Susanne I. Hintschich

Population and decay of keto states in conjugated polymers

Using time-resolved and steady-state photoluminescence techniques, fluorene/fluorenone copolymers have been studied to investigate the role of keto defects in degraded polyfluorene. Keto sites can be populated via migration from polyfluorene singlets, thereby quenching the polyfluorene fluorescence, and via direct photon absorption. In the former case, the migration process dominates all thermal and interchain variability in the efficiency of quenching. No annihilation process of fluorenone triplets and no interchain processes such as excimer formation participate in the defect emission itself.

Parabolic polarization splitting of Tamm states in a metal-organic microcavity

We observe hybrid states of cavity photons and Tamm plasmons in an organic microcavity with an incorporated thin silver layer of increasing thickness up to 40 nm. Via μ-photoluminescence spectroscopy, we investigate their angular dependence. At oblique angles, we observe a TE-TM polarization splitting of more than 40 meV for each mode. An analytical model is developed to describe the coupling of Tamm plasmons and cavity photons and to account for the splitting of the orthogonally polarized resonances.

Continuously tunable laser emission from a wedge-shaped organic microcavity

We present an organic microcavity laser with wide tunability in the range of 595–650nm, having a threshold as low as 2nJ/pulse. The active medium consists of the organic composite tris(8-hydroxy quinoline) aluminum (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) embedded in between two distributed Bragg reflectors. The inhomogeneously broadened emission from DCM is exploited for lasing by means of a tunable Fabry–Perot-type interferometer. Continuous tuning is achieved by varying the thickness of the active layer. The device shows a high photostability under direct excitation in the absorption band of the guest molecule, thus, allowing to monitor the optical gain spectrum of the active medium.

The Complex Excited-state Behavior of a Polyspirobifluorene Derivative: The Role of Spiroconjugation and Mixed Charge Transfer Character on Excited-state Stabilization and Radiative Lifetime

In this study, we report on the unusual fluorescence decay of an alkoxy-substituted polyspirobifluorene. Excited state behavior has been probed as a function of time, using femtosecond photobleaching, single photon counting, and streak camera techniques. Unusually complex decay kinetics are observed, which strongly depend on solvent viscosity and polarity, featuring decay components in both the tens of picoseconds and in the nanosecond time domain. These findings are explained by the consequences of spiroconjugation in combination with excited-state conformational relaxation. We propose that exciton wave function delocalization into the spiro units effectively traps the exciton, allowing it to relax further into a highly emissive state with a very long lifetime as compared to non-spiroconjugated polymer analogues. Frontier molecular orbitals and exciton orbitals have been calculated using a first principles density functional theory (DFT) approach. These results confirm the importance of spiroconjugation as both the highest occupied molecular orbital (HOMO) and the (lowest) exciton level are not localized on the polymer backbone but strongly extend into the side fluorene groups of the spirobifluorene units. The results of our calculations are very sensitive to the substitution pattern on the spirobifluorene units, in particular when oxygen is included. This finding may lead to new materials of this kind with optimized charge carrier transport properties and high luminescence quantum yields.

Intramolecular fluorescence quenching in luminescent copolymers containing fluorenone and fluorene units: a direct measurement of intrachain exciton hopping rate.

