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Excited State Absorption Properties of Pt(II) Terpyridyl Complexes Bearing π-Conjugated Arylacetylides† Xianghuai Wang, Se´bastien Goeb,‡ Zhiqiang Ji, and Felix N. Castellano* Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403 ReceiVed: February 19, 2010; ReVised Manuscript ReceiVed: April 26, 2010
The synthesis, photophysics, and excited state absorption properties of three platinum(II) terpyridyl acetylide charge transfer (CT) complexes possessing a lone ancillary ligand systematically varied in phenylacetylide (PA) π-conjugation length, [Pt(tBu3tpy)([CtC-C6H4]n-H)]ClO4 (n ) 1, 2, 3), are described. Density functional theory (DFT) calculations performed on the ground states of complexes 1, 2, and 3 reveal that their HOMOs reside mainly on the ancillary π-conjugated PA moiety, ranging from 86 to 97%, with LUMOs predominantly centered on the terpyridyl acceptor ligand (91-92%). This electronic structure leads to the production of a triplet ligand-to-ligand CT (3LLCT) excited state upon visible light excitation with minor contributions from the corresponding triplet metal-to-ligand CT (3MLCT) excited state. Unusually strong red-to-near-IR transient absorptions are produced in the excited states of these molecules following selective long wavelength visible excitation of the low energy CT bands that do not emanate from the terpyridyl radical anion produced in the CT excited state or from an arylacetylide-based triplet intraligand (3IL) excited state. The extinction coefficients of these low energy absorption transients were determined using the energy transfer method with anthracene serving as the triplet acceptor. A detailed theoretical investigation using DFT and TDDFT methods reveals that these intense near-IR transient absorptions involve transitions resulting from transient oxidation of the PA subunit. In essence, the production of the 3LLCT excited state transiently oxidizes the PA moiety by one electron, producing the corresponding highly absorbing radical cation-like species, analogous to that experienced in related intramolecular photoinduced electron transfer reactions. The computational work successfully predicts the oscillator strength and peak wavelength of the measured excited state absorption transients across this series of molecules. In the present effort, there is a convergence of theory and experiment given that the excited state absorption properties of these Pt(II) chromophores are determined by localized transitions that resemble open shell radical cation species. Introduction The excited state properties of Pt(II) acetylides continue to inspire both fundamental and applied interdisciplinary research in photonics.1-32 Of particular interest are square planar bipyridyl and terpyridyl charge transfer (CT) acetylide chromophores which produce an array of photophysical responses, tunable through both the electronic structure of the ligand framework and/or solvent medium.33-53 Such flexibility results in the ability to finely tune absorption/emission spectra, excited state lifetimes, emission quantum yields, and transient absorption features.54-60 These criteria are of the utmost importance as they present a unique opportunity for the development of systematic structureproperty relationships,61-67 potentially exploitable in broadband optical limiting chromophore design.68-73 We note that a variety of Pt(II) terpyridyl acetylide molecules have been shown to exhibit strong reverse saturable absorption (RSA),49,59,68-71 resulting from the fact that these chromophores generally produce large absorption transients throughout the visible and near-IR in the CT excited state. Interested in the origin of these strong transients, we published a comprehensive study in 2007 which demonstrated that the intense excited state absorption features observed in these species likely emanate from the †
Part of the “Michael R. Wasielewski Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Current address: Universite´ d Angers, CIMA UMR CNRS 6200-UFR Sciences, 2 Boulevard Lavoisier, 49045, Angers, France.
photogenerated hole (or associated transitions) delocalized across the Pt center and the acetylide unit and not from the terpyridylbased radical anion.74 This suggested that rational chemical modifications within the acetylide subunit would likely have a substantial impact on the resultant transient absorption properties and inevitably the RSA response. These combined experimental results inspired the current effort where we identify a clear-cut systematic trend of arylacetylide π-conjugation length on the intensity and spectral profile of the excited state absorptions in the corresponding Pt(II) complexes, thereby confirming our previous hypothesis. Specifically, three Pt(II) acetylide complexes {[Pt(tBu3tpy)(L)]ClO4, where tBu3tpy is 4,4′,4′′-tri(tert-butyl)-2,2′:6′,2′′terpyridine and L is CtC-Ph (1), CtC-C6H4-CtC-Ph (2), CtC-C6H4-CtC-C6H4-CtC-Ph (3)} were prepared and structurally characterized, and their static and dynamic photophysical properties were quantified. For the first time, the excited state extinction coefficients of CT chromophores 1-3 were determined using the energy transfer method with anthracene serving as the triplet acceptor. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were applied to compounds 1-3, [Pt(tBu3tpy)Cl]+, in addition to their open-shell analogues to evaluate the molecular orbitals responsible for producing the experimentally observed absorption transients in the CT excited state of these complexes. While TDDFT calculations have been applied to construct/rationalize
10.1021/jp101528z 2010 American Chemical Society Published on Web 05/14/2010
Excited State Properties of Pt(II) Terpyridyl the transient spectra of select organic molecules,75,76 to the best of our knowledge the present work represents the first attempt to emulate excited state absorption spectra of transition metal complexes. The combined experimental/theoretical studies reveal that the optical transitions producing the strong red-to-near-IR transient absorptions in 1-3 involve transitions to the oxidized cation radical-like phenylacetylide subunit which is produced in the CT excited state.
