Multiphoton spectroscopy of triplet states

Nonlinear absorption refers to the change in transmittance of a material as a function of the intensity of the radiation. Two-photon absorption is one of several multi-photon absorption (MPA) processes showing nonlinear effects in molecules. It is no doubt that the invention of the laser in 1960 was the key to observing such nonlinear optical phenomena, and numerous nonlinear effects have been discovered since.

Multi-photon transitions have several characteristic properties, such as intensity, wavelength and polarization dependence. Organic molecules containing conjugated pi-electron systems with charge asymmetry in a donor-acceptor structure, or in conjugated polymers, have been shown to have large second- and third-order nonlinearities. Like organic molecules, organometallic complexes can possess large NLO responses, due to the large molecular polarizability originating from the strong metal-ligand charge transfer in these molecules. With an even greater flexibility in the design stage and the possibility for variation in metal-ligand environment and geometry, it became possible to tune the NLO responses in organometallic complexes in ways not possible for purely organic or inorganic molecules.

Typical examples of organometallic system we worked with is square planar Pt(II) acetylides, often abbreviated Pt1, shown in Figure 1. A series of more than 20 different variants of the Pt-acetylides were studied in the thesis of Eirik Glimsdal during 2006-2009.

The spectra in Figure 2 show some of the most important results: by employing a flash photolysis system it was possible to determine the kinetics and absorbance properties of the triplet-excited states and assess their importance in applications of optical power limiting (laser protection devices). For more details, it is referred to the thesis of Eirik Glimsdal: [Spectroscopic characterization of some platinum acetylide molecules for optical power limiting applications, NTNU, Thesis 2009:165, ISBN: 978-82-471-1723-1].

Contact: Mikael Lindgren

Figure 1. Examples of compounds studied in the thesis of Dr. Eirik Glimsdal.

Fig. 2. Normalized ground state and triplet state absorption of the T3, T4, T6, T7 and Pt3 chromo-phores in THF solution studied in the thesis of Dr. Eirik Glimsdal.