Excited State Dynamics of Conjugated Metalloporphyrins. Laura Michelle Cleveland. Department of Chemistry Duke University.

Transcription

1 Excited State Dynamics of Conjugated Metalloporphyrins by Laura Michelle Cleveland Department of Chemistry Duke University Date: Approved: Michael J. Therien, Supervisor Warren S. Warren David Beratan Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the Department of Chemistry in the Graduate School of Duke University 2010

2 ABSTRACT Excited State Dynamics of Conjugated Metalloporphyrins by Laura Michelle Cleveland Department of Chemistry Duke University Date: Approved: Michael J. Therien, Supervisor Warren S. Warren David Beratan An abstract of a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the Department of Chemistry in the Graduate School of Duke University 2010

3 Copyright by Laura Michelle Cleveland 2010

4 Abstract The goal of this review is to highlight the unique and interesting aspects of conjugated metalloporphyrins. I will show the fundamental importance of these conjugated porphyrin systems by reviewing the excited state dynamics of some of the key systems that have been studied to date. The spectroscopic aspects I will be focusing on are the electronic absorption and emission and the transient absorption and emission. It is necessary to compare the ground and excited states in order to understand the significance of the excited state dynamics. Also important to discuss are the EPR and electrochemical data to further elucidate the unique qualities of conjugated porphryin oligomers. iv

5 Contents Abstract...iv List of Tables...vi List of Figures...vii 1. Introduction Types of Porphyrin Bridges Unique Qualities of Conjugated Porphyrin Oligomers Conclusion References.17 v

6 List of Tables Table 1: Fluorescence Quantum Yields of DA and DD series...12 vi

7 List of Figures Figure 1: Electronic absorption and fluorescence emission of butadiyne-linked porphyrins... 2 Figure 2: Diphenylethyne linked porphyrin dimer and pentamer arrays... 3 Figure 3: Structures of directly linked porphyrin and porphyrin tapes... 4 Figure 4: UV-Vis-IR absorption of (a) monomer 2 and directly meso-meso linked porphyrins S2-12 and (b) porphyrin tapes T Figure 5: Electronic Relaxation Dynamics of meso-meso linked porphyrin S2 and porphyrin tape T Figure 6: quinoidal spacers, supermolecular chromophores featuring spacers, and reference ethyne bridged porphyrins... 6 Figure 7: Electronic Absorption spectra of PZn-Sp-PZn compounds... 7 Figure 8: Structures of DD and DA series chromophores... 9 Figure 9: Potentiometric data showing (a) DD series redox potentials and (b) DA series redox potentials. Open circles are reduction potentials and filled circles are oxidation potentials Figure 10: Normalized electronic absorption and fluorescence spectra of DD and DA series chromophores Figure 11: Transient absorption of (left) DD at (a) 673nm and (b) 711nm and (right) DA at (a) 685nm and (b) 720nm Figure 12: NIR transient absorption of (a) DD series and (b) DA series Figure 13: Transient absorption for (a) MPZn, (b) MPZnA, and (c) MPZnM' Figure 14: Electronic absorption spectra of PZn, PPd, and PPt dimer and trimer Figure 15: Transient absorption spectra of PZn, PPd, PPt dimers (left), and trimers (right) vii