Experimental Section General. All reactions were carried out under an inert and dry argon atmosphere using standard techniques. Anhydrous CH2Cl2 and diisopropylamine (Aldrich Chemical Co.) were freshly distilled over CaH2. Spectroscopic grade CH3CN (Aldrich Chemical Co.) was used as received. All other reagents from commercial sources were used as received. 1H NMR and 13 C-{1H} NMR spectra were recorded on a Bruker Avance 300 (300 MHz) spectrometer. All chemical shifts are referenced to the residual solvent signals previously referenced to TMS, and splitting patterns are designated as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), and br (broad). EI mass spectra (70 eV) of the ligands were measured in-house using a direct insertion probe in a Shimadzu QP5050A spectrometer. Mass spectra on the Pt(II) complexes were measured in-house using a Bruker Daltonics Omniflex MALDI-TOF spectrometer. Elemental analyses were performed by Atlantic Microlab, Norcross, GA. Preparations. Caution! Perchlorate salts are potential explosion hazards and should be handled with care in small amounts. Complex 1,74 [Pt(tBu3tpy)Cl]ClO4,77 1-ethynyl-4-(phenylethynyl)benzene, and 1-ethynyl-4-((4-(phenylethynyl)phenyl)ethynyl)benzene,78 was synthesized according to literature procedures and yielded satisfactory 1H NMR and mass spectrometry data. General Synthetic Procedure. A Schlenk flask was charged with the platinum(II) chloride precursor, [Pt(tBu3tpy)Cl]ClO4, the appropriate terminal acetylene, CH2Cl2, and i-Pr2NH. The solution was stirred at room temperature for 24 h, and then the solvent was evaporated under vacuum. The residue was purified by column chromatography, followed by recrystallization from a mixture of CH2Cl2/n-hexanes. Complex 2. 2 was prepared using the general synthetic procedure from [Pt(tBu3tpy)Cl]ClO4 (60 mg, 0.08 mmol), 1-ethynyl-4-(phenylethynyl)benzene (33 mg, 0.16 mmol), CH2Cl2 (20 mL), and i-Pr2NH (1 mL). Chromatography was
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14441 performed on alumina, eluting with CH2Cl2 to 99.3/0.7 CH2Cl2/methanol (v/v) to give 63 mg (85%) of 2 as a yellow solid after recrystallization. 1H NMR (300 MHz, CDCl3): δ 9.13 (d, 3J ) 6.0 Hz, 2H), 8.40 (s, 2H), 8.34 (d, 4J ) 1.8 Hz, 2H), 7.63 (dd, 3J ) 6.0 Hz, 4J ) 1.8 Hz, 2H), 7.56-7.54 (m, 2H), 7.48 (s, 3H), 7.38-7.35 (m, 4H), 1.60 (s, 9H), 1.50 (s, 18H). MALDI-MS: m/z (nature of the peak, relative intensity): 797.52 ([M - ClO4]+, 100). Anal. Calcd for
C43H44ClN3O4Pt · H2O: C, 56.42; H, 5.07; N, 4.59. Found: C, 56.12; H, 4.89; N, 4.65. Complex 3. 3 was prepared using the general synthetic procedure departing from [Pt(tBu3tpy)Cl]ClO4 (60 mg, 0.08 mmol), 1-ethynyl-4-((4-(phenylethynyl)phenyl)ethynyl)benzene (50 mg, 0.16 mmol), CH2Cl2 (30 mL), and i-Pr2NH (1 mL); chromatography was performed on alumina, eluting with CH2Cl2 to 99.3/0.7 CH2Cl2/methanol (v/v) to give 65 mg (80%) of complex 3 as a yellow solid after recrystallization. 1H NMR (300 MHz, CDCl3): δ 9.12 (d, 3J ) 6.0 Hz, 2H), 8.40 (s, 2H), 8.34 (d, 4J ) 1.8 Hz, 2H), 7.62 (dd, 3J ) 6.0 Hz, 4J ) 1.8 Hz, 2H), 7.57-7.55 (m, 2H), 7.53 (s, 4H), 7.48 (s, 4H), 7.39-7.37 (m, 3H), 1.60 (s, 9H), 1.51 (s, 18H). MALDI-MS: m/z (nature of the peak, relative intensity): 897.51 ([M - ClO4]+, 100). Anal. Calcd for C55H58ClN3O4Pt · H2O: C, 60.32; H, 4.96; N, 4.14. Found: C, 60.59; H, 4.84; N, 4.08. Photophysical Measurements. UV-vis absorption spectra were recorded with a Shimadzu UV-3600 UV-vis-NIR spectrophotometer. Uncorrected luminescence spectra were obtained with a PTI Instruments spectrofluorimeter, of which the excitation source is a 75 W xenon lamp and the detector in the UV-vis region is a R-928 PMT detector. Photoluminescence lifetimes were measured by means of a nitrogen-pumped broad band dye laser system (PTI GL-3300 nitrogen laser, PTI GL301 dye laser, C-450 dye, 2-3 nm fwhm), which has been previously described.33 Photoluminescence intensity decays were analyzed in Origin 8.0 with goodness-of-fit determined by visual inspection of the residuals. Photoluminescence quantum yields were determined relative to [Ru(bpy)3]2+ in deaerated CH3CN (Φem ) 0.062)79 as the quantum counter. Transient absorption spectra were recorded with a Proteus spectrometer (Ultrafast Systems), which includes a 150 W Xe-arc lamp (Newport) as the probe light source, a Chromex (Bruker Optics) monochromator, and Si and InGaAs photodiode detectors for the visible and near-IR, respectively (DET 10A and DET 10C, Thorlabs),
