Symmetry Breaking in Platinum Acetylide Chromophores Studied by Femtosecond Two-Photon Absorption Spectroscopy.

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)2(NPh2), NH2, OCH3, t-butyl, CH3, H, F, CF3, CN, benzothiazole, and NO2, 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 S0 → Sn transitions to higher energy states, above the lowest-energy S0 → S1 transition, and the corresponding values increase systematically with increasing ED strength, reaching maximum value, σ2 ∼ 300 GM (1 GM = 10-50 cm4 s), for R = NPh2. At transition energies overlapping with the lowest-energy S0 → S1 transition in the one-photon absorption (1PA) spectrum, the same neutral and ED-substituted Pt complexes show weak 2PA, σ2 < 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 S0 → S1 transition gradually increases with increasing EW strength, reaching values σ2 = 700 GM for R = NO2, while in the S0 → Sn 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 S0 → S1 transition when ligands with strong EW substituents are twisted by 90° relative to the planar chromophore. Our NLT results in the S0 → S1 transition region are quantitatively similar to those obtained from the 2PEF measurement. However, at higher transition energy corresponding to S0 → Sn 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.

Spectroscopic structure-property relationships of a series of polyaromatic platinum acetylides.

To develop a structure-spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon) ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings (nFAR) and ligand topology (polyacene (L), polyphenanthrene (Z), or compact(C)). We measured ground-state absorption, fluorescence, and phosphorescence spectra. We also performed nanosecond and femtosecond transient absorption experiments. To extend the range of compounds in the structure-property relationship, we did DFT calculations on an expanded series of chromophores. Both the DFT results and experiments show that the S(1) and T(1) state energies are a function of both nFAR and the ligand topology. In the L chromophores, the S(1) and T(1) state energies decrease linearly with nFAR. In contrast, the S(1) and T(1) state energies of the Z chromophores oscillate around a fixed value with increasing nFAR. The C chromophores have behavior intermediate between the L and Z chromophores. A parallel series of calculations on the ligands shows the same behavior. The S(1)-S(n) energy obtained from ultrafast time-resolved spectra has a linear variation in nFAR. The rate constant for nonradiative decay, k(nr), was calculated from the S(1) state lifetime and decreases with an increasing number of π electrons in the aromatic ring. The result is consistent with the spin-orbit coupling caused by the central platinum heavy atom decreasing with larger nFAR. The present work shows that the framework developed for the analysis of poly(aromatic hydrocarbon) properties is useful for the understanding of the corresponding platinum acetylide complexes.

Photophysical properties of a series of electron-donating and -withdrawing platinum acetylide two-photon chromophores.

To explore spectroscopic structure-property relationships in platinum acetylides, we synthesized a series of complexes having the molecular formula trans-bis(tributylphosphine)-bis(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl)ethynyl)-R)-platinum. The substituent, R = NH(2), OCH(3), N(phenyl)(2), t-butyl, CH(3), H, F, benzothiazole, CF(3), CN, and NO(2), was chosen for a systematic variation in electron-donating and -withdrawing properties as described by the Hammett parameter σ(p). UV/vis, fluorescence, and phosphorescence spectra, transient absorption spectra on the fs-ps time scale, and longer time scale flash photolysis on the ns time scale were collected. DFT and TDDFT calculations of the T(1) and S(1) energies were performed. The E(S) and E(T) values measured from linear spectra correlate well with the calculated results, giving evidence for the delocalized MLCT character of the S(1) state and confinement of the T(1) exciton on one ligand. The calculated T(1) state dipole moment ranges from 0.5 to 14 D, showing the polar, charge-transfer character of the T(1) state. The ultrafast absorption spectra have broad absorption bands from 575 to 675 nm and long wavelength contribution, which is shown from flash photolysis measurements to be from the T(1) state. The T(1) energy obtained from phosphorescence, the T(1)-T(n) transition energy obtained from flash photolysis measurements, and the triplet-state radiative rate constant are functions of the calculated spin density distribution on the ligand. The calculations show that the triplet exciton of chromophores with electron-withdrawing substituents is localized away from the central platinum atom, red-shifting the spectra and increasing the triplet-state lifetime. Electron-donating substituents have the opposite effect on the location of the triplet exciton, the spectra, and the triplet-state lifetime. The relation between the intersystem crossing rate constant and the S(1)-T(1) energy gap shows a Marcus relationship with a reorganization energy of 0.83 eV. The calculations show that intersystem crossing occurs by conversion from a nonpolar, delocalized S(1) state to a polar, charge-transfer T(1) state confined to one ligand, accompanied by conformation changes and charge transfer, supporting the experimental evidence for Marcus behavior.

Platinum acetylide two-photon chromophores.

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.

Insight into the nonlinear absorbance of two related series of two-photon absorbing chromophores.

