Morgenroth, Marius

Measurements of the electronic circular dichroism (CD) are highly sensitive to the absolute configuration and conformation of chiral molecules and supramolecular assemblies and have therefore found widespread application in the chemical and biological sciences. Here, we demonstrate an approach to simultaneously follow changes in the CD and absorption response of photoexcited systems over the ultraviolet−visible spectral range with 100 fs time resolution. We apply the concept to chiral polyfluorene copolymer thin films and track their electronic relaxation in detail. The transient CD signal stems from the supramolecular response of the system and provides information regarding the recovery of the electronic ground state. This allows for a quantification of singlet−singlet annihilation and charge-pair formation processes. Spatial mapping of chiral domains on femtosecond time scales with a resolution of 50 μm and diffraction-limited steady-state imaging of the circular dichroism and the circularly polarised luminescence (CPL) of the films is demonstrated.

In this cumulative doctoral thesis, polymer-based film blends with large circular dichroism (CD) and large circularly polarized luminescence (CPL) are investigated using advanced steady-state and time-resolved optical spectroscopy and microscopy methods to understand their structure and dynamics upon photoexcitation. These systems include polymer thin films consisting of intrinsically chiral polyfluorene derivatives, films with induced circular dichroism consisting of achiral polyfluorene derivatives and helicene-like chiral inducers as well as an organic host material which could be used together with these chiral polymer emitters. First, a comprehensive picture of the photoinduced dynamics of the chiral helicene-like molecule (+)-/(−)-2,2’-dimethoxy-5,5’,6,6’-tetrahydro-1,1’-bidibenzo-[c,h]acridine and its methylene-bridged analogue (+)-/(−)-bis-5,5’,6,6’-tetrahydro-dibenzo[c,h]acridino[1,1’,2,2’-d,f]-[1,3]dioxepine is obtained. After photoexcitation, the depopulation of the S1 state is fast (< 50 ps) for both systems. The quantum yield for internal conversion (IC) is about 80%, resulting in the formation of hot ground state molecules (S0∗). Triplet formation via intersystem crossing (ISC) is responsible for the remaining 20%. The bridged system exhibits a reduced Stokes shift and a slightly slower S1 decay. This effect is likely due to its more rigid structure. A characteristic excited-state absorption band at 575 nm is assigned to the T1 state which decays on a time scale of more than 500 ps. Very efficient chiral induction by these two additives is found in their blends with achiral poly-[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo-[2,1,3]thiadiazol-4,8-diyl)] (F8BT), reaching CD signals with peak signals of up to 9000 mdeg. The less rigid structure of the unbridged derivative might be responsible for the significantly better chiral induction, possibly because of better adaptation to the F8BT scaffold during the formation of the supramolecular structure. Using a newly developed setup for the detection of transient circularly polarized luminescence (TrCPL) with a time resolution of 28 ps, a static mechanism for the generation of the CPL is confirmed. When studying enantiomer mixtures of a given chiral additive as inducing agent for F8BT, it is found that the ”sergeant and soldiers” concept and “majority rules” do not apply, as demonstrated by CD and CPL microscopy images of the different thin film blends. In addition, cholesteric thin films of the intrinsically chiral polymer {9,9-bis[(3S)-3,7-dimethyloctyl]fluorenyl-2,7-diyl}-alt-benzo[2,1,3]thiadiazol-4,8-diyl} (c PFBT) are produced which also reach large CD values of up to 10000 mdeg. These are characterized in the UV−Vis range upon photoexcitation using a newly developed setup for broadband transient circular dichroism (TrCD) spectroscopy. The ultrafast chiral response of the c-PFBT copolymer thin film was of supramolecular origin. TrCD signals only monitor the recovery of the ground state S0 without contributions from excited-state CD signals, which makes their interpretation easier than for the transient absorption (TA) response. The relaxation processes are assigned to mainly diffusive S1−S1 singlet–singlet annihilation (SSA) and a minor channel for Förster resonance energy transfer (FRET). In addition, charge pair formation is observed with a total quantum yield of 15%. The fluorescing polymer films may be applied as materials for OLEDs which emit circularly polarized light, so-called CP-OLEDs. Such devices require host materials for the transfer of electrical charges to the emitter. Understanding the photophysical processes in such a host material, namely 1,3-bis(N-carbazolyl)benzene (mCP), is therefore important. Thus, mCP is investigated in tetrahydrofuran (THF) solution and in poly(methyl methacrylate) (PMMA) film blends at different dopant concentrations. Time-correlated single photon counting (TCSPC) provides a monoexponential decay with lifetimes in the range of 6–7 ns for “isolated” mCP in solution and in the mCP:PMMA thin film with a mass ratio of 2:98. With increasing dopant concentration in the mCP:PMMA blends and also for the neat mCP thin film, the mCP emission is strongly quenched. Neat mCP films and films with high mCP content show three different bands, which are assigned to monomer, aggregate and excimer emission. At higher mCP concentration, an S1−S0 homo-FRET mechanism becomes operative. This way, the excitation energy is transferred via other mCP molecules to an energetically lower trap site, which can be either an mCP aggregate or excimer. For the neat mCP thin film, the average relaxation lifetimes for the monomer, aggregate and excimer bands are 0.92, 3.84 and 7.22 ns respectively. Complementary transient absorption measurements show that at high initial exciton number densities, triplet formation, which has a quantum yield of 20% in THF, is outperformed by SSA.

Morgenroth, Marius

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