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with signals amplified using a current amplifier (Femto HCA200M-20K-C). Nanosecond excitation pulses were generated from a computer-controlled Nd:YAG laser/OPO combination (Vibrant LD 355 II, OPOTEK), tuned to 470 nm and operating at 10 Hz. A neutral density filter (50% transmittance) was inserted between the excitation laser and sample to maintain the incident energy at 1.2 mJ/pulse, measured with a Molectron Power Max 5200 power meter. Single wavelength kinetic absorption transients were analyzed in Origin 8.0, with goodness-of-fit judged by visual inspection of residuals. All spectroscopic measurements were performed at ambient temperature, typically near 20 °C. All the photoluminescence samples were prepared with spectroscopic grade CH2Cl2 (Sigma-Aldrich) in 1 cm2 anaerobic quartz cells (Starna Cells), degassed by argon for at least 30 min and maintained under an argon blanket during the experiments. The solutions were preserved at an OD ∼ 0.1 for photoluminescence measurements and an OD ∼ 0.4 for transient absorption measurements at 470 nm. The molar extinction coefficients of the triplet excited states of compounds 1-3 were determined using the energy transfer method as previously described.80-82 Using nanosecond laser flash photolysis, this method involves the use of a donor in the presence of an acceptor whose concentration is sufficient to quench all the donor triplets. The transient absorption difference spectra of the donor (prompt signal) and the sensitized acceptor (peak value following energy transfer) are measured at their respective (well separated) wavelength maxima and compared using eq 1. Equation 1 correlates these values where 3ε(D) and 3 ε(A) are the triplet molar extinction coefficients of the donor (unknown) and acceptor (known), respectively, and ∆A(D) is the maximum optical density of the donor triplet in the absence of the acceptor and ∆A(A) is the maximum optical density of the acceptor triplet when both donor and acceptor are present. 3 3
ε(A)
ε(D)
)
∆A(A) ∆A(D)
(1)
In the current experiments, 1-3 served as independent donor molecules, selectively excited at 470 nm, which transferred their triplet excited state energy to anthracene. Anthracene possesses a triplet-to-triplet extinction coefficient of 45 500 M-1 cm-1 at 430 nm in benzene.83 The peak wavelength of this transient is centered at 420 ( 5 nm in our spectrometer and we make the assumption that the previously reported extinction coefficient is indeed correct. In CH2Cl2, the solvent used in all of the current transient absorption experiments, the peak of the anthracene triplet-to-triplet absorption, also peaks at 420 nm. For lack of an appropriate tabulated value in this particular solvent, the T1fTn extinction coefficient of anthracene at 420 nm in CH2Cl2 was determined to be ε420 nm ) 47 320 M-1 cm-1 by direct comparison to an optically matched solution in benzene (λex ) 355 nm, 1.2 mJ/pulse). The T1fTn anthracene extinction coefficient in CH2Cl2 is only a factor of 1.04 greater than that previously determined in benzene. DFT Calculations. The computational study performed on complexes 1, 2, and 3 was accomplished with DFT using the Gaussian 03 program suite.84 The B3LYP functional85,86 and the 6-31G(d,p) basis set were applied to the nonmetal atoms (hydrogen, carbon, nitrogen). The LANL2DZ effective core potential (ECP) and corresponding basis set functions87 were used for the platinum atom. The molecular orbital composition analyses were performed by GaussSum88 applied to the respective Gaussian output files. The lowest triplet excited state
Figure 1. Static absorption spectra of complexes 1-3 measured in CH2Cl2 at room temperature.
