lowest triplet state also has ICT character and is populated by spin–orbit coupled intersystem crossing.


Synthesis of [SbT(TPA)P(OMe)2]PF6 (2).
[SbT(TPA)PCl2]Cl (40 mg, 0.026 mmol) was dissolved in a mixture of dry CHCl3:CH3OH:anhydrous pyridine (= 2:2:1) and was refluxed for 5-6 days under a nitrogen atmosphere.The reaction mixture was occasionally monitored using mass spectrometry to confirm the product was being formed in the reaction mixture over the course of the reaction.After confirming that the product had been formed, the reaction mixture was dried under vacuum and purified using column chromatography on neutral Al2O3 where CH2Cl2:CH3OH (= 99:1) solvent mixture was used to elute the product on the column.The solvent was then removed under a vacuum.For conversion to PF6 salt, the product was dissolved in a minimum amount of methanol, and anhydrous NH4PF6 (100 mg) was then dissolved in the same solution.DI water (5x the amount of methanol) was added over the course of 30-40 minutes to precipitate the PF6 salt of the product by vacuum filtration and was further dried via vacuum to yield 32mg of the product.Yield = 32 mg (85%). 1

Physical methods
NMR and mass spectroscopy.NMR spectra were recorded on a Bruker Advance 400 MHz NMR spectrometer using CDCl3 as the solvent.ESI mass spectra were recorded on a Bruker MicroTOF-III mass spectrometer using direct injection from an UltiMate 3000 HPLC and acetonitrile as a solvent.
Absorption and emission spectroscopy.UV/Vis spectra were recorded with an Agilent Cary 100 UV/Vis spectrometer.The concentration of the samples used for these measurements ranged from 5 × 10 −6 M (porphyrin B-band (Soret)) to 5 × 10 −5 M (Q-bands) solutions.Steady-state fluorescence spectra were recorded using a Photon Technologies International Quanta Master 8075-11 spectrofluorometer, equipped with a 75 W Xenon lamp, running with FelixGX software.Emission spectra were collected using excitation in each of the major absorption bands.The samples were adjusted to 0.2 optical density at the selected excitation wavelength.Due to the low fluorescence quantum yields the excitation and emission slits were maintained at 3/3 nm.Constant sample concentration (4.5 ´ 10 -6 M) was maintained in all CH2Br2 titrations.

DFT calculations.
All the porphyrin structures were initially constructed on a local PC using the GaussView 6 (GV6.0)software.DFT computations were performed on a supercomputer using the Gaussian 16 software suite. 2 Since the investigation includes the theoretical study of the excited state and charge transfer properties where the highest excitation is to the LUMO+1, the B3LYP method was chosen.The 6-311+G(2df,pd) split-valence polarized basis set was used to model hydrogen and the period 2 elements (C, N, O) in the compounds.Since antimony is a period 5 element, the relativistic effects of the core electrons were modeled using effective core potentials (ECPs).The Stuttgart/Dresden ECPs, in combination with the triple-zeta polarized basis set (def2TZVPP), were chosen to model antimony for this study.Additionally, the Self-Consistent Reaction Field (SCRF) method, the Conductor Polarizable Continuum Model (CPCM), and the Gaussian 16 parameters for acetonitrile were included to model the structures in solution.Thus, the B3LYP method was coupled with a GenECP basis to form the B3LYP/GenECP model chemistry, and SCRF(CPCM, Solvent=Acetonitrile) was used to optimize the geometry of all the structures herein to a stationary point on the Born-Oppenheimer surface and calculate the first ten excited singlet states of all chemical species in the current study.All the structures were optimized sans symmetry constraints as +1 charged cations and closed-shell singlets.The self-consistent field (SCF) convergence constraints and the DFT grid utilized in the calculation were the G16 default values, "Tight" and "UltraFine" respectively.The optimization of the geometrical parameters of each of the chemical species in the study was continued until the maximum force, RMS force, maximum displacement, and RMS displacement reached or was less than the default Gaussian 16 minima and the predicted energy change upon a successive optimization cycle of the geometrical parameters was in the range of -5×10 -9 A.U.To generate the difference density maps shown in Fig. 1, additional TDDFT calculations were carried out using Orca (release 5.0.1). 3,46][7] ESP map color scale 4.0 -8.0 ´ 10 -2 V.The orbitals have been plotted with an isovalue of 0.02 and the DD maps at an isovalue of 0.0002.

