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MO: Fachverband Molekülphysik

MO 1: Photochemistry

MO 1.7: Vortrag

Montag, 6. März 2017, 16:15–16:30, N 6

New Insights into the Photodissociation of Bisphenyl(2,4,6-trimethylbenzoyl)phosphine oxide — •Steffen Straub, Cara Tabea Lohrmann, Jörg Lindner, and Peter Vöhringer — Rheinische Friedrich-Wilhelms-Universität Bonn

Benzoylphosphine oxides are widely known as industrial photoinitiators in photochemical curing processes. In the recent past, their photochemistry and photophysics have been studied, for example by Picosecond Pump Probe Spectroscopy. These efforts suggested a C-P-bond breakage taking place on a time scale of hundred picoseconds and being facilitated by an S₁-T₁ intersystem crossing.

Here, we unravel the primary processes of the photoinitiator Bisphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (BTPO) in great detail, using femtosecond UV-pump IR-probe spectroscopy. All experiments were carried out using pump pulses of 100 fs duration, centered at a wavelength of λ = 380 nm.

We found that the formation of the benzoyl radical occurs in a delayed fashion and cannot be fit to first order growth kinetics, as was assumed previously. A careful analysis of the spectro-temporal evolution in the mid-infrared region yields accurate time constants of 20 ps and 120  ps for the initial S₁-T₁ intersystem crossing and the ensuing α-cleavage with radical pair formation, respectively. Moreover, we were able to identify the C=C-stretching mode of the benzoyl radical in liquid solution, which has been accomplished previously only in the gaseous phase or through complex cryogenic matrix isolation techniques.