geometries of complexes 1, 2, and 3 were optimized using unrestricted formalisms. The initial guess for geometry optimization on all complexes placed the phenylacetylide and the terpyridine ring plane at a 45° dihedral angle, and optimization proceeded without imposed symmetry. The absorption spectra of the lowest triplet excited states were obtained using the TDDFT method, where 24 upper excited states were determined. All the calculations were performed using the polarizable continuum model89 (PCM) with implicit CH2Cl2 solvation. Results and Discussion Syntheses and Structures. The phenyleneethynylene synthons used in the preparation of 2 and 3 were prepared and characterized as described in the literature.78 The three Pt(II) complexes central to the present study were prepared in accordance with standard Pt-to-acetylide coupling procedures with final purification achieved by chromatography over alumina. Compounds 1-3 are air-stable solids at rt and are readily soluble in a variety of organic solvents. The newly prepared complexes were structurally characterized by 1H NMR, mass spectrometry, and elemental analysis. The stepwise variation in phenylacetylide conjugation length across 1-3 is intended to reveal the pivotal role these ligands play in the excited state absorption properties in Pt(II) terpyridyl acetylides. Static Electronic Spectra and Ground State DFT Calculations. The static UV-vis absorption spectra of complexes 1-3 in CH2Cl2 are each displayed in Figure 1. Extinction coefficients ranging between 10 000 and 70 000 M-1 cm-1 are observed between 300 and 360 nm and are assigned to ligand-localized π-π* transitions in the phenylacetylide and tBu3tpy moieties.90 Similar near visible transitions have been observed in bipyridyl analogues of these molecules48,91 in addition to the transdisposed bisphosphine chromophores bearing phenyleneethynylene oligomers.92 The broad absorption bands at longer wavelengths, with extinction coefficients between 4000 and 7000 M-1 cm-1 are assigned as combinations of overlapping MLCT [Pt (dπ) f tpy (π*)] and LLCT [CtCR (π) f tpy (π*)] transitions as ascertained by recent experimental results, DFT calculations on structurally related compounds,37,93,94 in addition to DFT calculations on these exact molecules (see below). The charge transfer transitions in 1-3 are markedly red-shifted with respect to their Pt(II)-bipyridyl analogues. Across the series, the extinction coefficients steadily increase with phenylacetylide conjugation length, and 2 and 3 attain the same low-energy absorption maximum within experimental error (475 nm). These data attest to the strong participation of the phenylacetylide subunit in the CT absorption transitions of Pt(II) terpyridyl acetylide chromophores. DFT calculations performed on complexes 1-3 in a CH2Cl2 continuum (PCM) reveal the composi-
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TABLE 1: Molecular Orbital Composition Analysis for the Ground States of 1, 2, and 3 in a CH2Cl2 Continuum Obtained from DFT Calculations composition (%) orbital 1 1 2 2 3 3
LUMO HOMO LUMO HOMO LUMO HOMO
energy (eV) -3.13 -5.92 -3.16 -5.61 -3.21 -5.45
components t
Pt
6 π*( Bu3tpy) d(Pt) + π(CtCR) 10 π*(tBu3tpy) 6 π(CtCR) 4 π*(tBu3tpy) 6 π(CtCR) 2
CtCR 3 86 3 94 3 97
t
Bu3tpy 91 4 91 2 91 1
tion of the molecular orbitals giving rise to the HOMO f LUMO transition responsible for the lowest energy CT absorption (Table 1 and Figure 2). It immediately becomes apparent that the orbital composition analysis (GaussSum) presented in Table 1 and the respective isodensity plots in Figure 2 argue strongly for HOMOs that are predominately localized on the phenylacetylide moiety and LUMOs focused on the tBu3tpy acceptor ligand in each case. Consistent with that calculated in related structures, the lowest energy CT transitions in these molecules should definitely be considered an admixture of metalto-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT), substantially favoring the latter. Interestingly, as the π-conjugation in the acetylide increases across the series, the DFT calculations place more electron density in the orbital admixture on the ancillary ligand, culminating to 97% in 3. In other words, 3 exhibits the most LLCT character in the series, whereas 1 possesses the least (86%). At the same time, the LUMO orbital composition is completely invariant across the series, terminating the CT transition predominately onto the t Bu3tpy moiety in 1-3. In agreement with previous DFT calculations on [Pt(tpy)(CtCPh)]+,93 we were unable to reproduce the long wavelength features of the experimental ground state absorption spectra of complexes 1-3 in CH2Cl2 as a result of the large extent of charge transfer associated with these transitions (Figure S1, Supporting Information). While it is possible to correct this deficiency using the ∆SCF approach which has been applied to some analogous Pt(II) bipyridyl acetylide complexes,38 it will not be discussed any further since this particular facet goes beyond the scope of the current manuscript. Static and Time-Resolved Photoluminescence. Steady-state photoluminescence spectra of 1-3 measured in argon-saturated CH2Cl2 are presented in Figure 3. The spectra are broad and structureless and are symmetrically quenched in the presence of dissolved dioxygen. Compound 1 displays the highest energy emission in the series, λmax ) 592 nm, whereas the spectra for 2 and 3 are red-shifted by ∼10 nm but remain completely superimposed with each other, λmax ) 603 nm. The photoluminescence quantum yields of 1-3 in degassed CH2Cl2 as determined relative to [Ru(bpy)3]2+ are 0.15, 0.042, and 0.044, respectively. The excited state photoluminescence intensity decay data closely parallel that of the quantum efficiencies. In each case, the intensity decays are first order in deaerated CH2Cl2 with lifetimes of 3.8 µs, 880 ns, and 820 ns, for 1, 2, and 3, respectively. All experimentally determined photoluminescence parameters and calculated radiative (kr) and nonradiative decay (knr) rate constants obtained in CH2Cl2 are collected in Table 2. These photoluminescence parameters assume an intersystem crossing quantum efficiency of unity due to spin-orbit coupling promoted by the Pt internal heavy atom.1-4 As the photoluminescence lifetimes and quantum yields decrease significantly from complex 1 to 2 and 3, knr concomitantly increases by a factor of 5 between 1 and 2 (and 3), whereas kr slightly increases