8 1. Introduction One of the basic principles that fuels scientific discovery is curiosity of nature s most amazing feats. How does nature make use of energy and how can humans copy that process? Photosynthesis is an extremely efficient process that plants use to convert light into energy. By trying to duplicate and improve this light-harvesting feature, we can learn how to more efficiently use energy across the solar spectrum and apply it to new types of electronic devices 1,2,3,4,5,6,7,8,9. The idea of implementing porphyrin-based materials in electronic devices is not a new concept 10. Research on conjugated porphyrin oligomers has grown immensely in the last fifteen years. Due to several unique spectral properties of these types of molecules, conjugated porphyrin systems draw much attention to the structure-property relationships and how to optimize this aspect LeCours, S. M.; Guan, H.-W.; DiMagno, S. G.; Wang, C. H.; Therien, M. J. J. Am. Chem. Soc. 1996, 118, Priyadarshy, S.; Therien, M. J.; Beratan, D. N. J. Am. Chem. Soc. 1996, 118, Karki, L.; Vance, F. W.; Hupp, J. T.; LeCours, S. M.; Therien, M. J. J. Am. Chem. Soc. 1998, 120, Anderson, H. L.; Martin, S. J.; Bradley, D. D. C. Angew. Chem., Int. Ed. Engl. 1994, 33, Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D. J. Am. Chem. Soc. 1996, 118, O Keefe, G. E.; Denton, G. J.; Harvey, E. J.; Phillips, R. T.; Friend, R. H.; Anderson, H. L. J. Chem. Phys. 1996, 104, Beljonne, D.; O Keefe, G. E.; Hamer, P. J.; Friend, R. H.; Anderson, H. L.; Bre das, J. L. J. Chem. Phys. 1997, 106, O Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L., III; Wasielewski, M. R. Science 1992, 257, Debreczeny, M. P.; Svec, W. A.; Wasielewski, M. R. Science 1996, 274, Wagner, R. W.; Lindsey, J. S. Journal of the American Chemical Society 1994, 116, Anderson, H. L. Chemical Communications 1999,

9 2. Types of Porphyrin Bridges Porphyrins have been linked together in several different ways. The bridge type and attachment position actually make significant differences in the photophysics of the porphyrin system. In the 1990 s, Therien reported ethynyl and butadiynyl linked porphyrins. Therien showed that the meso-to-meso linked porphyrin dimers have the strongest electronic coupling, followed by meso-to-β and β-to-β, as evidenced by the large excitonic splitting and red-shifted Q-band in the electronic absorption spectra 1,2. Figure 1: Electronic absorption and fluorescence emission of butadiyne-linked porphyrins. Other porphyrin bridges that have been explored over the years include phenylene bridges, directly linked porphyrin tapes, and more recently, quinoidal spacers. Lindsey has reported results on diphenylethyne linked porphyrins. However, spectral data reveal that electronic communication between porphyrins via the diphenylethyne bridge is weak in the excited and ground states. Despite this weak communication, there is evidence that the ethyne component of the bridge enables 1 Lin, V. S. Y.; Dimagno, S. G.; Therien, M. J. Science 1994, 264, Lin, V. S. Y.; Therien, M. J. Chemistry-a European Journal 1995, 1,

10 conformational changes that improves the conjugation between the porphyrin and diphenylethyne group in the excited state 3. Figure 2: Diphenylethyne linked porphyrin dimer and pentamer arrays. Kim and Osuka have studied directly linked porphyrin arrays, including some known as porphyrin tapes, in which the porphyrins are directly linked by three single bonds across the meso-meso, and the two B-B positions. This type of structure exhibits a dramatic red-shift dependent on oligomer length that extends well into the infrared region of the absorption spectra. Internal conversion is thought to be the main pathway 3 Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey, J. S.; Bocian, D. F. Journal of the American Chemical Society 1994, 116,

11 by which the lowest excited states are formed. These porphyrin tapes are more suited to molecular electric wires as opposed to molecular photonic wires due to the reduced band gap between the HOMO and LUMO 4. Figure 3: Structures of directly linked porphyrin and porphyrin tapes 4 Cho, H. S.; Jeong, D. H.; Cho, S.; Kim, D.; Matsuzaki, Y.; Tanaka, K.; Tsuda, A.; Osuka, A. Ibid.2002, 124,

12 Figure 4: UV-Vis-IR absorption of (a) monomer 2 and directly meso-meso linked porphyrins S2-12 and (b) porphyrin tapes T2-12 Figure 5: Electronic Relaxation Dynamics of meso-meso linked porphyrin S2 and porphyrin tape T2 5

