A4.1: Uncaging strategies based on the VIPER approach
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.