| People involved: | Ivan Scheblykin |
| Former members: | Daniel Thomsson, Hongzhen Lin, Robbert Bloem, Magdalena Forster, Seyed Tabaei, Ralph Hania, Cecilie Rønne, Oleg Mirzov, Yuxi Tian, Dewu Long |
This project is related to the following Fields, Subjects and Techniques:
| Fields: | Photochemistry and Photophysics |
| Subjects: | Organic Semiconductors |
| Techniques: | Single molecule spectroscopy, Time-resolved single molecule spectroscopy |
It has been realized that nanoscale organization is the key issue for operation of molecular electronic devices based on conjugated polymers (CPs) or so-called organic semiconductors. The conformation of individual polymer chains and their mutual packing can dramatically influence their properties. We use SMS to study energy migration, excited state relaxation and quenching in CPs as well as influence of chain conformations and environment on those processes. We also can measure optical response of individual chains to an external electric field, which mimics a single-molecule optoelectronic device. All these issues are of a great importance for understanding and improving CP functioning in light-emitting diodes, field-effect transistors and solar cells.
The structure of electronic states of conjugated polymers is such that several types of excited species (singlet and triplet excitons) and charges (polarons) can be easily created by visible light excitation. All these species participate in very complicated exciton dynamics, which is intrinsic feature of conjugated polymers. Actually, after photoexcitation a substantial part of energy (more than 50%) disappears via different nonradiative processes. Both radiative and nonradiative processes are strongly dependent on the individual internal organization of a particular polymer chain. That is why studying of conjugated polymers at SM level is very promising.
Probably the most known and fascinating feature of fluorescence of a single chromophore is so-called "blinking" or "on/off behavior" [W.E. Moerner, J. Phys. Chem. B 2002, 106, 910]. A chromophore, being a single quantum system, behaves somewhat "digitally". From time to time it goes to a long living (microseconds and more) non-emissive "dark" state. When in this state the molecule does not fluoresce. Thus, the time dependence of the fluorescence intensity under continuous excitation will look like a telegraph signal having "on" and "off" periods. In the simplest case this "dark state" is just a triplet state with a lifetime of the order of milliseconds.
It was a surprise to observe the blinking effect for polymer chains containing thousands of monomer units [J. Yu, D. Hu, P.F. Barbara, Science 2000, 289, 1327]. Nowadays a generally admitted model of this "collective on/off behavior" exploits an idea of the whole polymer chain working as an antenna for the excitation light, but only a very small part of the chain being responsible for the fluorescence. This part is called emitting site or fluorescent trap. The fluorescence from the emitting site can be quenched by a closely situated quencher, which leads to the blinking effect.
However, it is not known what the quencher is. The duration of dark periods in the fluorescence is ranging from milliseconds to minutes. The quencher can be a triplet exciton, a polaron [I. Scheblykin et al, ChemPhysChem, 2003, 4, 260], or a charged chemical defect. It is also possible that the different types of quenchers are working together at the same or different time scales. We can observe the process of exciton quenching at nanoscale experimentally by monitoring fluorescence intensity fluctuations of a polymer chain.
For a more detailed introduction into single-molecule spectroscopy of conjugated polymers see the Licentiate thesis by O. Mirzov.
Qualitatively the same blinking effect was observed in MEH-PPV at cryogenic temperatures . It showed that the role of energy migration to a single energy trap may be overestimated. A "direct" quenching mechanism was proposed assuming a geometrical size of the coiled polymer chain close to the Forster radius of energy transfer from any excited polymer segment to a quencher (e.g. hole polaron), which can easily be of the order of a few nanometers. Therefore energy migration towards any "special" energy trap is not a necessary requirement in order to have fluorescence blinking.
O.Mirzov, F.Cichos, C. von Borczyskowski and I.G.Scheblykin, "Direct exciton quenching in single molecules of MEH-PPV at 77 K", Chem.Phys.Lett, 2004, 386, 286
O.Mirzov, F.Cichos, C. von Borczyskowski and I.Scheblykin, "Fluorescence blinking in MEH-PPV single molecules at low temperature", J.Lum., 2005, 112, 353
MEH-PPV was deposited onto a surface from a toluene solution and covered with a polymer cap layer of poly (vinyl-alcohol) spin-coated from an aqueous solution for protection against air. Because MEH-PPV is insoluble in water, such sample preparation guarantees that MEH-PPV chains do not mix with the cap polymer. We found that this "host matrix free" environment results in substantially larger fluorescence spectral diffusion than has been observed for conjugated polymer single chains embedded into polymer matrices. The average spectral diffusion range was 500 cm-1 and the maximum registered value reached 1100 cm-1 which is about 6 times larger than the values reported before. We propose that the transition energy shifts are caused by the differences of the London dispersive forces in slightly different polymer chain conformations.