13 Benzothiadiazole and pentacene spacers have been shown to enhance electronic communication between (porphinato)zinc(ii) units (compared to butadiyne linked units). This property is a result of the spacer s ability to regulate frontier orbital energy levels and consequently intensifies the contribution of the quinoidal resonance to the ground and excited states. This class of spacers appears to present many qualities that exceed those of butadiyne linked porphyrins, including a further red-shifted Q-state, smaller HOMO-LUMO gap, and broad transient absorption in the near infrared, all of which indicate that these types of porphyrins have great potential in electronic devices 5. Figure 6: quinoidal spacers, supermolecular chromophores featuring spacers, and reference ethyne bridged porphyrins 5 Susumu, K.; Duncan, T. V.; Therien, M. J. Ibid.2005, 127,

14 Figure 7: Electronic Absorption spectra of PZn-Sp-PZn compounds 7

15 3. Unique Qualities of Conjugated Porphyrin Oligomers There are several qualities that conjugated porphyrin oligomers possess that make them unique and interesting systems to study. Among these qualities are localized triplet states, delocalized ground states, conformational differences in the ground and excited states, efficient donor-bridge-acceptor dynamics, and small band gaps. Most of this information has been determined by examining the excited state dynamics of different conjugated porphyrin systems. In 1995, Angiolillo et. al. determined via EPR spectroscopy that the triplet excited state of ethyne and butadiyne bridged (porphinato)zinc arrays is localized to a single chromophore; whereas, the singlet excited states are delocalized 1. Later in 2000, Shediac et. al. presented EPR and photophysical data elucidating a spin transition in the lowest photoexcited state. It is this spin transition that allows the meso-meso ethyne bridged porphyrins to be better electronically coupled between chromophores than the meso-β or β-β linked porphyrins. The reorientation causes a change in the dipolar axis from perpendicular to the porphyrin plane to parallel with the ethyne bridge across the meso carbons. This property is unique to the ethyne bridged porphyrin systems 2. Studies of the excited state dynamics of β-β and meso-meso linked porphyrins shows that singlet excited state of the β-β linked dimers are weakly coupled and localized. This is mostly due to the large dihedral angle between porphyrin units, which causes a large excitonic splitting in the B-band. In the meso-meso linked system electronic communication between chromophores is enhanced due to the minimal barrier of rotation about the ethyne bridge. This means that the meso-meso ethyne 1 Angiolillo, P. J.; Lin, V. S. Y.; Vanderkooi, J. M.; Therien, M. J. Ibid.1995, 117, Shediac, R.; Gray, M. H. B.; Uyeda, H. T.; Johnson, R. C.; Hupp, J. T.; Angiolillo, P. J.; Therien, M. J. Ibid.2000, 122,

16 bridged porphyrins exist in conformations with maximal conjugation, as evidence by the large red shift in the Q-band 3. In a series of ethyne bridged (porphinato)zinc(ii) compounds (dimer, trimer, and pentamer with DD, DA, DDD, DAD, DDDDD, and DADAD structures in Fig. 8), information about the band gap patterns was determined. Low band gap materials are desirable for many optoelectronic device-type applications. Potentiometric data (Fig. 9) indicate that with increasing oligomer length in the DD species, the radical anion is stabilized and the radical cation is destabilized, meaning that larger oligomers are easier to both oxidize and reduce because the positive and negative charges are distributed. However, in the DA compounds, the anion and cation stabilization and destablilization is independent of oligomer length. The D and A components also have differences in orbital energy levels that contribute to a globally delocalized singlet state 4. Figure 8: Structures of DD and DA series chromophores 3 Kumble, R.; Palese, S.; Lin, V. S. Y.; Therien, M. J.; Hochstrasser, R. M. Ibid.1998, 120, Susumu, K.; Therien, M. J. Ibid.2002, 124,

