On-off switch of charge-separated states of pyridine-vinylene-linked porphyrin-C₆₀ conjugates detected by EPR

Sabrina V Kirner¹, Danny Arteaga², Christian Henkel¹, Johannes T Margraf^{1

3}, Nuria Alegret⁴, Kei Ohkubo^{5

6}, Braulio Insuasty², Alejandro Ortiz², Nazario Martín⁷, Luis Echegoyen⁸, Shunichi Fukuzumi^{5

6

9}, Timothy Clark³, Dirk M Guldi¹

Affiliations

¹ Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials , Friedrich-Alexander-Universität Erlangen-Nürnberg , Egerlandstrasse 3 , 91058 Erlangen , Germany.
² Departamento de Química , Facultad de Ciencias Naturales , Universidad del Valle , A.A. 25360 Cali , Colombia.
³ Department of Chemistry and Pharmacy , Computer Chemistry Center , Friedrich-Alexander-University Erlangen-Nürnberg , Nägelsbachstr. 25 , 91052 Erlangen , Germany.
⁴ Departament de Química Física i Inorgànica , Universitat Rovira i Virgili , 43007 , Tarragona , Spain.
⁵ Department of Material and Life Science , Graduate School of Engineering , Osaka University , ALCA and SENTAN , Japan Science and Technology Agency (JST) , Suita , Osaka 565-0871 , Japan.
⁶ Department of Bioinspired Science , Ewha Womans University , Seoul 120-750 , Korea.
⁷ Departamento de Química Orgánica , Facultad de Química , Universidad Complutense 28040 , Madrid , Spain.
⁸ Department of Chemistry , University of Texas at El Paso , El Paso , Texas 79968-0519 , USA.
⁹ Faculty of Science and Technology , Meijo University and ALCA and SENTAN , Japan Science and Technology Agency (JST) , Tempaku , Nagoya , Aichi 468-8502 , Japan.

PMID: 29449913
PMCID: PMC5669314
DOI: 10.1039/c5sc02051d

On-off switch of charge-separated states of pyridine-vinylene-linked porphyrin-C₆₀ conjugates detected by EPR

Sabrina V Kirner et al. Chem Sci. 2015.

. 2015 Oct 1;6(10):5994-6007.

doi: 10.1039/c5sc02051d. Epub 2015 Jul 9.

Authors

Affiliations

¹ Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials , Friedrich-Alexander-Universität Erlangen-Nürnberg , Egerlandstrasse 3 , 91058 Erlangen , Germany.
² Departamento de Química , Facultad de Ciencias Naturales , Universidad del Valle , A.A. 25360 Cali , Colombia.
³ Department of Chemistry and Pharmacy , Computer Chemistry Center , Friedrich-Alexander-University Erlangen-Nürnberg , Nägelsbachstr. 25 , 91052 Erlangen , Germany.
⁴ Departament de Química Física i Inorgànica , Universitat Rovira i Virgili , 43007 , Tarragona , Spain.
⁵ Department of Material and Life Science , Graduate School of Engineering , Osaka University , ALCA and SENTAN , Japan Science and Technology Agency (JST) , Suita , Osaka 565-0871 , Japan.
⁶ Department of Bioinspired Science , Ewha Womans University , Seoul 120-750 , Korea.
⁷ Departamento de Química Orgánica , Facultad de Química , Universidad Complutense 28040 , Madrid , Spain.
⁸ Department of Chemistry , University of Texas at El Paso , El Paso , Texas 79968-0519 , USA.
⁹ Faculty of Science and Technology , Meijo University and ALCA and SENTAN , Japan Science and Technology Agency (JST) , Tempaku , Nagoya , Aichi 468-8502 , Japan.

