J. Phys. Chem. A 2010, 114, 3431–3442
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Luminescence, Singlet Oxygen Production, and Optical Power Limiting of Some Diacetylide Platinum(II) Diphosphine Complexes Eirik Glimsdal,† Marcus Carlsson,‡ Tomas Kindahl,‡ Mikael Lindgren,*,† Cesar Lopes,§ and Bertil Eliasson*,‡ Department of Physics, Norwegian UniVersity of Science and Technology (NTNU), NO-7491 Trondheim, Norway, Department of Chemistry, Umeå UniVersity, SE-901 87 Umeå, Sweden, and DiVision of Information Systems, Swedish Defence Research Agency (FOI), SE-581 11 Linko¨ping, Sweden ReceiVed: September 23, 2009; ReVised Manuscript ReceiVed: January 4, 2010
A series of four new trans-diphosphine Pt(II) diacetylide complexes, with a thiophene and two benzenoid rings in each acetylide ligand, have been synthesized and characterized with respect to optical absorption, spectrally and time-resolved luminescence, and optically nonlinear properties such as two-photon absorption cross section and optical power limiting. Density functional theory (DFT) calculations of a few ground state conformations of three Pt(II) diacetylide structures showed similar total energy for each geometry-optimized rotamer but some differences in the vertical excitation energies and in the ligand-to-metal charge-transfer character. The wavelengths of the calculated excitations were found to be red-shifted compared with peaks in the optical absorption spectra, but the general trends and shifts of wavelengths between the different structures are well reproduced. Static emission spectra for degassed samples in THF solution of the larger compounds showed small Stokes shifts and low fluorescence quantum yields, indicating fast intersystem crossing to the triplet manifold. More pronounced differences between the compounds were displayed in the phosphorescence data, in terms of spectral emission wavelengths and decay times. For instance, the phosphorescence decay of the compound with the thiophene ring close to the Pt center was found to be significantly faster than for the other compounds. A possible relationship between triplet lifetime and conformation of the compounds is discussed. It was also demonstrated that the quenching of the excited triplet states in air-saturated samples involves energy transfer to the oxygen triplet state, and subsequent generation of singlet oxygen showing the typical emission at approximately 1275 nm. The amount of produced singlet oxygen followed the phosphorescence yields of the solute molecules. Two-photon absorption cross sections (σ(2)) were measured and showed values on the order of 10 GM at 780 nm for all compounds. Optical power limiting measurements of the new complexes in THF using 5 ns pulses, showed only slightly better performance at the wavelength of 532 nm compared to that of similar platinum compounds with only two aryl rings in each ligand. At 600 nm the complexes with three aryl rings were significantly better optical limiters than the smaller compounds with two aryl rings in the ligands. Introduction Previous reports have shown that organometallic compounds, in which the transition metal contributes to the delocalized π-electron system, can have interesting photophysical properties.1,2 Square planar Pt(II) acetylides constitute a class of such compounds, which has been extensively studied with respect to NLO properties and optical power limiting (OPL) applications.3-23 The heavy metal atom can provide an efficient intersystem crossing (ISC) from the singlet to the triplet manifold as a consequence of strong spin-orbit coupling.24 In general, we are interested in understanding mechanisms of nonlinear absorption and how the energy is dissipated in the process. We also seek better knowledge of the effects which reduce or quench radiative emission from photoexcited states, as similar compounds might be utilized in other photophysical applications. Thus, a series of four new trans-diphosphine Pt(II) * Corresponding authors. B.E.: e-mail,
[email protected]; phone, (+46)907866837; fax, (+46)90136310. M.L.: e-mail, Mikael.