17 Figure 9: Potentiometric data showing (a) DD series redox potentials and (b) DA series redox potentials. Open circles are reduction potentials and filled circles are oxidation potentials. The excited state dynamics of bis- [(5,5ʹ -10,20-bis[3,5-bis(3,3-dimethyl-1- butyloxy)phenyl]porphinato)zinc(ii)]ethyne (DD) and [(5,-10,20-bis- [3,5-bis(3,3- dimethyl-1-butyloxy)phenyl]porphinato)zinc(ii)]-[(5ʹ,-15ʹ -ethynyl-10ʹ,20ʹ - bis(heptafluoropropyl)- porphinato)zinc(ii)]ethyne (DA) were studied in The most important spectral property gleaned from this data was the intense near infrared (NIR) absorption observed for the S 1 S n transition. This is significant because this type of absorption had not been seen before from multiporphyrin compounds. Other interesting conclusions from this study were the conformational heterogeneity of DA and DD species that stems from the dihedral angle distribution between chromophores and leads to enhanced conjugation 5. 5 Rubtsov, I. V.; Susumu, K.; Rubtsov, G. I.; Therien, M. J. Ibid.2003, 125,

18 Figure 10: Normalized electronic absorption and fluorescence spectra of DD and DA series chromophores. Figure 11: Transient absorption of (left) DD at (a) 673nm and (b) 711nm and (right) DA at (a) 685nm and (b) 720nm In continuing the study of these DA and DD type PZn compounds, another important property was discovered. These chromophores display remarkably high NIR quantum yields along with a low band gap and intense, tunable absorption in the excited state. Also important to note is that the fluorescence quantum yields are 11

19 dependent on the increasing maximum wavelength of the S 1 S 0 transition. This effect is caused by the decreased triplet state yield with increased oligomer length 6. Figure 12: NIR transient absorption of (a) DD series and (b) DA series Table 1: Fluorescence Quantum Yields of DD and DA series φ f DD 0.16 DA DDD 0.22 DAD 0.17 DDDDD 0.14 DADAD 0.19 Another class of supermolecular ethyne bridged porphyrins were studied in 2004, exhibiting strong T 1 T n NIR absorbance, high oscillator strengths, and other interesting excited state dynamics. This set of molecules include (polypyridyl)metal- (porphinato)zinc(ii) compounds (MPZn or MPZnM or MPZnM, where M or M can be 6 Duncan, T. V.; Susumu, K.; Sinks, L. E.; Therien, M. J. Ibid.2006, 128,

20 Osmium or Ruthenium). The lifetimes of these compounds are also very long compared to (porphinato)zinc(ii) precursor 7. Figure 13: Transient absorption for (a) MPZn, (b) MPZnA, and (c) MPZnM' Further studies of this class of chromophores show that adjusting the molecular framework slightly can drastically affect the excited triplet state absorption maxima while maintaining large absorption bandwidth. Also highlighted is that the excited state lifetime can be modified across four orders of magnitude (from ns regime to µs regime) 8. As discussed previously, (Porphinato)zinc(II) units were linked using various proquinoidal spacers (PZn-Sp-PZn structures). The main goal of adding these types of spacers to the PZn units was to decrease further the HOMO-LUMO band gap. This class of supermolecular chromophores displays broad NIR S 1 S n absorbances and increased electronic communication between PZn units. The wide range of tunability of the absorbance reveals the role of the quinoidal resonance in contributing to the electronically excited singlet states 9. Lastly, a class of (porphinato)palladium(ii) and (porphinato)platinum(ii) chromophores has been explored. These compounds exhibit intense, broad NIR excited 7 Duncan, T. V.; Rubtsov, I. V.; Uyeda, H. T.; Therien, M. J. Ibid.2004, 126, Duncan, T. V.; Ishizuka, T.; Therien, M. J. Ibid.2007, 129, Susumu, K.; Duncan, T. V.; Therien, M. J. Ibid.2005, 127,

21 state absorption similar to the MPZn compounds; however, these heavier metal porphyrin systems can produce excited triplet states at unit quantum yield due to fast singlet to triplet intersystem crossing. Also, these systems show that the excited state absorption maximum can be varied, while sustaining microsecond triplet state lifetimes. The purely π-π* characteristic of the palladium and platinum porphyrin systems also is important in non-linear optical material applications 10. Figure 14: Electronic absorption spectra of PZn, PPd, and PPt dimer and trimer. 10 Duncan, T. V.; Frail, P. R.; Miloradovic, I. R.; Therien, M. J. The Journal of Physical Chemistry B, null-null. 14

