Electron Transfer Reactions in Sub- Porphyrin-Naphthyldiimide Dyads

A series of donor-acceptor compounds based on a sub-porphyrin (SubP) as an electron donor and naphthyldiimide (NDI) as an acceptor has been designed, synthesized and investigated by time-resolved emission and transient absorption measurements. The donor and acceptor are separated by a single phenyl spacer substituted by methyl groups in order to systematically vary the electronic coupling. The electron transfer reactions in toluene are found to be quite fast; charge separation is quantitative and occurs within 5-10 ps and charge recombination occurs in 1-10 ns, depending on the substitution pattern. As expected, when steric bulk is introduced on the adjoining phenyl group, electron transfer rates slow down because of smaller electronic coupling. Quantum mechanical modelling of the potential energy for twisting the dihedral angles combined with a simplified model of the electronic coupling semi-quantitatively explains the observed variation of the electron transfer rates. Investigating the temperature variation of the charge separation in 2-methyltetrahydrofuran (2-MTHF) and analyzing using the Marcus model allow experimental estimation of the electronic coupling and reorganization energies. At low temperature, relatively strong phosphorescence is observed from the donor-acceptor compounds with onset at 660 nm signaling that charge recombination occurs, at least partially, through the sub-porphyrin localized triplet excited state. Finally, it is noted that charge separation in all SubP-NDI dyads is efficient even at cryogenic temperatures (85 K) in 2-MTHF glass.


13:
This compound was synthesized according to the reported procedure. [S2] 2c. General procedure for the synthesis of SubP-XMe-NDI-derivatives: A 25 ml Schlenk tube was charged with 5-bromo-10,15-diphenylsubporphyrin 13 (10 mg, 0.020 mmol, 1 eq), NDI-Ar-Bpin (8-12) (3 eq), Pd(PPh3)2Cl2 (10 mol%), and Na2CO3 (10 eq) and purged with N2. A 10 mL mixture of THF/H2O (8:2) was added via syringe, and the resulting solution was purged with N2 for 10 min. The reaction mixture was heated at 65 °C for 6 h. After the completion of reaction, the solution was cooling down to room temperature and products were extracted with DCM and the extract was dried over Na2SO4 and concentrated by a rotary evaporator. The resulting crude mixture was subjected to the usual axial exchange conditions (DCM/MeOH 50 °C) [S3] followed by removal of the solvent by a rotary evaporator. The crude mixture was partially purified by column chromatography through a silica gel column (DCM:n-hexane:Et2O 1:2:1). The final purification was carried out through recycle GPC by eluting CHCl3 as a solvent and yielding pure target molecules SubP-XMe-NDI in 50-60% yield.

Quantum mechanical calculations
Density functional calculations (DFT) were performed using the Gaussian 16 and 9 software packages [S9] using the hybrid functional B3LYP and the basis set 6-311G**. Full optimization of the ground-state structures and dihedral scans were performed using the 6-31G** basis set. Excited state energies (20 lowest) were calculated using the time-dependent formulism (TDDFT) and vertical excitation energies for the lowest electronic transitions are given in Table S4 and compared to experiments. There is a consistent shift of 0.3-0.4 eV in all the calculated values compared to experiments which is not surprising since stabilizing solvent effects are neglected in the calculation. Fig. S53 shows the optimized geometry of the SubP-NDI compound and Fig. S54 shows the frontiers orbitals of the SubP-3Me-NDI compound. The potential energy surfaces displayed in Fig. 7 were calculated by scanning the dihedral angles α or β between -90 and 90 in steps of 10 while fully optimizing the remaining internal coordinates. To avoid hysteresis for the highest potential energy barriers, re-optimizations approaching the summit from both sides were performed. Table S4. State energies for SubP in toluene as estimated from spectroscopic and electrochemical [S7-S8] measurements. Experimental results are compared to TDDFT calculated values. S1 / eV S2 / eV T1 / eV CSS a / eV Experiments 2.5 3.3 1.8 2.1 Theory b 2.9 (degenerate) 3.7 (degenerate) 2.1 (degenerate) ---a Energy of the charge separated state is based on the oxidation and reduction potentials in DCM and corrected with the Weller equation (Eq. 1) to toluene conditions. Singlets calculated to be 0.4 eV higher in energy compared to experiments. If the calculated triplet energy is shifted by the same amount, agreement with the observed phosphorescence becomes excellent. b TDDFT/B3LYP/6-311G** calculations on DFT/B3LYP/6-31G** optimized structures.

Fluorescence Lifetime Measurement
Fluorescence lifetime of the SubP donor was determined with time-correlated single photon counting upon 483 nm excitation.  [S5, S6] . It is clearly seen that these radical cation and anion signals rising with time delay that indicates charge separation formation while exciting singlet state of subporphyrin. The transient characteristics are similar for all Sub-X-NDI compounds, but the charge separation rate is different since it depends on electronic coupling of donor (SubP) and acceptor (NDI) units. Further, transient absorption spectra for SubP has only singlet and triplet ESA signals without any radical characteristics (Fig. S60). In Fig. S61, it is shown that the charge separation rate is different for the the different dyads. Forming and decaying of the CSS slows down with increasing dihedral angle between donor and acceptor.

Temperature Dependent Emission Measurements
Emission intensity of the Sub-XMe-NDI compounds increase between 15 to 40 times upon lowering the temperature and phosphorescence is observed for all dyads at 85 K ( Fig. S62-S66). For the SubP (donor alone), temperature dependency of the fluorescence is not pronounced, and very weak relative phosphorescence is also seen at 85 K (Fig. S67).

Phosphorescence Quantum Yield Calculation
For the derivation of equation 7 we combine measurements on the dyads and the SubP donor. All rate constants are defined in connection to Fig. 7 and below.

Formation of triplet in SubP.
The fluorescence quantum yield and lifetime in absence of acceptor are given by 0 = + + 0 = 1 + + where kF, kIC, and kISC are the rate constants for fluorescence (radiative), internal conversion and intersystem crossing, respectively. The quantum yield of phosphorescence is where kP is the radiative rate constant for phosphorescence and k´ISC the rate constant for intersystem crossing back to the ground state. The phosphorescence lifetime for both the SubP donor and the dyads is By combining Eqs. S1, S2 and S3 we get 0 = . 0 . . 0 (S4)

Formation of triplet in the dyads.
The fluorescence quantum yield and lifetime for the dyads are given by = + + + = 1 + + + (S5) where kCS is the rate constant for charge separation. In the dyads the triplet is formed both from the direct intersystem crossing and through the charge separated state and the phosphorescence quantum yield is, thus, given by where kCR1 and kCR2 are the rate constants for recombination through the triplet state and directly to the ground state, respectively (cf. Fig. 7).