across the series. These differences indicate that the nature of the excited state(s) producing the photoluminescence in 2 and 3 is rather similar but should be considered distinct with respect to 1. Qualitative evidence of these differences emerges from measuring the photoluminescence properties of 1-3 in degassed CH3CN, a well-established Lewis basic quencher of Pt(II) terpyridyl-based CT excited states2,4,74 (Table 3). The excited state lifetimes of 1, 2, and 3 in CH3CN are 526, 308, and 760 ns, respectively. Interestingly, the molecule with the least metallic character in its calculated ground state orbital structure (Table 1 and Figure 2), 3, possesses the longest lifetime in the series in this solvent. The lifetime of 2 is attenuated by a factor of 2.8 relative to CH2Cl2, whereas 1 is quenched by a significantly larger margin, 7.2 times in relation to the noncoordinating solvent. These data indirectly support the DFT calculations as follows. The HOMO retains less metal character as the π-conjugation in the ancillary ligand increases which should systematically attenuate the Pt(II) Lewis acidity in the CT excited state; the same effect renders the CT transition more LLCT in nature in progressing from 1 to 3. In the CT excited state, electron density in the HOMO is depleted, and the effect of Lewis basic quenching by CH3CN should attenuate as this orbital retains more acetylide ligand character. In other words, the Lewis acidity of the metal in the excited state should follow the trend of 1 > 2 > 3, and this is indeed observed through Lewis basic, CH3CN-induced, dynamic quenching of the CT excited state in these molecules. Nanosecond Transient Absorption Spectroscopy. Emission-corrected transient absorption difference spectra were measured for complexes 1-3 in argon-saturated CH2Cl2 at room temperature throughout the visible and near-IR. Figure 4 presents the absorption transients obtained for isoabsorbing CH2Cl2 solutions of 1-3 measured at 100 ns delay after a 470 nm excitation laser pulse. In each case, the single-wavelength transients that were kinetically analyzed exhibited single exponential kinetics, and the lifetimes (τTA) determined are collected in Table 2. As has been routinely observed in this class of chromophores, the excited state lifetimes determined by laser flash photolysis (at higher concentration) are slightly attenuated (by 1.15-1.26-fold) relative to those measured (optically dilute) through transient photoluminescence.56,74 The results here suggest that there is a minor amount of selfquenching in 1-3, but under the current experimental conditions used in all laser flash photolysis experiments, we do not observe any photoluminescence from such species at long wavelengths. Therefore, their contribution to the transient absorption difference spectra obtained in the current study is believed to be negligible and will no longer be considered. The emitting state in each molecule is also presumed to be the same as that which generates the absorption transients in all instances. We note that the transient features measured for 1 are consistent with those obtained by us previously in addition to that measured in closely related structures, some of which have been proposed as optical limiters as a result of their rather intense absorption transients with concomitant broad spectral bandwidth at long wavelengths. Two major positive absorption bands were observed in all three complexes. One major band occurs in the vicinity of 500 nm, and it is so intense that there are no significant ground-state bleaching features observed at all. The lower energy band position varies with each complex, producing a maximum transient absorption peak at 760, 930, and 990 nm, for 1, 2, and 3, respectively. This long wavelength transient feature indeed red shifts with increasing conjugation length in the ancillary acetylide ligand, and isoabsorbing solutions of these
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Figure 2. DFT-calculated isodensity plots of the HOMO (left) and LUMO (right) of ground state 1 (top), 2 (middle), and 3 (bottom) in a CH2Cl2 continuum (PCM).
TABLE 2: Room-Temperature Photophysical Properties of 1-3 in CH2Cl2 λabs
τemb
λem
(nm) compd (ε, (M-1cm-1)) (nm) Φema,b (µs) 1
2
Figure 3. Normalized emission spectra ((2 nm) of complexes 1-3, recorded in argon-saturated CH2Cl2 at room temperature (λex ) 470 ( 2 nm). The emission spectra of 2 and 3 are completely superimposed.
3
315 (17214) 340 (14811) 412 (4612) 459 (4287) 310 (52874) 330 (49649) 412 (5278) 475 (5961) 337 (68969) 410 (6662) 474 (7054)
krc
knrd
(s-1)
τTAe
(s-1)
3.9 × 10
4
(µs)
2.2 × 10
5
592
0.15
3.8
3.0
603
0.042
0.88 4.8 × 104 1.1 × 106 0.74
603
0.044
0.82 5.4 × 104 1.2 × 106 0.71
a
chromophores (at 470 nm) clearly produce significantly stronger excited state absorptions in 2 and 3 with respect to 1 (Figure 4). To make direct quantitative comparisons, the excited state extinction coefficients of 1-3 were determined using the triplet energy transfer method with anthracene as the acceptor (see Experimental Section for specific details). Since the extinction coefficient of the peak T1fTn transition in anthracene (420 nm) has not been previously determined in CH2Cl2, this value was measured here, yielding ε420 nm ) 47 320 M-1 cm-1 by direct comparison to flash photolysis experiments performed on isoabsorbing anthracene solutions in benzene (λex ) 355 nm). Table 4 collects the transient absorption peaks along with their calculated extinction coefficients as ascertained through energy
Quantum yield of photoluminescence measured relative to [Ru(bpy)3](ClO4)2 (Φem ) 0.062) in acetonitrile. b Emission quantum yields and lifetime decays, (5%. c kr ) Φ/τ. d knr ) (1 - Φ)/τ. e Transient absorption determined excited state lifetimes.