Transient EPR spectroscopy.
Transient EPR time/field data sets were recorded at 80 K using a modified Bruker EPR 200D-SRC X-band spectrometer equipped with a Flexline dielectric resonator and a CF 935 cryostat.A frequency-doubled Continuum Surelite Nd:YAG laser was used for pulsed light excitation at 532 nm with at a repetition rate of 10 Hz and 10 ns pulse width.The samples were prepared by dissolving each porphyrin in 200 µL of 2-methylTHF to a concentration of approximately 0.7 mM.The resulting solutions were degassed by several freeze-pump-thaw cycles.The samples were then frozen in a clear glass and placed in the cryostat.The spin-polarized TREPR spectra were extracted from the full-time/field dataset by taking the average signal intensity in a 500 ns time window centered at 750 ns after the laser flash and subtracting the average signal level before the laser flash.The spectra were simulated using EasySpin 8 is described below.
Femtosecond laser flash photolysis.Femtosecond transient absorption experiments were performed using an ultrafast femtosecond laser source (Libra) by Coherent incorporating a diode-pumped, mode-locked Ti:sapphire laser (Vitesse) and a diode-pumped intracavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. Samples were excited at 410 nm.A Helios transient absorption spectrometer coupled with a femtosecond harmonics generator, provided by Ultrafast Systems LLC, was used for optical detection.The sources for the pump and probe pulses were derived from the fundamental output of Libra (Compressed output 1.45 W, pulse width 100 fs) at a repetition rate of 1 kHz; 95% of the fundamental output of the laser was introduced into a TOPAS-Prime-OPA system with a 290−2600 nm tuning range from Altos Photonics Inc., (Bozeman, MT), while the rest of the output was used for generation of a white light continuum.Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data.Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems.All measurements were conducted in degassed solutions at 298 K.The estimated error in the reported rate constants is ±10%.

UV-Visible Absorption Data
Table S1.Absorption wavelengths and extinction coefficients of the investigated compounds in CH3CN.

Electrochemistry
To better comprehend the electrostatic potential differences, the electrochemical studies of 1 and 2 were performed in CH3CN with 0.1 M TBA•PF6, (see Figure S6), and Table S2.The cathodic scan reveals two oneelectron reduction processes between -0.53 and -1.01 V for 1 and −0.37 and −0.84 V for 2, corresponding to the successive addition of two electrons to the LUMO localized primarily on the porphyrin ring.The positively shifted potentials indicate these systems' strong electron acceptor nature.Moreover, the shift is greater in 2 compared to 1, indicating that its porphyrin ring is even more electron deficient, making it a superior electron acceptor.The anodic scan reveals oxidation processes at 0.96 and 1.15 V for 1 and 1.04 V for 2, corresponding to the removal of electrons from the HOMO.The current values for the oxidation are significantly higher than their corresponding reduction currents suggesting that the HOMO is localized on the TPA units.The absence of porphyrin-centered oxidation processes is consistent with the electron deficiency on the central ring.Table S3.Absorption spectrum of 1 from TD-DFT.

Triplet Quencher and Thermally Activated Delay Fluorescence (TADF) Studies
In order to understand the nature of the 850 nm emission band, triplet quenching experiments were performed using compound 1 in toluene, see Figure S17a.The emission was collected in the presence and absence of triplet quencher O2 with excitation at 500 nm.The samples were purged with O2 and N2 gases for 30 min before collecting the emission.It is anticipated the proposed 3 CT state would undergo energy transfer with 3 O2 to produce 1 O2 with an energy of 0.97 eV. 1 Based on the energetics (Figure 3) the proposed energy transfer process is thermodynamically favorable under these conditions, and the intensity of the 850 nm band should decrease in the presence of O2 gas.However, as shown in Figure S17a, the emission intensities are not very different with and without O2 gas.These inconclusive results prompted us to attempt the same studies with ferrocene (Fc), see Figure S17b.Ferrocene's lowest energy triplet state is around 1.16 eV, 9 therefore, a triplet-triplet energy transfer from 3 CT to 3 Fc is thermodynamically feasible.In Figure S17b, it is evident that the emission intensity remains relatively stable even in the presence of a large excess.Overall, the results from experiments involving triplet quenchers are inconclusive.This may be due to these triplet quenchers reacting with the dark triplet states of the porphyrins, or the singlet/triplet states being mixed and not as susceptible to being quenched by the chosen triplet quenchers.
Temperature-dependent emission studies were performed in CH2Cl2 (Figure S17c) and in toluene (Figure S17d) to examine the thermally activated delay fluorescence (TADF).Compound 1 was excited at 500 nm and the emission was collected while increasing the temperature.If TADF is present in the molecule, the emission intensity would decrease.However, in CH2Cl2 solvent the mission intensity slightly increases whereas in toluene marginally decreases.Once again, the observed trends do not confirm the TADF mechanism in compound 1.
The TCSPC fluorescence lifetime measurements at various emission wavelengths were carried out in toluene, see Table S5.These lifetimes are in the ns range, and as such, they do not show clearly whether the emission arises from the singlet or triplet state.

