To explore the photophysics of platinum acetylide chromophores with strong two-photon absorption cross-sections, we have investigated the synthesis and spectroscopic characterization of a series of platinum acetylide complexes that feature highly pi-conjugated ligands substituted with pi-donor or -acceptor moieties. The molecules (numbered 1-4) considered in the present work are analogs of bis(phenylethynyl)bis(tributylphosphine)platinum(II) complexes. Molecule 1 carries two alkynyl-benzothiazolylfluorene ligands, and molecule 2 has two alkynyl-diphenylaminofluorene ligands bound to the central platinum atom. Compounds 3 and 4 possess two dihexylaminophenyl substituents at their ends and differ by the number of platinum atoms in the oligomer "core" (one vs two in 3 and 4, respectively). The ligands have strong effective two-photon absorption cross-sections, while the heavy metal platinum centers give rise to efficient intersystem crossing to long-lived triplet states. Ultrafast transient absorption and emission spectra demonstrate that one-photon excitation of the chromophores produces an S1 state delocalized across the two conjugated ligands, with weak (excitonic) coupling through the platinum centers. Intersystem crossing occurs rapidly (Kisc approximately 1011 s-1) to produce the T1 state, which is possibly localized on a single conjugated fluorenyl ligand. The triplet state is strongly absorbing (epsilonTT > 5 x 104 M-1 cm-1), and it is very long-lived (tau > 100 micro s). Femtosecond pulses were used to characterize the two-photon absorption properties of the complexes, and all of the chromophores are relatively efficient two-photon absorbers in the visible and near-infrared region of the spectrum (600-800 nm). The complexes exhibit maximum two-photon absorption at a shorter wavelength than 2lambda for the one-photon band, consistent with the dominant two-photon transition arising from a two-photon-allowed gerade-gerade transition. Nanosecond transient absorption experiments carried out on several of the complexes with excitation at 803 nm confirm that the long-lived triplet state can be produced efficiently via a sequence involving two-photon excitation to produce S1, followed by intersystem crossing to produce T1.
Sterically hindered fullerenyl chromophore dyad and triads, C 60 (>DPAF-C 9 ) x (x ) 1 and 2, respectively), in an acceptor-donor (A-D) molecular linkage of C 60 -(keto-fluorene) x were synthesized and fully characterized. Attachment of two 3,5,5-trimethylhexyl groups on C 9 of the fluorene ring moiety greatly improves their solubility and makes direct intermolecular aromatic stacking contacts more difficult. They are the first series of fullerene derivatives showing high three-photon absorptivity (3PA). Accordingly, C 60 (>DPAF-C 9 ) 2 exhibits 2PA and 3PA cross sections in the values of 0.824 × 10 -48 cm 4 s (or 82.4 GM) and 6.30 × 10 -25 cm 6 /GW 2 , respectively, in femtosecond region among the highest ones reported for many diphenylaminofluorene-derived AFX chromophores. Utilization of a keto linker located immediately between C 60 cage and fluorene chromophore moieties facilitates molecular polarization of the DPAF ring toward the C 60 cage. That may serve as the fundamental cause for correlation of enhanced A-D electron interactions to, ultimately, observed multiphoton absorption cross sections. By using nanosecond laser flash photolysis results taken at 355 nm as the reference, transient absorption data obtained from femtosecond pump-probe experiments at 800 nm unambiguously verified the occurrence of two-photon excitation processes of C 60 (>DPAF-C 9 ) in air-saturated benzene and subsequent efficient energy transfer from the two-photon pumped DPAF-C 9 moiety to the C 60 cage moiety.
In this work we detail the photophysical properties of a series of butadiynes having the formula H-(C6H4-C[triple bond]C)n-(C[triple bond]C-C6H4)n-H, n=1-3 and ligands H-(C6H4-C[triple bond]C)n-H, n=1-3 and compare these to previous work done on a complimentary series of platinum-containing complexes having the formula trans-Pt[(PC4H9)3]2[(C[triple bond]-C6H4)n-H]2, n=1-3. We are interested in understanding the role of the platinum in the photophysical properties. We found that there is conjugation through the platinum in the singlet states, but the triplet states show more complex behavior. The T1 exciton, having metal-to-ligand charge-transfer character, is most likely confined to one ligand but the Tn exciton appears to have ligand-to-metal charge-transfer character. The platinum effect was largest when n=1. When n=2-3, the S0-S1,S1-S0,T1-S0, and T1-Tn spectral properties of the platinum complex are less influenced by the metal, becoming equivalent to those of the corresponding butadiynes. When n=1, platinum decreases the triplet state lifetime, but its effect diminishes as n increases to 2.
[structure: see text] A vinylene-linked porphyrin dimer, with no substituents at the beta-positions, has been synthesized by CuI/CsF promoted Stille coupling. In the crystal structure of this dimer, the C(2)H(2) bridge is twisted by 45 degrees relative to the plane of the porphyrins. The absorption, emission spectra, and electrochemistry reveal substantial porphyrin-porphyrin pi-conjugation. The triplet excited-state absorption spectrum of this dimer makes it suitable for reverse saturable absorption at 710-900 nm.