O.Mirzov, T.Pullerits, F.Cichos, C. von Borczyskowski and I.G.Scheblykin, "Large spectral diffusion of conjugated polymer single molecule fluorescence at low temperature", Chem.Phys.Lett, 2005, 408, 317
T.Pullerits, O.Mirzov, and I.G.Scheblykin, "Conformational Fluctuations and Large Fluorescence Spectral Diffusion in Conjugated Polymer Single Chains at Low Temperatures", J.Phys.Chem.B, 2005, 109, 19099
The purpose of this work is to address the issue of applicability of single-molecule spectroscopy (SMS) results for conjugated polymers to bulk samples, e.g. conjugated polymer films. We studied a series of photoluminescence spectra of thin films of conjugated polymer MEH-PPV with wide range of thickness. The thickness was varied from ~20 nm to the value corresponding to well-separated single molecules (SMS sample). The thickness variation resulted in a strong (~2000 cm-1) blue-shift and broadening of the spectrum. This result was reproduced on isolated molecules embedded into PMMA matrix. We performed a comprehensive comparison of presented and elsewhere published spectra of MEH-PPV polymer and oligomers in different samples: films, solutions, isolated-molecule coatings and standard SMS samples. The comparison allows arguing that the main reason behind the blue shift is conformational disorder, which is largely dependent on the sample.
O.Mirzov, and I.G.Scheblykin, "Photoluminescence spectra of a conjugated polymer: from films and solutions to single molecules", Phys.Chem.Chem.Phys., 2006, 8, 5569
See also a project related to this one: Polarisation single molecule spectroscopy of conjugated polymers
In order to understand exciton migration and fluorescence intensity fluctuation mechanisms in conjugated polymer single molecules, we studied fluorescence decay dynamics at "on" and "off" fluorescence intensity levels with 20 ps time resolution using MEH-PPV dispersed in PMMA. Two types of intensity fluctuations were distinguished for single chains of conjugated polymers. Abrupt intensity fluctuations (blinking) were found to be always accompanied by corresponding changes in fluorescence lifetime. On the contrary, during "smooth" intensity fluctuations no lifetime change was observed. Time-resolved data in combination with data on fluorescence emission and excitation anisotropy lead to a picture where a single polymer molecule is seen as consisting of several energy transfer domains. Exciton migration is efficient within a domain and not efficient between domains. Each domain can have several emitting low-energy sites over which the exciton continuously migrates until it decays. Emission of individual domains is often highly polarized. Fluorescence from a domain can be strongly quenched by Forster energy transfer to a quencher (hole polaron) if the domain overlaps with the quenching sphere.
H. Lin, S.R. Tabaei, D. Thomsson, O. Mirzov, P.-O. Larsson and I. G. Scheblykin, "Fluorescence Blinking, Exciton Dynamics and Energy Transfer Domains in Single Conjugated Polymer Chains", J.Am.Chem.Soc.2008, 130, 7042
O. Mirzov, R. Bloem, P. R. Hania, D. Thomsson, H. Lin, and I. G. Scheblykin, "2D polarisation single molecule imaging of multichromophoric systems with energy transfer", Small, 2009, 5, 1877
The paper describes in details the 2D polarization technique (link here) and illustrates it on conjugated polymers single molecules and single aggregates.
H Lin, Y. Tian, K. Zapadka, G. Persson, D. Thomsson, O. Mirzov, P.-O. Larsson, J. Widengren, I. G. Scheblykin, "Fate of excitations in conjugated polymers: single-molecule spectroscopy reveals nonemissive "dark" regions in MEH-PPV individual chains", Nano Letters, 2009, 9, 4456
The most striking experimental finding was that long conjugated polymer chains of PPV family loose their ability to fluorescence when isolated in a host polymer matrix or on a glass surface. Note, that usually the opposite effect happens: when the system gets diluted fluorescence quantum yield goes up! In order to prove this several series of experiments where fluorescence excitation cross-sections of polymers molecules with different molecular weights were compared to a standard dye were carried out. It turned out that emission of a chain of 400 monomer units long is as weak as emission of a 1-2 laser dye molecules. It means that the chain either does not absorb the excitation light anymore or has the fluorescence quantum yield as low as 0.1% (compare to 10% fluorescence yield of pristine conjugated polymer films). This result is very important because it implies that interpretation of all the experimental data published over last 10 years on single molecule spectroscopy of PPV derivatives has to be reconsidered. This especially concerns ideas about "surprisingly efficient energy funnelling" in conjugated polymers, observation of zero-phonon lines and so on.
H.Lin et al Manuscript was submitted to PCCP
We also examined carefully sample preparation conditions. It turned out many well-known publications in literature was dealing with not single chains, but with aggregates of several chains. Basically, much more dilution is needed to get truly isolated chains. This simple effect allowed us to explain existing contradictions in literature concerning the fluorescence properties of isolated chains of PPV derivatives.
Fluorescence intensity of an individual polymer chains is fluctuating (e.g. blinking effect); Its polarization properties like modulation depth and phase are fluctuating too:
M.Forster, D. Thomsson, R.Hania and I.G. Scheblykin, "Redistribution of emitting state population in conjugated polymers probed by single-molecule fluorescence polarization spectroscopy" Phys.Chem.Chem.Phys, 2007, 9, 761
We applied correlation analysis to fluorescence intensity and fluorescence polarization properties of individual chains of two structurally different conjugated polymers.
D. Thomsson, H.Lin, I.G. Scheblykin, "Correlation Analysis of Fluorescence Intensity and Fluorescence Anisotropy Fluctuations in Single-Molecule Spectroscopy of Conjugated Polymers" ChemPhysChem 11 (2010) 897-904; DOI: 10.1002/cphc.200900724 ChemPhysChem, 2010, 11, 897