The quenching process of fluorescence emission in polyfluorene (PF) due to the presence of intramolecular 9-fluorenone (9 FL) moieties is studied in dilute toluene solution as a function of 9 FL content in eight copolymers containing both fluorene and fluorenone units (PF/FL(x)). The absorption spectrum of PF/FL(x) copolymers clearly shows a new absorption band, redshifted relatively to the PF and 9-fluorenone absorption, which increases in intensity when the fluorenone fraction increases and also decreases with solvent polarity. Fluorescence emission spectra of PF/FL(x) show that this redshifted and unstructured emission does not coincide with the 9-fluorenone emission and, with increasing solvent polarity, it further redshifts and decreases in intensity. An isoemissive point is clearly observed on the fluorescence emission spectra of PF/FL(x) as a function of fluorenone content, showing that the new emission band is formed at the expense of PF. We propose the formation of an intramolecular charge transfer complex (ICTC) between PF units and 9-fluorenone to explain the appearance of the new emission band. Global analysis of time resolved fluorescence decays collected at 415 nm (PF emission) and 580 nm (the ICTC emission) show that three exponentials are generally needed to achieve excellent fits. Two of the components (420 ps and 6.5 ns) are independent of 9-fluorenone fraction. A further fast component is strongly dependent on fluorenone fraction and ranges between 280 and 70 ps. This component appears as a decay time at 415 nm and as a rise time at 580 nm and is ascribed to the migration of exciton to quenching sites (formation of intramolecular CT complex or exciton ionization at CT complex). A kinetic mechanism involving three different kinetic species, quenched PF units kinetically coupled with the ICTC complex, and unquenched PF units is proposed to explain the experimental data and the quenching rate constant is obtained, k(1) congruent with 10(11) s(-1). This is an experimental measurement of the intrachain exciton hopping rate.

Spectroscopic investigation of the different long-lived photoexcitations in a polythiophene

Using time-resolved photoinduced transient absorption and gated emission techniques long-lived excitations of the solid-state conjugated polymer poly(3-methyl-4-octyl-thiophene) (PMOT) have been detected and analyzed at different temperatures. At 15 K phosphorescence and delayed fluorescence resulting from triplet and geminate pair decay, respectively, are observed. A redshifted emission (with respect to the prompt fluorescence) detected at ambient temperatures is assigned to excimer fluorescence. We also observed excitations with extremely long lifetimes of 300 ms at 289 K that we allocate to triplet excimer formation.

Dispersion tomography of an organic photonic-wire microcavity

We investigate the complex mode structure in microcavities with multidimensional optical confinement. Our active material is composed of the organic blend Alq3:DCM, embedded into a microcavity containing arrays of photonic wires, facilitating strong lateral confinement. We directly record the energy dispersion for one k→ vector component while the second lateral k→ component is scanned. Thereby, we obtain a detailed dispersion tomogram of the cavity resonances, showing excellent agreement with our optical model. We are able to exceed the lasing threshold and observe stimulated emission not only from the bottom of the cavity parabola, but also from higher order modes.

Multimode laser emission from laterally confined organic microcavities

Room-temperature multimode laser emission is observed in a microcavity consisting of dielectric mirrors and small-molecular-weight organic photonic dots as a cavity layer. The structure shows simultaneous lasing of a wide variety of transverse modes. A comparison of the laser operating characteristics with those of unpatterned structures shows an enhancement in the spontaneous emission coupling factor by more than two orders of magnitude due to the lateral confinement. The spectral features are in quantitative agreement with calculations of quantized photonic states in three-dimensional optical cavities.

Pressure dependent radiative quantum yields of the prompt and delayed luminescence of polyfluorene films

Abstract We have measured the prompt and delayed fluorescence and the phosphorescence quantum yields (PLQYs) of thin spin coated films of the conjugated polymer poly(9,9-di(ethylhexyl)fluorene) (PF2/6) at 20 K either in vacuum or under pressure. Only in the latter case, which was achieved by sandwiching the polymer film between two quartz discs, was the observation of delayed luminescence possible. In this case the PLQYs are of the order of 10 −4 . Additionally the prompt fluorescence is enhanced by a factor of 2, more resolved and spectrally red shifted when compared to non-sandwiched films. We attribute our observations to pressure induced trap formation preventing excitons from migration to quenching sites.

Mode discretization in an organic microcavity including a perforated silver layer

Two optical Tamm plasmons and a discretized microcavity state are observed simultaneously in an organic microcavity by angle-resolved photoluminescence spectroscopy. The Tamm plasmons form as a result of a 40 nm silver layer placed between the bottom distributed Bragg reflector and the λ/2 cavity layer. This silver layer is perforated by round holes of a few microns size, generating optical mesas from which discretized microcavity states are observed concurrently. The discretization and the intensity of the different states are studied as a function of angle and hole diameter and compared to analytical calculations.