[email protected]; phone, (+47)73593414; fax, (+47)73597710. † Norwegian University of Science and Technology (NTNU). ‡ Umeå University. § Swedish Defence Research Agency (FOI).
diacetylide complexes, with a thiophene and two benzenoid rings in each acetylide ligand, was synthesized, and characterized with respect to optical absorption, luminescence, and optically nonlinear properties such as two-photon absorption cross section and OPL based on the efficient absorption of excited triplet states.3 Specifically, we synthesized the thiophene-containing transdiarylalkynyl-bis(tri-n-butylphosphine) Pt(II) compounds 1a-1d (Chart 1). Earlier studies have indicated that the thiophene ring can be more effective than a phenyl/phenylene ring or rings with heteroatoms such as furan, pyrrole, and pyridine for increasing the third-order NLO properties of a conjugated π-system.25-27 This effect could be due to the increased polarizability of the heavier sulfur atom and the greater ease of the thiophene ring to attain coplanarity with adjacent aromatic rings. In a recent study of 1f, 1g, and 1i (Chart 1),14,15 we found the rates of phosphorescence decay to be greater for the thiophenyl-containing compounds than for 1i and the previously reported 1h,3 with the rates increasing in the order 1i ≈ 1h < 1g < 1f. In addition, UV-vis, NMR and IR spectroscopy data, and quantum chemistry calculations gave support for stronger interactions between Pt and the alkynyl
10.1021/jp9091514 2010 American Chemical Society Published on Web 02/11/2010
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CHART 1: Structures of Pt(II) Acetylides Discussed in This Studya
a
The newly synthesized compounds are 1a-1d.
ligands in 1f than in 1g and 1h. Although the OPL performance of 1f and 1i was very similar at 532 nm for 5 ns pulses, and at this wavelength somewhat better than for 1g, the results of 1f and 1g for wavelengths of 550 and 610 nm were clearly inferior to those of 1i.14,15 Previous studies have shown that the increased length of the ligand results in a smaller triplet quantum yield and increased lifetime of the triplet state.8,10 These effects were suggested to be caused by a larger singlet-to-triplet energy gap and a lower rate for ISC for the longer Pt(II) acetylides.8 To investigate further the properties of triplet states and nonlinear absorption, we increased the number of conjugated rings in the arylalkynyl ligands from that of 1f and 1g to 1a-1d. The thiophene unit was placed at various positions relative to the platinum core to impose alterations to the conjugated system and reveal the changed photophysical properties of the compounds. As methoxy substitution in some cases is known to increase the third-order hyperpolarizability,26 and because an increased solubility was anticipated, compound 1d was also prepared for comparison with 1c. In addition, the earlier reported platinum compound 1e6,8 was prepared to allow comparison under identical experimental conditions. Linear and nonlinear absorption and emission properties of 1a-1e were measured by static and time-resolved spectroscopy in the near-UV and visible region. Specifically, we investigate the quenching process by dissolved oxygen from the luminescence of singlet oxygen. OPL properties were recorded at wavelengths of 532 and 600 nm, and TPA cross sections were measured at 780 nm. Ground state geometries and excitation energies for several conformations of models of 1a, 1b, and 1e were obtained from timedependent DFT calculations.
Results and Discussion Preparation of New Dialkynyl Platinum(II) Compounds. Compounds 1a-1d (Chart 1) were synthesized by initial preparation of the alkyne ligands 9a, 9b, 12, and 16 (Schemes 1 and 2), followed by coupling of these alkynes to the platinum core by a reaction developed by Sonogashira et al.28 (Scheme 3). In the final reaction, a TEA/THF solution of the arylalkyne, PtCl2(P(n-Bu)3)2, and CuI as catalyst was rapidly heated to 60 °C for 5 min, which resulted in good yield of the complex. Details of the synthesis are given in the experimental section. Optical Spectroscopy and Luminescence. The parameters obtained from a detailed photophysical characterization of the Pt(II) organometallic compounds 1a-1g as outlined in Chart 1, using THF as solvent, are summarized in Table 1. These include absorption and emission wavelengths, fluorescence quantum yields, and excited state lifetimes as well as data of optical power limiting measurements carried out at 532 and 600 nm. The results of the novel thiophene ring derivatives 1a-1d along with their spectral features are discussed in more detail below with reference to the compounds 1e-1i. The linear absorption spectra of 1a-1e are shown in Figure 1. The main absorption band of all five compounds seems to have at least two overlapping peaks in the region for S0 f S1 transitions.6,29 The absorption band of 1c is similar to that of 1e; the former with its thiophene ring furthest away from the Pt atom. The λmax values of 1c and 1e are 382 and 378 nm, respectively, and these appear at shorter wavelengths than the absorption bands of 1a, 1b, and 1d. When structural differences
Diacetylide Pt(II) Diphosphine Complexes
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SCHEME 1: Synthesis of Alkyne Ligands 9a and 9ba
a
Yields of isolated products in brackets.