transfer-based transient absorption actinometry. We note that the values presented at the respective short wavelength maxima represent the lower limits of the measured extinction coefficients since contributions from ground state bleaching cannot be quantitatively assessed. Fortunately, the molar absorptivities determined at the longer peak wavelengths are considered to be a true representation of these values, keeping in mind the uncertainties in these measurements ((10%). The extinction coefficients associated with the excited state absorptions in 1-3 increase with increasing phenylacetylide conjugation length
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TABLE 3: Room-Temperature Photoluminescence Properties of 1-3 in CH3CN λem compd 1 2 3
(nm) 576 590 589
Φema,b 0.030 0.017 0.020
τemb
k rc
knrd
(ns)
(s-1)
526 308 760
5.7 × 10 5.5 × 104 2.6 × 104
(s-1) 4
1.8 × 106 3.2 × 106 1.3 × 106
a Quantum yield of photoluminescence measured relative to [Ru(bpy)3](ClO4)2 (Φem ) 0.062) in acetonitrile. b Emission quantum yields and lifetime decays, (5%. c kr ) Φ/τ. d knr ) (1 Φ)/τ.
Figure 4. Transient absorption difference spectra of complexes 1, 2, and 3 recorded in argon-saturated CH2Cl2 at room temperature measured 100 ns following a 470 nm (5-7 ns fwhm), 1.2 mJ laser pulse.
TABLE 4: Triplet Excited State Molar Extinction Coefficients of 1-3 in CH2Cl2 Determined by the Energy Transfer Method Using Anthracene as an Acceptor 1 2 3 a
excited state absorption peak
ε* (M-1 cm-1)
520 nm 760 nm 460 nm 930 nm 520 nm 990 nm
24860a 32030 34320a 69260 50000a 77270
Considered to be a lower limit estimate (see text for details).
across the series of molecules. These data, combined with the DFT calculations presented earlier along with our previous comprehensive study on 1, leave little doubt that the excited state absorptions observed in 1-3 must be intimately tied to the electronic structure resident on the appended acetylide subunits. Considering that oxidative spectroelectrochemistry measurements are not possible in these molecules due to the irreversible nature of the first observable oxidation process, TDDFT calculations were performed (see below) to reveal the orbital structures responsible for the intense transient absorption features observed in this class of chromophores. TDDFT Calculations: Correlations to Excited State Absorption Spectra A. Doublet State Calculations. In metal-bearing charge transfer complexes, particularly MLCT chromophores, it is common practice to utilize spectroelectrochemistry to elucidate the spectroscopic characteristics of both oxidized and reduced forms of the complex.95 These data can be correlated to the transient absorption properties of the molecule and typically resemble the major transient features of the complex. Since the Pt(II) complexes at the heart of the present study cannot be subjected to oxidation on electrochemical time scales, similar experimental comparisons are not possible. Therefore, we
Figure 5. Absorption spectra of one-electron reduced (a) and oneelectron oxidized (b) complex 1 obtained from TDDFT calculations using the PCM (CH2Cl2 continuum) formalism. The simulated absorption spectra were generated with a Gaussian profile, fwhm ) 1665 cm-1. The calculated β-orbitals involved in the strongest longwavelength transition indicated by the blue vertical line are also inset.
performed TDDFT calculations on the respective one-electron reduced and one-electron oxidized forms of 1 (the ground state doublets) to simulate the anticipated spectroelectrochemical results (Figure 5). In both open shell doublet and triplet states, electrons can be divided into two groups according to spin-up and spin-down: the n spin-up electrons are referred to as the R electrons, and spin-down electrons (n-1 for doublet and n-2 for triplet) are referred to as β electrons.96 Scheme 1 presents the specific R and β designations for all pertinent frontier orbital configurations discussed herein. For the one-electron reduced complex 1 (Figure 5a), the strongest transition in the red-tonear-IR region is calculated to occur at 784 nm (blue vertical line) with a rather weak oscillator strength (f ) 0.0613). This particular transition involves electron promotion from the R HOMO orbital to the R LUMO+2 orbital, which both possess t Bu3tpy π orbital character as shown in the Figure 5a inset. Since this transition is relatively weak, it can be ruled out as responsible for producing the major transient long-wavelength features observed in Figure 4. The other strong absorption feature calculated to occur at higher energy in the blue and green portions of the spectrum seems at first glance coincident with that experimentally observed in the one-electron reduced complex 1 in addition to the excited state absorption difference spectra of 1-3 (Figure 4). However, TDDFT calculations reveal the true nature of this seemingly recursive spectral feature. In one-electron reduced 1, this higher energy transition occurs between the β HOMO orbital, which is in fact unoccupied in the one-electron oxidized and triplet excited complex (Scheme 1), and the β LUMO orbital. In the triplet excited state of complexes 1-3 (see below), the analogous transitions involve promotion of an electron from the R HOMO orbital to upper unoccupied R orbitals, which is quite similar to the weaker, shoulder-producing transition
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SCHEME 1: Singlet, Doublet, and Triplet r and β Frontier Orbital Configurations
predicted to occur at 523 nm in one-electron reduced 1 (Figure 5a). The TDDFT calculations predict that a transition initiating from the R HOMO orbital will necessarily produce a high energy feature (in the blue/green region) in the absorption spectrum. However, a distinct transition from the β HOMO to the β LUMO of reduced 1 (at higher energy) contributes significantly more oscillator strength than the corresponding R HOMOoriginating transition. The doublet state calculation on reduced 1 is therefore largely consistent with our previous spectroelectrochemical and reductive quenching results, collectively suggesting that the transient absorption data presented in Figure 4 cannot have its origin with optical transitions in the one-electron reduced complexes. Figure 5b presents the corresponding TDDFT data for oneelectron oxidized 1. This calculation positions the strongest optical transition in the red at 619 nm with corresponding oscillator strength of 0.42 (indicated by the blue vertical line), which is blue-shifted relative to the 691 nm maximum observed in the transient difference spectrum of 1, Figure 4. The strength of this calculated