Ensemble

In this part of our research, the sample of interest does not consist of just one single molecule, but of rather many (in most cases very many). Knowing the properties of the ensemble helps to understand the properties at the single molecule level and vice versa. In the lab, in most cases „ensemble“ means a solution of the sample of interest in a specific solvent. In other cases, specially prepared surfaces are investigated.

Spectroscopy

The general method of (optical) spectroscopy uses the interaction with light (photons) to initiate processes in a sample, which are then monitored in order to gain information. In most cases, again photons are used as a carrier of this information (exception: Photoacoustic spectroscopy). The wavelength distribution of the intensities (spectrum) is recorded as the most obvious data set, however also the polarization state of the electric fields involved and the changes thereof can yield important information. On top, other measurement conditions can be varied systematically. Among others, the temperature, solvent, and concentration of the samples are all used as critical parameters.

When light (photons) interacts with sample matter (electrons), fundamentally three different processes are possible: it can (a) be transmitted, (b) reflected or (c) absorbed. While (a) and (b) leave the sample (largely) unchanged, case (c) will yield an excited state (S¹) and a changed ground state S⁰. The former makes up the starting point of many different possible pathways; most of these are investigated in great detail in the lab and listed below.

Steady State Spectroscopy

The S¹ state has a limited lifetime; in many cases, the electron will return to the ground state by emitting a fluorescence photon. Thus, this state cannot be investigated with constant light illumination. Doing such an experiment will only yield properties of the changes in the ground state (absorption) and the (steady state) fluorescence. Both these types of measurements are routinely and extensively performed in the lab.

Excited State Spectroscopy

In order to investigate excited states in molecules, it is necessary to employ pulsed light sources, which allow for interactions with the sample only at specific times and short durations. As the processes mentioned above go on in very different time domains, they all ask for specific measuring equipment including appropriate pulsed laser and detections systems.

In the lab, times in the range from nano- to microseconds can be investigated using a flash photolysis apparatus built around a Nd:YAG-laser pumped dye laser.

For the time range from ca. 30 ps to ca. 10 ns, two independent single photon timing laser systems are available. Although the time resolution attainable is similar in both, the experiments are slightly different with respect to the usable wavelength range. Both systems measure the time resolved fluorescence from the samples of interest.

Furthermore, for even shorter times, a dual-OPA regeneratively amplified femtosecond laser system is in use. Depending on the specific type of experiments, the time range which can be covered by this equipment stretches from ca. 100 fs to ca. 2 ns.

With this system, various types of experiments are performed, which employ different detection schemes (e.g. Upconversion, polychromatic transient absorption, …). Of course, very often it is useful to perform measurements using various of these schemes after one another to investigate and clarify fast kinetic processes in the samples of interest.

Excited State kinetics

As mentioned above, the state generated by the optical excitation is the starting point for many possible pathways. The simplest case is, of course, the return of the electron to the ground state by emission of a fluorescence photon. This process is observed in many samples; however others are – dependent on the sample – seen on top.

Under suitable conditions, in some samples the energy put into the S¹ state can be transferred to other parts of the molecule(s). Depending on the conditions, this transfer can appear as dissipative (energy hopping) or directional, both types are investigated extensively in the lab in various different samples.

Under again different specific conditions in a sample, a transfer of an electron from one part within a molecule (donor) to a different part (acceptor) can be initiated. Also this type of kinetics is studied at large within the lab.