Fs-Transient Absorption Studies
Changing the solvent from polar CH3CN to nonpolar toluene revealed appreciable changes in photodynamics, as shown in Figure S20.The expected ESA and GSB peaks complimenting the absorption peak maxima were also observed.For 1, the DAS revealed four components, and from spectral comparison, the first spectrum at t1 = 3.9 ps is attributed to the S2 state, the second one at t2 = 42.2 ps to the S1 state, and the third one at t3 = 94.8 ps to the charge transfer state (Figure S20a).A long-lived component at t4>3 ns was also observed with features of the triplet excited state.From the initial growth and decay of the population time profiles (Figure S20b and c), the following sequence of occurrence: S2 à S1 à CT à T1 could be suggested; however, looking at the initial time of growth, the S1 state to simultaneously populate both CT and T1 state cannot be ruled out.In the case of 2, a four-component fit also provided satisfactory results (Figure S20d).In this case, the spectra at t1 H NMR (CDCl3, 400 MHz): δ, ppm 9.68 (s, 8H), 8.14 (d, 8H, J = 8.4 Hz), 7.55 (d, 8H, J = 8.4 Hz), 7.47 (d, 32H, J = 4.3 Hz), 7.23 (m, 8H), -2.10 (s, 6H). 19F NMR (CDCl3, 377 MHz): δ, ppm -73.1 (d, 6F, J = 713 Hz). 31 P NMR (CDCl3, 162 MHz): δ, ppm -144.5 (sept, J = 712.8).ESI MS: m/z 1463.3904 for [M -PF6] + , calculated 1463.4654 for C94H70O2N8Sb + .

Figure S7 .
Figure S7.Natural transition orbitals (isovalue 0.02) of the lowest excited singlet states of 1 (panels a and b) and 2 (panels c and d).Panels a and c are the NTOs of the electron hole.Panels b and d are the NTOs of the excited electron.

-Figure S9 .
Figure S9.Density difference maps (isovalue 0.0002) of the eight lowest singlet states of 1.The surfaces indicate the change in electron density in going from the ground state to the excited singlet state.Red: electron density decrease, green: electron density increase.

Figure S10 .
Figure S10.Density difference maps (isovalue 0.0002) of the eight lowest singlet states of 2. The surfaces indicate the change in electron density in going from the ground state to the excited singlet state.Red: electron density decrease, green: electron density increase.

S 1 ( 1 .
Figure S11.Density difference maps (isovalue 0.0002) of the eight lowest triplet states of 1.The surfaces indicate the change in electron density in going from the ground state to the excited triplet state.Red: electron density decrease, green: electron density increase.

Figure S12 .
Figure S12.Density difference maps (isovalue 0.0002) of the eight lowest triplet states of 2. The surfaces indicate the change in electron density in going from the ground state to the excited triplet state.Red: electron density decrease, green: electron density increase.

Figure S19 .
Figure S19.Spectral changes observed during chemical oxidation of (a) 1 and (c) 2; and reduction of (b) 1 and (d) 2 in acetonitrile.Nitrosonium tetrafluoroborate and cobaltocene were used as oxidizing and reducing agents.

= 3 .
6 ps to the S2 state, t2 = 26.3ps to the S1 state, and t3 = 161.9ps for the CT state and t4>3 ns to the triplet state were possible to assign (FigureS20e).From the initial growth and decay of the population time profiles (FigureS20f), the following simultaneous sequence of occurrence: S2 à S1 à CT and S2 à S1 à T1 was possible to envision.

Figure S20 .
Figure S20.Fs-TA spectra at the indicated delay times of (a) 1 and (d) 2 in toluene at the excitation wavelength of 410 nm.Their corresponding decay-associated spectra (DAS) (b) and (e) and the population time profiles (c) and (f) are on the right.

Table S2 .
Redox data of the investigated compounds in CH3CN with 0.1 M [TBA]PF6.Sample Potentials (V vs. SCE)

Table S5 .
TCSPC lifetime measurements of 1 in toluene at different temperatures.The sample was excited at 494 nm, and emission was collected at various wavelengths.