An Optical Power Limiting and Ultrafast Photophysics Investigation of a Series of Multi-Branched Heavy Atom Substituted Fluorene Molecules

Hampus Lundén,Jean-Christophe Mulatier,Mikael Lindgren,Didrik K Hopen,Adrien Liotta,Iulia Minda,Delphine Pitrat,Pavel Chábera,Cyrille Monnereau,Cesar Lopes,Tönu Pullerits,Chantal Andraud,Stéphane Parola

doi:10.3390/inorganics7100126

Abstract

A common molecular design paradigm for optical power limiting (OPL) applications is to introduce heavy atoms that promote intersystem crossing and triplet excited states. In order to investigate this effect, three multi-branched fluorene molecules were prepared where the central moiety was either an organic benzene unit, para-dibromobenzene, or a platinum(II)–alkynyl unit. All three molecules showed good nanosecond OPL performance in solution. However, only the dibromobenzene and Pt–alkynyl compounds showed strong microsecond triplet excited state absorption (ESA). To investigate the photophysical cause of the OPL, especially for the fully organic molecule, photokinetic measurements including ultrafast pump–probe spectroscopy were performed. At nanosecond timescales, the ESA of the organic molecule was larger than the two with intersystem crossing (ISC) promoters, explaining its good OPL performance. This points to a design strategy where the singlet-state ESA is balanced with the ISC rate to increase OPL performance at the beginning of a nanosecond pulse.

Highlights

Organic and organometallic dyes with nonlinear optical properties are successfully being used for self-activated optical power limiting (OPL) [1,2,3]
(3–5 their ns), the relatively long lived state was Platinum(II)–alkynyl complexes are pulse known to derive performance in thetriplet red wavelength expected to play an important role in performance, as in similar
THF solutions of approximately 1–5 μM were prepared for luminescence spectra, luminescence lifetimes, and linear absorbance, and microsecond excited state absorption measurements

Summary

Introduction

Organic and organometallic dyes with nonlinear optical properties are successfully being used for self-activated optical power limiting (OPL) [1,2,3]. A good OPL material shows low linear attenuation, but at higher intensities/fluences above a certain “cut-off level”, the optical radiation is efficiently attenuated [1,2,3]. A heavy-metal atom, such as platinum, is often used to facilitate efficient intersystem crossing (ISC) into a highly absorbing and long-lived triplet state [1,2,3]. Examples of heavy-metal atoms used in organometallic dyes include platinum, gold, mercury, ruthenium, and palladium [1,2,3,4,5,6,7,8,9,10]. Among pure organic ISC promoters para-dibromophenyl has been used for singlet O2 generation in applications of photodynamic therapy [11,12]

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