transition appears to be qualitatively consistent with that observed experimentally and results from the promotion of an electron from the β HOMO-5 orbital to the β LUMO orbital as indicated in the Figure 5b inset. As can be inferred directly from the isodensity plot, the transition terminates on a β-LUMO that is strongly localized on the acetylide ligand portion of the molecule suggesting this is indeed where one electron oxidation takes place in the ground state. These specific orbitals are practically identical in terms of isodensity distribution, transition energy, and oscillator strength to the molecular orbitals involved in the strongest red-to-near-IR triplet-to-triplet transition (T1fTn) of 1 presented below in Figure 6. The combined doublet state TDDFT calculations performed on the one electron reduced and oxidized states in 1 suggest that the photogenerated hole (the oxidized component of the excited triplet state complex) is responsible for the intense absorption transients measured for 1-3 at long wavelengths (Figure 4). B. Triplet State Calculations. The optimized structure of the lowest triplet state of complex 1 was obtained and used to calculate its T1fTn spectrum (Figure 6). The TDDFT calculation performed on 1 agrees well with the transient absorption spectral
data, and the strong near-IR absorption band is assigned to the β HOMO-1 f β LUMO transition (indicated with a blue vertical line), presented in Figure 6. The β LUMO has significant phenylacetylide π orbital character as implied from the isodensity plots presented in the Figure 6 inset. In all triplet state geometry optimizations across complexes 1-3, the tBu3tpy and phenylacetylide subunits always adopt a coplanar arrangement. To demonstrate that other structural variations would not reproduce the absorption intensities observed in the experimental TA difference spectra, the tBu3tpy and phenylacetylide groups were forced into a perpendicular geometric arrangement, and this alternative set of TDDFT calculations was performed. The oscillator strength of the associated transition at 763 nm dropped severely to 0.0625, merely 1/7 of the intensity calculated in the coplanar structure and assigned to the R HOMO f R LUMO+3 transition, with orbitals primarily corresponding to terpyridine π character. Thus, we believe that the coplanar structure obtained by the present triplet state calculations should be representative of the true molecular structure producing the strong excited state absorption transients in the red and near-IR in molecules 1-3. We note that geometry optimization always produces a structure where the terpyridyl and PA ligand are coplanar, regardless of the initial dihedral angle used.
Figure 6. TDDFT calculated T1fTn absorption spectrum of 1 using the PCM (CH2Cl2 continuum) formalism. The simulated absorption spectrum was generated with a Gaussian profile, fwhm ) 1665 cm-1. The calculated β-orbitals involved in the strongest long wavelength transition indicated by the blue vertical line are also inset.
Excited State Properties of Pt(II) Terpyridyl
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Figure 7. TDDFT calculated T1fTn absorption spectrum of the [Pt(tBu3tpy)Cl]+ complex using the PCM (CH2Cl2 continuum) formalism. The simulated absorption spectrum was generated with a Gaussian profile, fwhm ) 1665 cm-1. The calculated β-orbitals involved in the strongest long wavelength transition indicated by the blue vertical line are also inset. Note that the oscillator strengths of all associated transitions in this molecule are substantially attenuated with respect to the PA-bearing structures 1-3.
To further strengthen our argument that the absorption transients can only be accounted for in these molecules when the PA subunit is present, similar calculations were performed on the triplet excited state of the model chromophore [Pt(tBu3tpy)Cl]+ (Figure 7). The strongest calculated absorption band is found at 750 nm (β HOMO-5 f β LUMO) which is only 1/7 of the oscillator strength (f ) 0.0630) relative to that calculated in 1. In addition, both of these MOs have primarily terpyridine π orbital character, and previous experiments from our laboratory have already confirmed the small excited state absorption cross section exhibited by this complex.74 The present calculation combined with past experiments on [Pt(tBu3tpy)Cl]+ definitively eliminates the possibility of terpyridine π orbital character being responsible for the strong red-to-near-IR transient absorptions in 1-3. The TDDFT calculated oscillator strengths of the triplet excited states of complexes 1, 2, and 3 are presented in Figure 8 along with the measured transient difference spectra measured at 100 ns delay superimposed for comparative purposes. The triplet state calculations reveal that a systematic red-shift should occur in the strongest T1fTn transient feature (blue vertical lines) in the long wavelength band observed experimentally as the conjugation length increases in the phenylacetylide moiety (Figure 8). The calculated absorption peak position and relative intensities exhibited by complexes 1 and 2 align remarkably well with the experimental data, whereas there is a disparity of approximately 0.27 eV between the experimental and calculated longest wavelength T1fTn feature in 3. The strongest near-IR transient absorption position in 3 is clearly underestimated by this calculation; however, the features exhibited at shorter wavelengths are well reproduced by the TDDFT triplet state calculations. The specific molecular orbitals involved in the strongest long wavelength absorption transients are shown as isodensity plots inset within Figure 8, and their respective transition energies and oscillator strengths are presented in Table 5. The combined computational data from the three molecular structures demonstrate that the majority of the LUMO orbital components is resident on the phenylacetylide subunit in each instance. For the most part, the computational work successfully predicts the oscillator strength and peak wavelength of the measured excited state absorption transients across this series of molecules. In the present work, convergence of theory and experiment is realized since the excited state absorptions of Pt(II) chromophores 1-3 are predominantly determined by high
Figure 8. Calculated oscillator strength of the transient excited state of complexes 1 (A), 2 (B), and 3 (C) (red lines) and the experimental difference spectra taken at a delay of 100 ns following a 470 nm laser pulse (black lines). The orbitals involved in the strongest red-to-nearIR transition (blue vertical lines) are also presented in each instance.