22 Figure 15: Transient absorption spectra of PZn, PPd, PPt dimers (left), and trimers (right). 15

23 4. Conclusion As this review article has shown, conjugated metalloporphyrins have several unique properties that make them interesting to investigate. The structural-property relationships of these molecules yields vast amounts of information in the spectra. Studying the excited state dynamics of conjugated porphyrin materials is paramount to understanding how these molecules utilize energy and how we can apply these features to new devices. 16

24 References Anderson, H. L. Chemical Communications 1999, Anderson, H. L.; Martin, S. J.; Bradley, D. D. C. Angew. Chem., Int. Ed. Engl. 1994, 33, Angiolillo, P. J.; Lin, V. S. Y.; Vanderkooi, J. M.; Therien, M. J. Journal of the American Chemical Society 1995, 117, Beljonne, D.; O Keefe, G. E.; Hamer, P. J.; Friend, R. H.; Anderson, H. L.; Bre das, J. L. J. Chem. Phys. 1997, 106, Cho, H. S.; Jeong, D. H.; Cho, S.; Kim, D.; Matsuzaki, Y.; Tanaka, K.; Tsuda, A.; Osuka, A. Journal of the American Chemical Society 2002, 124, Debreczeny, M. P.; Svec, W. A.; Wasielewski, M. R. Science 1996, 274, Duncan, T. V.; Frail, P. R.; Miloradovic, I. R.; Therien, M. J. The Journal of Physical Chemistry B, null-null. Duncan, T. V.; Ishizuka, T.; Therien, M. J. Journal of the American Chemical Society 2007, 129, Duncan, T. V.; Rubtsov, I. V.; Uyeda, H. T.; Therien, M. J. Journal of the American Chemical Society 2004, 126, Duncan, T. V.; Susumu, K.; Sinks, L. E.; Therien, M. J. Journal of the American Chemical Society 2006, 128, Karki, L.; Vance, F. W.; Hupp, J. T.; LeCours, S. M.; Therien, M. J. J. Am. Chem. Soc. 1998, 120, Kumble, R.; Palese, S.; Lin, V. S. Y.; Therien, M. J.; Hochstrasser, R. M. Journal of the American Chemical Society 1998, 120, LeCours, S. M.; Guan, H.-W.; DiMagno, S. G.; Wang, C. H.; Therien, M. J. J. Am. Chem. Soc. 1996, 118, Lin, V. S. Y.; Dimagno, S. G.; Therien, M. J. Science 1994, 264, Lin, V. S. Y.; Therien, M. J. Chemistry-a European Journal 1995, 1, O Keefe, G. E.; Denton, G. J.; Harvey, E. J.; Phillips, R. T.; Friend, R. H.; Anderson, H. L. J. Chem. Phys. 1996, 104, O Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L., III; Wasielewski, M. R. Science 1992, 257, Priyadarshy, S.; Therien, M. J.; Beratan, D. N. J. Am. Chem. Soc. 1996, 118,

25 Rubtsov, I. V.; Susumu, K.; Rubtsov, G. I.; Therien, M. J. Journal of the American Chemical Society 2003, 125, Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey, J. S.; Bocian, D. F. Journal of the American Chemical Society 1994, 116, Shediac, R.; Gray, M. H. B.; Uyeda, H. T.; Johnson, R. C.; Hupp, J. T.; Angiolillo, P. J.; Therien, M. J. Journal of the American Chemical Society 2000, 122, Susumu, K.; Duncan, T. V.; Therien, M. J. Journal of the American Chemical Society 2005, 127, Susumu, K.; Therien, M. J. Journal of the American Chemical Society 2002, 124, Wagner, R. W.; Lindsey, J. S. Journal of the American Chemical Society 1994, 116, Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D. J. Am. Chem. Soc. 1996, 118,