In this work, we describe the spectroscopic properties of a series of platinum complexes containing one acetylide ligand per platinum, having the chemical formula trans-Pt(PBu(3))(2)((C[triple bond]CC(6)H(4))(n)()-H)Cl, n = 1-3 (designated as half-PEn-Pt) and compare their spectroscopic behavior with the well-characterized series trans-Pt(PBu(3))(2)((C[triple bond]CC(6)H(4))(n)-H)(2), n = 1-3 (designated as PEn-Pt). This comparison aims to determine if the triplet state of PEn-Pt is confined to one ligand or delocalized across the central platinum atom. We measured ground-state absorption spectra, fluorescence spectra, phosphorescence spectra, and triplet-state absorption spectra. The ground-state absorption spectra and fluorescence spectra both showed a blue shift when comparing half-PEn-Pt with PEn-Pt, showing the S(1) state is delocalized across the platinum. In contrast, the phosphorescence spectra of the two types of compounds had the same 0-0 band energy, showing the T(1) state was confined to one ligand in PEn-Pt. The triplet state absorption spectra blue shifted when comparing half-PEn-Pt with PEn-Pt, showing the T(n) state was delocalized across the central platinum. This comparison supports recently published work that suggested this confinement effect (Rogers, J. E et al. J. Chem. Phys. 2005, 122, 214701).
Utilization of two-photon (TP) excited fluorescence resonance energy transfer (FRET) within a light-harvesting dendrimer has proven to be a reliable method for the enhancement of the effective TP absorption efficiency of many FRET acceptor molecules. This light-harvesting approach has enabled TP photosensitization of singlet oxygen from a porphyrin in both aqueous and organic media using wavelengths more transmissive to human body tissue (750−1000 nm). This ability to utilize near-infrared irradiation to induce photochemical reactions is especially attractive for applications including in vivo photochemistry, oxygen sensing, and photodynamic cancer therapy. In efforts to further the applicability of this concept, we synthesized an array of novel photosensitizers by metalation of the porphyrin core with aluminum, silver, and zinc. Time-resolved fluorescence, transient absorption measurements, and TP excitation experiments demonstrated efficient TP excited FRET to produce the porphyrin excited state triplet which subsequently generates singlet oxygen by the aluminum and zinc metalated species. Singlet oxygen photosensitization efficiency was found to be most efficient using aluminum followed by zinc and least efficient using silver. In fact, silver metalated photosensitizers were found to be nonfluorescent and incapable of generating a measurable amount of singlet oxygen. With the proper choice of inserted metals, it was possible to tune the efficiency of TP induced singlet oxygen production.
We study instantaneous two-photon absorption (2PA) in a series of nominally quasi-centrosymmetric trans-bis(tributylphosphine)-bis-(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl) ethynyl)-R)-platinum complexes, where 11 different substituents, R = N(phenyl)(NPh), NH, OCH, t-butyl, CH, H, F, CF, CN, benzothiazole, and NO, represent a range of electron-donating (ED) and electron-withdrawing (EW) strengths, while the Pt core acts as a weak ED group. We measure the 2PA cross section in the 540-810 nm excitation wavelength range by complementary femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods and compare the obtained values to those of the Pt-core chromophore and the corresponding noncentrosymmetric side group (ligand) chromophores. Peak 2PA cross sections of neutral and ED-substituted Pt complexes occur at S → S transitions to higher energy states, above the lowest-energy S → S transition, and the corresponding values increase systematically with increasing ED strength, reaching maximum value, σ ∼ 300 GM (1 GM = 10 cm s), for R = NPh. At transition energies overlapping with the lowest-energy S → S transition in the one-photon absorption (1PA) spectrum, the same neutral and ED-substituted Pt complexes show weak 2PA, σ < 30-100 GM, which is in agreement with the nearly quadrupolar structure of these systems. Surprisingly, EW-substituted Pt complexes display a very different behavior, where the peak 2PA of the S → S transition gradually increases with increasing EW strength, reaching values σ = 700 GM for R = NO, while in the S → S transition region the peak 2PEF cross section decreases. We explained this effect by breaking of inversion symmetry due to conformational distortions associated with low energy barrier for ground-state rotation of the ligands. Our findings are corroborated by theoretical calculations that show large increase of the permanent electric dipole moment change in the S → S transition when ligands with strong EW substituents are twisted by 90° relative to the planar chromophore. Our NLT results in the S → S transition region are quantitatively similar to those obtained from the 2PEF measurement. However, at higher transition energy corresponding to S → S transition region, the NLT method yields effective multiphoton absorption stronger than the 2PEF measurement in the same systems. Such enhancement is observed in all Pt complexes as well as in all ligand chromophores studied, and we tentatively attribute this effect to nearly saturated excited-state absorption (ESA), which may occur if 2PA from the ground state is immediately followed by strongly allowed 1PA to higher excited states.
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