Stereochemical Effects on Platinum Acetylide Two-Photon Chromophores

Abstract

A series of cis-platinum(II) acetylide complexes containing two-photon-absorbing chromophores have been synthesized and characterized to explore the effects of stereochemistry on the nonlinear absorption properties. The molecules feature 4-(phenylethynyl)phenylethynylene (PE2), diphenylaminofluorene (DPAF), and benzothiazolylfluorene (BTF) ligands. The photophysical properties were investigated under one-and two-photon conditions and compared to the known trans analogues via UV−visible absorption, photoluminescence, femtosecond and nanosecond transient absorption (TA), nanosecond z-scan, and femtosecond two-photon absorption (2PA). The bent cis complexes exhibit blue shifts in the absorption, emission, femtosecond, and nanosecond TA spectra along with lower molar extinction coefficients and lower phosphorescence yields relative to the trans complexes suggesting less efficient Pt-induced spin−orbit coupling and intersystem crossing in the cis configuration. The cis chromophores are noncentrosymmetric and therefore show dipolar behavior with a pronounced 2PA in the 0−0 transition of the S 0 → S 1 band, while the trans complexes show quadrupolar behavior with a forbidden 0−0 transition. In the S 0 → S n region, both cis and trans complexes show intense two-photon-absorption bands (up to 3700 GM by the peak cross section for cis-BTF) which contain a significant contribution from the excited state absorption (S 1 → S n ). All six complexes exhibit comparable nonlinear absorption response with a significant contribution from triplet−triplet absorption that slightly favors trans complexes but is more strongly dependent upon the structure of the π-conjugated chromophore.

trans-N-(Heterocyclic Carbene) Platinum(II) Acetylide Chromophores as Phosphors for OLED Applications

“…Since the first report of a convenient synthetic route, − platinum acetylide − molecules, oligomers, and polymers have been considered as model organometallic complexes in photophysics − for fundamental investigation of triplet excited states in π-conjugated materials. In addition, studies have explored potential applications of platinum acetylides in phosphorescent organic-light-emitting diodes (OLED) − and solar cells − and as nonlinear absorption chromophores. − Platinum acetylide materials are air stable, have good thermal stability, and are soluble in common organic solvents, properties which render them processable from solution and by vacuum thermal deposition. One reason for the interest in platinum acetylide materials is fundamental investigation and application of triplet states in organic electronics. ,, Excitation of these complexes is accompanied by rapid singlet → triplet intersystem crossing that affords triplet excitons in high yield. , In some cases, the polymers and complexes exhibit moderate to high room temperature phosphorescence yields.…”

Polymeric Nonlinear Absorption Chromophore Array from Controlled Radical Polymerization and “Click” Chemistry

“…An excitation wavelength of 600 nm was selected due to the lack of appreciable ground-state absorption (Figure ) coupled with the fact that the triplet excited state absorbs appreciably at this wavelength (Figure ). In addition, previous experiments have shown that the FB and Pt-substituted FB chromophores exhibit pronounced two-photon absorption in the 600–800 nm region. , …”

“…In addition, previous experiments have shown that the FB and Ptsubstituted FB chromophores exhibit pronounced two-photon absorption in the 600−800 nm region. 32,38 The z-scan experiment provides qualitative insight into the nonlinear response of a sample by probing the transmittance of optical pulses through the sample at different power density (power per unit area). 61−63 The power density is varied by translating the sample through a focused beam.…”

“…Due to the dual-mode TPA/ESA mechanism (Figure a), several families of organometallic chromophores exhibit efficient nonlinear absorption, including metalloporphyrins and platinum­(II) and iridium­(III) complexes. − Among these, platinum acetylides have attracted particular interest. ,,,− The NLA platinum acetylides have the structure of trans -(R 3 P) 2 Pt­(−CC–Ar) 2 , in which R 3 P is a trialkylphosphine and Ar is a π-conjugated ligand containing a two-photon absorption moiety, such as benzothiazole-2,7-fluorene (FB) and diphenylamino-2,7-fluorene (DPAF). ,,− Platinum acetylide complexes display pronounced nonlinear absorption for optical pulse widths ranging from femtoseconds to nanoseconds due to a combination of TPA by the π-conjugated ligands, combined with ESA by the long-lived triplet excited state that is populated efficiently due to the spin–orbit coupling provided by the heavy metal center. , …”

“…10,32,35−37 Platinum acetylide complexes display pronounced nonlinear absorption for optical pulse widths ranging from femtoseconds to nanoseconds due to a combination of TPA by the πconjugated ligands, combined with ESA by the long-lived triplet excited state that is populated efficiently due to the spin−orbit coupling provided by the heavy metal center. 10,38 For many applications of NLA materials as optical power limiters (Figure 1b), 39,40 these materials need to be incorporated into nonconjugated organic polymer matrices, either by physical mixing 41 or by covalent bonding between the polymers and chromophores. 42 Solid-state materials of this type were previously developed with platinum acetylides as guest and poly(methyl methacrylate) (PMMA) matrix as host.…”