| People involved: | Ivan Scheblykin |
| Former members: | Daniel Thomsson, Dewu Long, Yuxi Tian |
This project is related to the following Fields, Subjects and Techniques:
| Fields: | Photochemistry and Photophysics |
| Subjects: | |
| Techniques: | Single molecule spectroscopy |
The main advantage of single molecule spectroscopy (SMS) is that it eliminates ensemble averaging. Having this in mind, studying of antennas (large macromolecules and ensembles of molecules) by SMS methods does not look promising. It seems that due to averaging over the ensemble all individual properties will vanish and inherent single-quantum system effects will disappear. However, the difference between a multi-chromophoric system in general and an antenna is that in the latter all chromophores are "connected" with each other via energy transfer. An antenna provides collective response to excitation and the chromophores in the antenna cannot be considered as independent.
An ideal antenna would be an ensemble of coherently coupled chromophores. Such an antenna behaves like a single quantum system because the excitation (Frenkel exciton) is delocalized over the all chromophores. In reality, the exciton wavefunction is still localized on a limited number of chromophores (often called coherent length), which can be as large as several tens of dye molecules for some specific J-aggregates at low temperature (see below). An exciton, delocalised over several chromophores can migrate over the antenna via coherent and incoherent (hopping) energy transfer over a distance up to several tens of nanometers. It is the exciton migration distance that determines the "useful" antenna size.
Because the chromophores "talk to each other" antennas can still reveal individual properties. Indeed, light absorption, excited state delocalization and energy transfer efficiency depend on interaction between individual chromophores, which in its turn depends on their mutual arrangement and the antenna conformation as a whole. Despite of a large physical size of an antenna there may be just a few places where charge transfer states (or other dark long-living states) can be formed. Therefore, the exciton quenching phenomenon becomes very sensitive to the organization /conformation of the whole system again. Classical phenomena inherent to single quantum systems like blinking and spectral diffusion have been already reported for several multi-chromophoric antenna systems. All these justify that SMS is as a very promising technique to apply to study exciton dynamics, energy migration and organization of light-harvesting antennas. Moreover, geometrical parameters of individual antennas such as number of chromophores, chromophore orientation and packing and the shape of the whole antenna become accessible when SMS approach is applied. (2D-polarization single molecules imaging)
Since 1936 molecular J-aggregates have attracted substantial attention in scientific community due to their unique optical properties and their application in photography as sensibilizators. These systems can be imagined as 1,2 or 3 dimensional nano-ensembles (with a high degree of order) of organic dye molecules (Fig.3). It is remarkable that such well-ordered systems are built by molecular self-assembling. J-aggregates have been used for years in photographic materials where they absorb light and then transfer the excited state towards surfaces of silver halide crystals where electrons transfer reaction occurs and a "hidden image" forms.
Due to the resonance Coulombic interaction between closely packed dye molecules collective excitations (Frenkel excitons) delocalised over tens of molecules are formed. Coherent coupling between chromophores leads to increasing of the extinction coefficient and the radiative rate constant (so called super radiance), absorption spectrum shift and line narrowing both in absorption and fluorescence. As it has been demonstrated for some J-aggregates in solutions excitons can migrate by hopping and coherent energy transfer over as many as 104 - 106 dye molecules. Such a system of coherently coupled chromophores can work as extremely efficient light-harvesting antenna and energy transporting "wire" (or photonic wire, as it is sometimes called) to be used for solar cells and other applications. Hence, single J-aggregates are very interesting objects to study individually. However, only very recently J-aggregates, which could be immobilized on a surface at very low concentration (requirement for applying SMS techniques) without loosing of their properties, became available. We studied J-aggregates of PDI dyes developed in the group of Prof. Frank Wuerthner (Wuerzburg, Germany). (see the figure below).
The presence of disorder leads to localization of the collective excited state (exciton) on just several monomers. The number of monomers carrying the exciton wavefunction is often called coherent length. It is quite difficult to know what the coherent length is. For different types of J-aggregates the coherent length varies from 3-4 in disordered aggregates at room temperature to several tens of monomers for well-packed aggregates like e.g. PIC aggregates, THIATS aggregates at low temperature. There have been many theoretical works to calculate the influence of disorder, photons, temperature and aggregate dimension on spectral and energy transport properties (see e.g. publications of Prof. Jasper Knoester and co-workers). From the experimental point of view a particular informative way to study those effects is to look at temperature dependence of the optical properties.
See for example Scheblykin et al "Excitons in molecular aggregates of 3,3'-bis-[3-sulfopropyl]-5,5'-dichloro-9 ethylthiacarbocyanine (THIATS): Temperature dependent properties" J.Phys.Chem.B, 2001, 20, 4636 and references there in.
Recently we studied temperature dependence of absorption and fluorescence spectra of PDI J-aggregates and found many interesting analogies between PDI and THIATS J-aggregates despite of much larger level of disorder in the former system. We also observed a mysterious scaling phenomenon, which has not been explained so far. Our paper become the cover article for J.Phys.Chem.B:
Kaiser et al. "Temperature-Dependent Exciton Dynamics in J-Aggregates-When Disorder Plays a Role" J.Phys.Chem.B, 2009, 113, 15836
So-called collective blinking effect.... For many years since the original work of Paul Barbara it was believed that fluorescence blinking of MEH-PPV single conjugated polymer chains is due to collective quenching of hundreds of chromophores at the same time. However, we have proved that this is not the case [Lin et al, Nano Letters, 2009, 9, 4456]. It turned out that the blinking of MEH-PPV single chains corresponds to switching on/off 1-2 chromophores only.
So, does the collective blinking of hundreds of chromophores exists at al?
The answer is YES! MEH-PPV is just not the right system to look for this effect!
In this paper we reported fluorescence blinking corresponding to collective quenching of up to 100 dye monomers is reported for individual J-aggregates of a perylene bisimide (PBI) dye. It is the first time when collective quenching of so many chromophores is proved in a direct exoeriment. This implies an exciton diffusion length up to 70 nm in these one-dimensional assemblies. The number of quenched monomers was directly measured by comparing the fluorescence brightness of the J-aggregates with that of noncoupled PBI molecules. This brightness analysis technique is useful for unraveling photophysical parameters of any individual fluorescent nanosystem.