SCHEME 2: Synthesis of Alkyne Ligands 12 and 16
SCHEME 3: Synthesis of Arylalkynyl Platinum(II) Compounds
are compared between 1c and 1b and 1a, that is, as the thiophene unit appears closer to the platinum atom, the associated absorption spectrum is red-shifted. Although the value of λmax is shifted only 3 nm from 1c to 1b, but 23 nm comparing 1b with 1a, the long-wavelength side of the band of both 1b and 1a are shifted to a similar extent, by slightly more than 10 nm for each successive change of the thiophene ring position. This is in parallel with a red shift of 16 nm of the absorption band of 1f compared to that of 1g, as reported earlier.14,15 The main absorption of 1d is further red-shifted, with a λmax value of 419 nm. Hence, the 2,5-dimethoxy substitution of the ring affects the absorption more than the presence of a thiophenylene replacing a phenylene ring close to the Pt nucleus, as in 1a. Luminescence spectra of 10 µM samples of 1a-1e in THF were obtained using the SHG of a Ti:sapphire laser for excitation at 390 nm (Figure 2). The wavelengths for the emission peaks
are summarized in Table 1. In accordance with the findings of previous studies, we can separate these into fluorescence and phosphorescence, to be further discussed below. The fluorescence peak at highest energy (406 nm) belongs to 1e, which also displays a weak emission at 555 nm. Both values are in agreement with those previously obtained by Cooper and Rogers et al., with the latter being assigned to a T1-S0 transition.6,8 In resemblance with the absorption spectra, the fluorescence peaks of 1a-1d are red-shifted compared to that of 1e. With reference to 1e, the shifts are 8, 18, 34, and 40 nm for 1c, 1b, 1a, and 1d, respectively. The relatively broad emission feature of 1c indicates closely overlapping peaks. All five compounds showed only weak phosphorescence in air-saturated samples. The bands of 1e and 1b at 555 and 630 nm, respectively, are shown magnified in Figure 2 to emphasize their line shape.
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TABLE 1: Optical Power Limiting Data, Absorption and Emission Wavelengths, Fluorescence Quantum Yields, and Excited State Lifetimes of Pt(II) Complexes in THF OPL, Eout at Ein ) 150 (µJ)a 532 nm
experimental series (A-D) compd
A
B
1a 1b 1c 1d 1e 1fh 1gh
4.7
3.3 3.7 3.4
6.5 4.1 4.8
C
D
A
c
D
4.9 3.5 2.5 4.5 5.0
emissionb
600 nm
4.9 3.5 2.9 5.0 7.6
25i 26i
absorptionb λmax (nm) [ε/104 (M-1 cm-1)] λfl (nm) 408 [10.4] 385 [11] 382 [10.1] 419 [12.7] 378d [8.99]e 378 [8.9] 362 [11]
440 424 412 446 406d 420 392
τphos (µs) Qfl
τfl (ps)
0.026 0.034 0.020 0.054 0.012d 0.0045 0.0007
9 (99%), 200 (1%) 30 (90%), 400 (10%) 90 (40%), 300 (60%) 15 (94%), 700 (6%) 13 (95%), 200 (5%)
