Porphyrin Dye Media for Optical Limiting
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Defence and Evaluation Research Agency, St Andrews Rd. Malvern, WR14 3PS, U.K. ** Department of Chemistry, University of Oxford, Dyson Perrins Laboratory, South Park Rd, Oxford, OXI 3QY, U.K. ABSTRACT In this report we study nine structurally similar porphyrins and show how the material parameters that control the optical limiting depend on the molecular structure. Optical limiting measurements at 532 nm on the metal-free, zinc and lead analogues of tetraphenyl porphyrin (TPP), tetra(trimethylsilyethynyl) porphyrin (TTMSAP) and tetra(4-n-butylphenylethynyl) porphyrin (TNBBAP) reveal a strong dependence on the molecular structure. A double pump/cw probe technique is used to measure the material parameters that control the optical limiting and the results give a strong insight into how the molecular structure influences the individual parameters. These measurements revealed that one of the materials TTMSAP(Pb) has a ordear, ratio of -45 at 532 rim, which is one of the largest values ever reported. Finally, broadband measurements with a picosecond white-light source show that the excited state absorption profile for the porphyrins examined is much broader than the ground state features and is centred to the long wavelength side of the ground state Soret band. INTRODUCTION Nature uses porphyrins in two processes that are crucial to life on the earth. Magnesium porphyrins give plants their green colour and play the light gathering role (photons to electrons) in photosynthesis. Iron porphyrins transport and store oxygen in the body and give blood its red colour. Hence porphyrins are sometimes referred to as the "colours of life [1]". Man is beginning to realise the potential of porphyrins and a prime example is their application to photo-dynamic therapy. In this case, it is the biological compatibility and the ability to form singlet oxygen and kill cancer cells that is exploited. In optical limiting the strong excited state absorption and long excited state lifetime coupled with good chemical and photo-stability and enormous design potential mean that porphyrins are an important class of materials. Optical limiting was first reported in porphyrin dye media in 1985 by Blau et al [2]. Laser pulses of 80 ps duration at 532 nim were used to show that the excited state absorption cross-
section, o-, is larger than the ground state absorption cross-section, metal-free
(H 2 )
-g,, in zinc, cobalt and
tetra-phenyl porphyrins (TPP). Beddard et al [3] studied a wider range of tetra-
phenyl porphyrins and noted that porphyrins with closed-shell metal atoms have relatively long excited state lifetimes. Porphyrins with open shell metal atoms generally have short excited state
lifetimes due to rapid charge transfer from the excited porphyrin to the metal atom. This preliminary work was not followed up immediately but over the last 8 years progress has been
rapid. This was stimulated by the development of a series of tetra-benzo porphyrins (TBPs) [4]
and the demonstration that zinc meso-tetra (p-methoxyphenyl) TBP has a much l
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