Symmetric Versus Unsymmetric Platinum(II) Bis(aryleneethynylene)s with Distinct Electronic Structures for Optical Power Limiting/Optical Transparency Trade-off Optimization

A new series of symmetric and unsymmetric Pt(II) bis(acetylide) complexes of the type D-C C-Pt(PBu3)(2)-C C-D (D-Pt-D), A-C C-Pt(PBu3)(2)-C C-A (A-Pt-A) and D-C C-Pt(PBu3)(2)-C C-A (D-R-A) (D, donor groups; A, acceptor groups) are synthesized, and show superior optical power limiting (OPL)/optical transparency trade-offs. By tailoring the electronic properties of the aryleneethynylene group, distinct electronic structures for these metalated complexes can be obtained, which significantly affect their photophysical behavior and OPL properties for a nanosecond laser pulse at 532 nm. Electronic influence of the ligand type and the molecular symmetry of metal group on the optical transparency/nonlinearity optimization is thoroughly elucidated. Generally, aryleneethynylene ligands with pi electron-accepting nature will effectively enhance the harvesting efficiency of the triplet excited states. The ligand variation to the OPL strength of these Pt(II) compounds follows the order: D-Pt-D > D-Pt-A > A-Pt-A. These results could be attributed to the distinctive excited state character induced by their different electronic structures, on the basis of the data from both photophysical studies and theoretical calculations. All of the complexes show very good optical transparencies in the visible-light region and exhibit excellent OPL responses with very impressive figure of merit sigma(ex)/sigma(o) values (up to 17), which remarkably outweigh those of state-of-the-art reverse saturable absorption dyes such as C-60 and metallophthalocyanines with very poor transparencies. Their lower optical-limiting thresholds (0.05 J cm(-2) at 92% linear transmittance) compared with that of the best materials (ca. 0.07 J cm(-2) for InPc and PbPc dyes) currently in use will render these highly transparent materials promising candidates for practical OPL devices for the protection of human eyes and other delicate electro-optic sensors.