Record Multiphoton Absorption Cross‐Sections by Dendrimer Organometalation

Abstract

Large increases in molecular two-photon absorption, the onset of measurable molecular three-photon absorption, and record molecular four-photon absorption in organic π-delocalizable frameworks are achieved by incorporation of bis(diphosphine)ruthenium units with alkynyl linkages. The resultant ruthenium alkynyl-containing dendrimers exhibit strong multiphoton absorption activity through the biological and telecommunications windows in the near-infrared region. The ligated ruthenium units significantly enhance solubility and introduce fully reversible redox switchability to the optical properties. Increasing the ruthenium content leads to substantial increases in multiphoton absorption properties without any loss of optical transparency. This significant improvement in multiphoton absorption performance by incorporation of the organometallic units into the organic π-framework is maintained when the relevant parameters are scaled by molecular weights or number of delocalizable π-electrons. The four-photon absorption cross-section of the most metal-rich dendrimer is an order of magnitude greater than the previous record value.

Stellar Multi‐Photon Absorption Materials: Beyond the Telecommunication Wavelength Band

“…The molecular weight‐scaled data confirm that the 2PE‐containing (PE=phenyleneethynylene) arms have generated the most MPA‐efficient molecules when also considering synthetic expense. Indeed, 2‐Cl exhibits superior σ 2 /M values and similar σ 3 /M values to recently reported triruthenium and nonaruthenium dendrimers . This suggests a synthetically more straightforward “stellar” approach toward MPA‐efficient materials has advantages over dendritic approaches.…”

“…For a summary of earlier 3PA and 4PA data, see Ref. . In contrast to these precedents, 2‐C 2 C 6 H 4 NO 2 exhibits a maximal value at much longer wavelengths, and maintains measurable activity at 2000 nm and beyond.…”

“…There have been many reports of 3PA processes resulting from 2PA followed by excited‐state absorption (ESA), but far fewer describing instantaneous 3PA employing low repetition‐rate femtosecond pulses . Of these, very few have undertaken nonlinear‐absorption spectral‐dependence studies, although such studies are needed to identify the specific wavelengths in the telecommunications band at which the instantaneous 3PA coefficient is maximized ,–,,,. From the limited studies thus far, fluorene‐based monomers and oligomers have been shown to be particularly efficient three‐photon absorbers,,, exhibiting three‐photon‐excited fluorescence (3PEF)‐derived 3PA cross‐sections as high as 4.6×10 −76 cm 6 s 2 at 1330 nm .…”

“…From the limited studies thus far, fluorene‐based monomers and oligomers have been shown to be particularly efficient three‐photon absorbers,,, exhibiting three‐photon‐excited fluorescence (3PEF)‐derived 3PA cross‐sections as high as 4.6×10 −76 cm 6 s 2 at 1330 nm . Porphyrin‐phthalocyanine compounds exhibit effective 3PA cross‐sections as high as 7.6×10 −76 cm 6 s 2 at 1020 nm, metal alkynyl complexes (particularly dendrimers) exhibit instantaneous 3PA cross‐sections as high as 1.0×10 −76 cm 6 s 2 at 1200 nm,–, and oligothiophenes with ligated ruthenium termini exhibit 3PA cross‐sections as high as 4.4×10 −78 cm 6 s 2 at 1450 nm . Despite these successes, broadly applicable design rules for efficient 3PA materials are lacking, and materials efficient at the longer telecommunications wavelengths and beyond are scarce.…”

Long‐Range Corrected DFT Calculations of First Hyperpolarizabilities and Excitation Energies of Metal Alkynyl Complexes

“…In particular, coordination and organometallic complexes have attracted significant attention because they expand the possibilities for tuning NLO properties by varying the metal center (and its oxidation state and co‐ligands) when compared to organic chromophores . Metallocenyl and metal alkynyl complexes are the two most scrutinized classes of NLO‐active organometallic complexes, with the latter being found to exhibit record values of nonlinear optical coefficients …”