A comprehensive photophysical study of the linear and nonlinear absorption properties has been carried out on two series of two-photon absorbing dyes to gain insight into how structure-property relationships influence observed nonlinear absorption. The materials studied consist of an electron accepting benzothiazole group connected to an electron donating diphenylamine via a fluorene bridging group. Two series differ from each other by the addition of one phenyl group and for each series one-arm (dipolar, AF240 and AF270), two-arm (quadrupolar, AF287 and AF295), and three-arm (octupolar, AF350 and AF380) versions were studied. Overall the AF240 series exhibits higher intrinsic two-photon absorption (TPA) cross-sections than the AF270 series as well as enhanced nanosecond nonlinear absorption, with an increase with number of branches. The enhanced nanosecond nonlinearity is understood by taking into account the contribution from the singlet and triplet excited states and was verified by a two-photon assisted excited-state absorption model that satisfactorily predicts the nonlinear absorption of the chromophores.

Asymmetry in platinum acetylide complexes: confinement of the triplet exciton to the lowest energy ligand.

To determine structure-optical property relationships in asymmetric platinum acetylide complexes, we synthesized the compounds trans-Pt(PBu3)2(C[triple bond]CC6H5)(C[triple bond]C-C6H4-C[triple bond]CC6H5) (PE1-2), trans-Pt(PBu3)2(C[triple bond]CC6H5)(C[triple bond]C-C6H4-C[triple bond]C-C6H4-C[triple bond]CC6H5) (PE1-3) and trans-Pt(PBu3)2(C[triple bond]C-C6H4-C[triple bond]CC6H5)(C[triple bond]C-C6H4-C[triple bond]C-C6H4-C[triple bond]CC6H5) (PE2-3) that have different ligands on either side of the platinum and compared their spectroscopic properties to the symmetrical compounds PE1, PE2 and PE3. We measured ground state absorption, fluorescence, phosphorescence and triplet state absorption spectra and performed density functional theory (DFT) calculations of frontier orbitals, lowest lying singlet states, triplet state geometries and energies. The absorption and emission spectra give evidence the singlet exciton is delocalized across the central platinum atom. The phosphorescence from the asymmetric complexes comes from the largest ligand. Time-dependent (TD) DFT calculations show the S1 state has mostly highest occupied molecular orbital (HOMO) --> lowest unoccupied molecular orbital (LUMO) character, with the LUMO delocalized over the chromophore. In the asymmetric chromophores, the LUMO resides on the larger ligand, suggesting the S1 state has interligand charge transfer character. The triplet state geometries obtained from the DFT calculations show distortion on the lowest energy ligand, whereas the other ligand has the ground state geometry. The calculated trend in the triplet state energies agrees very well with the experimental trend. Calculations of triplet state spin density also show the triplet exciton is confined to one ligand. In the asymmetric complexes the spin density is confined to the largest ligand. The results show Kasha's rule applies to these complexes, where the triplet exciton moves to the lowest energy ligand.

Spectroscopic characterization of a series of platinum acetylide complexes having a localized triplet exciton.

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).

Effect of platinum on the photophysical properties of a series of phenyl-ethynyl oligomers.

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-optical property relationships in organometallic sydnones.

As part of an effort to develop a spectroscopic structure-property relationship in platinum acetylide oligomers, we have prepared a series of mesoionic bidentate Pt(PBu3)2L2 compounds containing sydnone groups. The ligand is the series o-Syd-(C6H4-C[triple bond]C)n-H, where n = 1-3, designated as Syd-PEn-H. The terminal oligomer unit consists of a sydnone group ortho to the acetylene carbon. We synthesized the platinum complex (Syd-PEn-Pt), the unmodified ligands (PEn-H), and the unmodified platinum complexes (PEn-Pt). The compounds were characterized by various methods, including X-ray diffraction, 13C NMR, ground-state absorption, fluorescence, phosphorescence, and laser flash photolysis. From solving the structure of Syd-PE1-Pt, we find the angle between the sydnone group and the phenyl group is 45 degrees . By comparison of the 13C NMR spectra of the sydnone-containing ligands, the sydnone complexes with the corresponding unmodified ligands and complexes not containing the sydnone group, the sydnone group is shown to polarize the nearest acetylenes and have a charge-transfer interaction with the platinum center. Ground-state absorption spectra of the complexes in various solvents give evidence that the Syd-PE1-Pt complex has an excited state less polar than the ground state, while the PE1-Pt complex has an excited state more polar than the ground state. In all the higher complexes the excited state is more polar than the ground state. The phosphorescence spectrum of the Syd-PE1-Pt complex has an intense vibronic progression distinctly different from the PE1-Pt complex. The sydnone effect is small in Syd-PE2-Pt and negligible in Syd-PE3-Pt. From absorption and emission spectra, we measured the singlet-state energy E(S), the triplet-state energy E(T), and the singlet-triplet splitting Delta E(ST). By comparison with energies obtained from the unmodified complexes, attachment of the sydnone lowers E(S) by approximately 0.1 eV and raises E(T) by approximately 0.1 eV. As a result, the sydnone group lowers Delta E(ST) by approximately 0.2 eV. The trends suggest one of the triplet-state singly occupied molecular orbitals (SOMOs) is localized on the sydnone group, while the other SOMO resides on the rest of the ligand.