Ultra-High to Ultra-Low Drug-Loaded Micelles: Probing Host-Guest Interactions by Fluorescence Spectroscopy.

Polymer micelles are an attractive means to solubilize water insoluble compounds such as drugs. Drug loading, formulations stability and control over drug release are crucial factors for drug-loaded polymer micelles. The interactions between the polymeric host and the guest molecules are considered critical to control these factors but typically barely understood. Here, we compare two isomeric polymer micelles, one of which enables ultra-high curcumin loading exceeding 50 wt.%, while the other allows a drug loading of only 25 wt.%. In the low capacity micelles, steady-state fluorescence revealed a very unusual feature of curcumin fluorescence, a high energy emission at 510 nm. Time-resolved fluorescence upconversion showed that the fluorescence life time of the corresponding species is too short in the high-capacity micelles, preventing an observable emission in steady-state. Therefore, contrary to common perception, stronger interactions between host and guest can be detrimental to the drug loading in polymer micelles.

fs-ps Exciton dynamics in a stretched tetraphenylsquaraine polymer.

A tetraphenylsquaraine was synthesized whose structure was elucidated by single crystal X-ray structure analysis. Unlike all known indolenine squaraines, the tetraphenylsquaraine shows an unusual nonplanar structure with the four phenyl groups pointing away from the squaric acid core in order to avoid steric congestion. This tetraphenylsquaraine was polymerized by a Yamamoto coupling to form a conjugated polymer with Xn = 38. The absorption spectra of this polymer are red-shifted compared to that of the monomer and show a J-type absorption band due to exciton coupling. Transient absorption spectra with fs-time resolution display a strong ground state bleaching signal with a peak on the red side rising concurrently with the decay of a peak on the blue side of an isosbestic point at 12 000 cm-1. This behavior is caused by energy transfer between two slightly different sections of the polymer with time constants of 0.3 and 2.6 ps. According to semiempirical calculations these different sections are stretched and slightly bent conformations of the polymer strand. Power dependent transient absorption measurements indicate exciton annihilation which also proves the excitons to be very mobile.

Photophysical and quantum chemical study on a J-aggregate forming perylene bisimide monomer.

Perylene bisimides (PBIs) are excellent dyes and versatile building blocks for supramolecular structures. Only recently have PBIs been shown to depict absorption characteristics of J-aggregates. We apply electronic structure calculations and femtosecond pump-probe spectroscopy to the monomeric, bay-substituted building-block of a PBI aggregate in dichloromethane to investigate its electronically excited states in order to provide the ingredients for the description of excitons in the aggregates and their annihilation processes. The PBI S(1)←S(0) absorption spectrum and the S(1)→S(0) emission spectrum have been assigned based on time-dependent Density Functional Theory calculations for the geometry-optimized electronic ground state and excited state structures in the gas phase. The monomeric absorption spectrum contains a strong transition at 580 nm and a broad shoulder between 575-500 nm, both features are attributed to a vibrational progression with an effective vibrational mode of 1415 cm(-1) whose major contributing vibrational normal modes are breathing modes of the perylene body. The effective vibrational mode of the emission spectrum is characterized by a frequency of 1369 cm(-1), whose major contributing vibrational normal modes are characterized by perylene and phenol (bay-substituent) CH bendings. The S(n)←S(1) excited state absorption spectrum is assigned based on Multi-Reference Configuration Interaction methodology. Here, we identify three transitions which give rise to two broad experimental features, one being located between 500 and 600 nm and the other one ranging from 650 to 750 nm.

One-dimensional exciton diffusion in perylene bisimide aggregates.

The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton-exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Förster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm(2)/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. The findings indicate that J-aggregates of perylene bisimides are promising building blocks to facilitate directed energy transport in optoelectronic organic devices or artificial light-harvesting systems.