TABLE 5: Triplet State TDDFT Calculation Results for the Strongest Transitions Exhibited by Complexes 1, 2, and 3a 1 2 3
transition
oscillator strength
participating MOsb
691 nm 923 nm 1270 nm
0.4225 0.8879 1.4628
β HOMO-1f β LUMO (0.69) β HOMOf β LUMO (0.71) β HOMOf β LUMO (0.86)
a Complete data presented as Supporting Information. largest expansion coefficient is shown.
b
Only the
oscillator strength localized transitions that resemble open shell radical cation species. If one considers the collective photophysical behavior of Pt(II) complexes bearing the acetylide subunit CtC-C6H4CtC-C6H4-CtC-Ph, a number of interesting observations emerge. In the present effort, molecule 3 exhibits 3CT (3LLCT) photophysics with excited state absorptions dominated by the transient vacancy produced in this ligand following visible light excitation. In the related Pt(dbbpy)(CtC-C6H4-CtC-C6H4CtC-Ph)2 complex, the lowest excited state is indeed 3CT in nature in low polarity solvents, albeit with predominant 3MLCT character, which undergoes complete state inversion to one of the neutral 3CtC-C6H4-CtC-C6H4-CtC-Ph* ligandlocalized compositions in solvents of modest polarity such as CH2Cl2.52,91 The latter is spectroscopically indistinguishable from
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the ligand localized triplet states produced in the trans- and cisdisposed Pt(II) bisphosphine bisacetylide structural analogues, all of which produce long-lived room-temperature phosphorescence.34,52,53,91,92 It is easily concluded that the terpyridyl structure exclusively yields a CT-based excited state, whereas the bipyridyl complex produces an array of excited states spanning pure 3CT to pure 3IL to configuration mixed compositions, controlled through solvent composition, and/or temperature. At the same time, the two phosphine-supported structures, regardless of geometric disposition, exclusively produce 3ILbased photophysics. It is therefore important to explore the spectroscopic behavior of conjugated acetylide ligands tethered to Pt(II) in a variety of ligand environments as minor structural perturbations can indeed unleash a wealth of photophysics in these structures.1,2 Facile synthetic alterations that produce marked changes in excited state absorption properties are quite desirable in emerging RSA materials, and π-conjugated Pt(II) acetylides appear poised to significantly contribute in this area. Conclusion The synthesis, photophysics, and excited state absorption properties of three platinum(II) terpyridyl CT chromophores bearing π-conjugated PA ligands have been reported. The combination of DFT and TDDFT electronic structure calculations along with the spectroscopic data provide support for a predominant 3LLCT excited state parentage in these molecules producing unusually strong red-to-near-IR transient absorptions upon long wavelength visible excitation of the low energy CT bands. The spectral profiles red-shifted, and the corresponding extinction coefficients of the absorption transients were found to increase with increasing π-conjugation length in the PA ligand. Density functional computations revealed that the intense near-IR transient absorptions involve optical transitions produced from the one-electron oxidation of the PA subunit. In essence, the production of the 3LLCT excited state oxidizes the PA moiety by one electron, thereby generating the corresponding highly absorbing radical cation-like species. The computational work provided good estimates of the oscillator strength and peak wavelength of the measured excited state absorption transients across this series of molecules. The excited state absorptions of these Pt(II) chromophores were well modeled by TDDFT since these properties are dictated by strongly localized transitions that resemble open shell radical cation species. Acknowledgment. This research was supported by the Air Force Office of Scientific Research (FA9550-05-1-0276), the NSF (CHE-0719050), and the BGSU Research Enhancement Initiative. The Ohio Supercomputer Center is acknowledged for providing the computational platform necessary for the successful completion of this work. Dr. Tanya N. Singh-Rachford (BGSU) and Prof. J. R. Cable (BGSU) are gratefully acknowledged for their assistance with the excited state extinction coefficient determinations and insightful discussions, respectively. Supporting Information Available: Simulated ground state absorption spectra of 1-3 in a CH2Cl2 continuum (PCM), detailed computational results of 1-3, and complete reference 84. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Muro, M. L.; Rachford, A. A.; Wang, X.; Castellano, F. N. Top. Organomet. Chem. 2010, 29, 159–191. (2) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro, M. L.; Rajapakse, N. Coord. Chem. ReV. 2006, 250, 1819–1828.
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