Wavelength-orthogonal caging groups provide ultimate flexibility for lightcontrol. However, synthesizing orthogonal caging groups is difficult because UV/VIS absorption bands are typically broad and tend to overlapp. In the mid-IR, selective excitation could be realized for a larger number of compounds in parallel, because vibrational bands are narrow compared to the useful spectral range. However, mid-IR excitation usually does not result in photochemistry. To combine mid-IR selectivity with UV/VIS-induced photochemistry, the PhD student will explore our recently introduced VIPER 2D-IR pulse sequence for uncaging, where VIPER stands for vibrationally promoted electronic resonance. In this double resonance scheme, molecules are selectively excited by a narrow picosecond mid-IR pulse and a subsequent femtosecond UV/VIS pulse, that is resonant only with the IR-excited molecules (making use of the fact that vibrational excitation and vibrational energy transfer change the UV/VIS spectrum). IR selection thus determines which molecules undergo photochemistry.
The PhD student will develop orthogonal uncaging strategies based on the VIPER pulse sequence. He or she will work with various caging groups synthesized in project A2.1. The VIPER yield and selectivity using different vibrational modes of the cage will be determined and compared to theoretical predictions from PhD thesis A1.1. The cages will be furthermore investigated by femtosecond 1D and 2D-IR spectroscopies. The experimental results will serve as benchmarks for the development of theoretical methods in PhD thesis A1.1 to model the VIPER excitation, including vibrational energy transfer, and the resulting 2D-IR spectra.
- "Mixed IR/VIS two-dimensional spectroscopy: chemical exchange beyond the vibrational lifetime and sub-ensemble selective photochemistry",
L. J. G. W. van Wilderen, A. T. Messmer, J. Bredenbeck, Angew. Chem. Int. Ed. 2014, 126, 2705–2710.
- "Ultrafast Hopping from Band to Band: Assigning Infrared Spectra Based on Vibrational Energy Transfer",
H. M. Müller-Werkmeister, Y.-L. Li, E.-B. W. Lerch, D. Bigourd, J. Bredenbeck, Angew. Chem. Int. Ed. 2013, 125, 6214–6217.
The VIPER yield and selectivity depend on the extent to which the IR excitation modulates the UV/Vis spectrum of the caging group. While the focus of Daniela Kern-Michler (A4.1) lies on the investigation of different caging groups and on the exploitation of isotope editing, this PhD thesis aims at deepening our understanding of the physics of the VIPER effect in order to optimize it and at developing a dedicated optical detection setup. In order to directly monitor the effect of vibrational excitation on the UV/Vis spectrum, the PhD student will build a femtosecond IR-pump UV/Vis-probe setup. The measurements performed with this setup will provide the optimal UV/Vis wavelength for the VIPER excitation and the optimal timing between IR and UV/Vis pulses for different vibrational modes of a given caging group, which will be synthesized by Matiss Reinfelds (A2.1).
The interpretation of these measurements will take place in close collaboration with theory, which is required to facilitate the interpretation and optimization of the experiments. The time-dependence of the UV/Vis spectra will be important benchmark data for Jan von Cosel (A1.1), who will develop theoretical methods that take into account anharmonicity and vibrational energy transfer, which is expected to play an important role for VIPER.
Further optimization potential that will be explored by the PhD student lies in the relative polarization of the IR and UV/Vis excitation and the energies of these pulses. Together with the influence of different excitation wavelengths of the two pump pulses and their relative timing, this are the parameters that can be optimized for the VIPER pulse sequence. Furthermore, the sample will be optimized by finding the concentration and optical path length which yield the best VIPER signal.
A4.3: Optimizing uncaging groups for VIPER and exploring higher order excitation
In the first term of CLiC Daniela Kern-Michler (A4.1) and Carsten Neumann successfully demonstrated the principle of selecting among different isotopologues of an uncaging group by exploiting differences in their IR spectra, while the UV/VIS spectra where indistinguishable – a new strategy for selective uncaging. In their study of a coumarin-based uncaging group they identified general criteria for a good VIPER uncaging group which will be put to use by the PhD student working on this project for developing the next generation of optimized groups for VIPER uncaging in close collaboration with synthesis (A2.5) and theory (A1.1).
VIPER experiments so far employed one-quantum vibrational excitation and one photon for electronic excitation. The PhD student working on this project will explore the use of two-photon electronic excitation and multi-quantum excitations of vibrations, both of which will allow the use of more powerful, more accessible near IR light sources with high penetration depth and potentially improved selectivity. Understanding the role of vibronic couplings in these higher order experiments requires support from theory (A1.1). Systems optimized for two-photon excitation will be developed in project A2.6.
In the first term of CLiC Carsten Neumann (A4.2) investigated the role of parameters such as pulse energies, timings, wavelengths and polarizations for VIPER 2D-IR spectroscopy and in particular VIPER uncaging. Importantly, he found that our laser setup operates in a range where yield and selectivity still are proportional to and limited by the IR pulse energy. This calls for the development of a new powerful IR light source optimized for VIPER experiments. The PhD student working on this project will explore a new approach for IR light generation which directly utilizes the laser fundamental. This light source will be used for VIPER experiments on the novel systems developed in projects A2.5 and A2.6 of the Heckel group.
Also in the first term, Carsten Neumann developed an IR-pump VIS-probe setup for investigating the effect of IR excitation on the UV/VIS spectrum. The PhD student working on this project will use this setup to investigate vibronic couplings and the role of vibrational energy transfer in uncaging systems of interest in close collaboration with theory (A1.1). Results of this study provide important feedback for the synthesis of the next generation of uncaging groups in A2.5 and A2.6.