PMID: 29449913
PMCID: PMC5669314
DOI: 10.1039/c5sc02051d

Abstract

The design, synthesis, and electronic properties of a new series of D-π-A conjugates consisting of free base (H₂P) and zinc porphyrins (ZnP) as electron donors and a fullerene (C₆₀) as electron acceptor, in which the two electroactive entities are covalently linked through pyridine-vinylene spacers of different lengths, are described. Electronic interactions in the ground state were characterized by electrochemical and absorption measurements, which were further supported with theoretical calculations. Most importantly, charge-transfer bands were observed in the absorption spectra, indicating a strong push-pull behavior. In the excited states, electronic interactions were detected by selective photoexcitation under steady-state conditions, by time-resolved fluorescence investigations, and by pump probe experiments on the femto-, pico-, and nanosecond time scales. Porphyrin fluorescence is quenched for the different D-π-A conjugates, from which we conclude that the deactivation mechanisms of the excited singlet states are based on photoinduced energy- and/or electron transfer processes between H₂P/ZnP and C₆₀, mediated through the molecular spacers. The fluorescence intensity decreases and the fluorescence lifetimes shorten as the spacer length decreases and as the spacer substitution changes. With the help of transient absorption spectroscopy, the formation of charge-separated states involving oxidized H₂P/ZnP and reduced C₆₀ was confirmed. Lifetimes of the corresponding charge-separated states, which ranged from ∼400 picoseconds to 165 nanoseconds, depend on the spacer length, the spacer substitution, and the solvent polarity. Interestingly, D-π-A conjugates containing the longest linkers did not necessarily exhibit the longest charge-separated state lifetimes. The distances between the electron donors and the acceptors were calculated by molecular modelling. The longest charge-separated state lifetime corresponded to the D-π-A conjugate with the longest electron donor-acceptor distance. Likewise, EPR measurements in frozen media revealed charge separated states in all the D-π-A conjugates investigated. A sharp peak with g values ∼2.000 was assigned to reduced C₆₀, while a broader, less intense signal (g ∼ 2.003) was assigned to oxidized H₂P/ZnP. On-off switching of the formation and decay of the charge-separated states was detected by EPR at 77 K by repeatedly turning the irradiation source on and off.

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Figures

Chart 1. Above: Porphyrin–fullerene conjugates linked by pyridine-vinylene units at *meta*- and *para*-positions **15–20a** (H₂P-*n-m*C₆₀, ZnP-*n-m*C₆₀, n = 1–3) and **15–20b** (H₂P-*n-p*C₆₀, and ZnP-*n-p*C₆₀, n = 1–3). Below: Corresponding porphyrin–pyridine-vinylene references **9–14a** (H₂P-*n-m*CHO, ZnP-*n-m*CHO, n = 1–3) and **9–14b** (H₂P-*n-p*CHO, ZnP-*n-p*CHO, n = 1–3).

Scheme 1. Synthetic route for the preparation of pyridine-vinylene linkers: top **2–6a** and bottom **2–6b**. Reagents and conditions: (a) Pd(PPh₃)₄, tributyl(vinyl)tin, toluene, reflux, 20–22 h, yields 82–86%. (b) 2,6-Dibromopyridine 3, Pd(OAc)₂, Bu₄NBr, K₂CO₃, DMF, reflux, 24 h, yields 65–68%. (c) Pd(PPh₃)₄, tributyl(vinyl)tin, toluene, reflux, 20 h yields 75–80% (d) 2,6-dibromopyridine 3, Pd(OAc)₂, Bu₄NBr, K₂CO₃, DMF, reflux, 24 h, yields 62–70%.

Scheme 2. Synthesis of new porphyrin–fullerene conjugates: top **15–20a** (H₂P-*n-m*C₆₀, ZnP-*n-m*C₆₀, n = 1-3); bottom **15–20b** (H₂P-*n-p*C₆₀, ZnP-*n-p*C₆₀, n = 1–3). Reagents and conditions: (a) C₆₀, N-octylglycine, toluene, reflux, 4–5 h, yields (**15a**, 32%); (**16a**, 35%); (**17a**, 30%); (**18a**, 38%); (**19a**, 34%); (**20a**, 36%); (b) C₆₀, N-octylglycine, toluene, reflux, 4–5 h, yields (**15b**, 31%); (**16b**, 38%); (**17b**, 37%); (**18b**, 34%); (**19b**, 45%); (**20b**, 42%).

Fig. 1. Left: CVs for porphyrin–fullerene conjugates **15–17a** (H₂P-*n-m*C₆₀, n = 1–3) and **15–17b** (H₂P-*n-m*C₆₀, n = 1–3) in DCM solutions (0.1 M TBAPF₆) at room temperature. Right: CVs for porphyrin–fullerene conjugates **18–20a** (ZnP-*n-p*C₆₀, n = 1–3) and **18–20b** (ZnP-*n-p*C₆₀, n = 1–3) in DCM solutions (0.1 M TBAPF₆) at room temperature. Oxidative scans between 0 and 1.25 V; reductive scans between 0 and –2.10 V.

**Fig. 2. Absorption spectra of (**18b**) ZnP-1-pC₆₀ (cyan); (**19b**) ZnP-2-pC₆₀ (blue); (**20b**) ZnP-3-pC₆₀ (navy) and ZnTPP (pink) as the reference in THF at room temperature.**

Fig. 3. Above: Fluorescence spectra of (**15a**) H₂P-1-mC₆₀ (cyan); (**16a**) H₂P-2-mC₆₀ (blue); (**17a**) H₂P-3-mC₆₀ (navy) and H₂TPP (pink) (excitation at 420 nm; OD = 0.04) in THF at room temperature. Below: Fluorescence spectra of (**18a**) ZnP-1-mC₆₀ (cyan); (**19a**) ZnP-2-mC₆₀ (blue); (**20a**) ZnP-3-mC₆₀ (navy) and ZnTPP (pink) (excitation at 420 nm; OD = 0.05) in THF at room temperature.

Fig. 4. Above: Differential absorption spectra (visible and near-infrared) observed upon femtosecond flash photolysis (420 nm, 150 nJ) of 9a (H₂P-1-mCHO) in THF with time delays between 0 ps (black) and 7.5 ns (wine) at room temperature. Below: Time-absorption profile of the spectra above at 1070 nm, monitoring the deactivation of the porphyrin singlet excited state.

Fig. 5. Above: Differential absorption spectra (visible and near-infrared) observed upon femtosecond flash photolysis (387 nm, 200 nJ) of **16a** (H₂P-2-mC₆₀) in THF with time delays between 0 ps (black) and 7.5 ns (wine) at room temperature. Below: Time-absorption profiles of the 1010 nm decay for (**15a**) H₂P-1-mC₆₀ (cyan); (**16a**) H₂P-2-mC₆₀ (blue) and (**17a**) H₂P-3-mC₆₀ (navy) upon femtosecond flash photolysis (387 nm, 200 nJ) in THF at room temperature, monitoring the charge recombination.

Fig. 6. Above: Differential absorption spectra (visible) observed upon nanosecond flash photolysis (355 nm, 10 mJ) of **16a** (H₂P-2-mC₆₀) in THF with time delays between 150 ns (purple) and 2.0 μs (wine) at room temperature under aerobic conditions. Below: Differential absorption spectra (near-infrared) observed upon nanosecond flash photolysis (355 nm, 10 mJ) of **16a** (H₂P-2-mC₆₀) in THF with time delays between 60 ns (purple) and 2.0 μs (wine) at room temperature under aerobic conditions.

Fig. 7. Above: Time-absorption profile of the 1010 nm decay – Fig. 6 – of **16a** (H₂P-2-mC₆₀) upon nanosecond flash photolysis (355 nm, 10 mJ) in THF under aerobic conditions, monitoring the charge recombination process. Below: Time-absorption profile of the 1010 nm decay of **16b** (H₂P-2-pC₆₀) upon nanosecond flash photolysis (355 nm, 10 mJ) in THF under aerobic conditions, monitoring the charge recombination.

Fig. 8. Above: Differential absorption spectra (visible and near-infrared) observed upon femtosecond flash photolysis (387 nm, 1 μJ) of **16a** (H₂P-2-mC₆₀) in THF with time delays between 0 ns (black) and 1.0 μs (wine) at room temperature under aerobic conditions. Below: Time-absorption profile of the spectra above at 1010 nm, monitoring the charge recombination.

Fig. 9. Above: Differential absorption spectra (visible and near-infrared) observed upon femtosecond flash photolysis (387 nm, 200 nJ) of **19a** (ZnP-2-mC₆₀) in THF with time delays between 0 ps (black) and 7.5 ns (wine) at room temperature. Below: Time-absorption profiles of the 1010 nm decay for (**18a**) ZnP-1-mC₆₀ (cyan); (**19a**) ZnP-2-mC₆₀ (blue) and (**20a**) ZnP-3-mC₆₀ (navy) upon femtosecond flash photolysis (387 nm, 200 nJ) in THF at room temperature, monitoring the charge recombination.

**Fig. 10. EPR signals observed under photoirradiation of **15a** (H₂P-1-mC₆₀) in benzonitrile at 77 K.**

**Fig. 11. On–off switch of the EPR signal due to charge separation of **17a** (top) and **20a** (bottom) in PhCN at 77 K by turning on and off the irradiation from a high-pressure mercury lamp.**

**Fig. 12. Lowest-energy conformations of (**17a**) H₂P-3-mC₆₀ (left) and (**17b**) H₂P-3-pC₆₀ (right).**

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On-off switch of charge-separated states of pyridine-vinylene-linked porphyrin-C₆₀ conjugates detected by EPR

Affiliations

On-off switch of charge-separated states of pyridine-vinylene-linked porphyrin-C₆₀ conjugates detected by EPR

Authors

Affiliations

Abstract

Figures

References

LinkOut - more resources

Research Materials