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J. Phys. Chem. A 2010, 114, 7003–7013

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Optimizing Simultaneous Two-Photon Absorption and Transient Triplet-Triplet Absorption in Platinum Acetylide Chromophores Kye-Young Kim,† Abigail H. Shelton,† Mikhail Drobizhev,‡ Nikolay Makarov,‡ Aleksander Rebane,*,‡,§ and Kirk S. Schanze*,† Department of Chemistry, UniVersity of Florida, P.O. Box 117200, GainesVille, Florida 32611, Physics Department, Montana State UniVersity, Bozeman, Montana, 59717, and National Institute of Chemical Physics and Biophysics, Tallinn, EE 12618, Estonia ReceiVed: January 20, 2010; ReVised Manuscript ReceiVed: May 16, 2010

A series of platinum-containing organometallic dimer complexes has been synthesized and the photophysical properties have been investigated under one- and two-photon (2PA) absorption conditions. The complexes have the general structure [DPAF-CtC-Pt(PBu3)2-CtC-Ar-CtC-Pt(PBu3)2-CtC-DPAF], where Ar is a π-conjugated unit, Bu ) n-butyl, and DPAF ) diphenylamino-2,7-fluorenylene. The core Ar units include 1,4-phenylene, 2,5-thienylene, 5,5′-(2,2′-bithienylene), 2,5-(3,4-ethylenedioxythiophene, 2,1,3-benzothiadiazole, and 4,7-dithien-2-yl-2,1,3-benzothiadiazole. Absorption and photoluminescence spectroscopy indicates that the complexes feature low-lying excited states based on both the core [-Pt(PBu3)2CtC-Ar-CtC-Pt(PBu3)2-] chromophore as well as the DPAF units. Photoexcitation of the complexes produces a singlet state excited state, which rapidly undergos intersystem crossing to afford a triplet state that has a lifetime in the microsecond time domain. In most cases, the lowest energy triplet state is localized on the core chromophore. Femtosecond 2PA spectra are measured along with triplet-triplet absorption spectra and nanosecond intensity-dependent transmission for solutions of the complexes. Each of the complexes features a 2PA absorption band in the near-infrared region (λ ≈ 700-750 nm) with a cross section 50-200 GM that is ascribed to the DPAF chromophore. The complexes also feature broad triplet-triplet absorption throughout the visible and near-infrared regions (λ ≈ 500-800 nm, TT ≈ 5-10 × 104 M-1 cm-1). Each of the complexes exhibits efficient nonlinear absorption of nanosecond pulses in the near-infrared region (600-800 nm), and we demonstrate that effect is most efficient in the chromophores where the 2PA cross section maxima coincides spectrally with the excited triplet state absorption. Introduction Designing chromophores for optical power limiting1–4 constitutes a challenging task because practical considerations imply several, often mutually contrasting, photophysical and photochemical properties. Organometallic compounds incorporating platinum (Pt) and other transition metals exhibit a combination of advantageous singlet-singlet and triplet-triplet absorption properties for application as nonlinear absorption chromophores.5–16 A high quantum yield for triplet state formation, ΦISC ≈ 1, is a key component of reverse saturable absorption (RSA), which is observed when a weak S0 f S1 one-photon absorption (1PA) from the ground state induces a much stronger T1 f Tn excited state absorption (ESA) in the triplet manifold.17–20 Heavy transition metal atoms such as Pt facilitate efficient singlet-triplet intersystem crossing (ISC), which in turn augments efficient formation of a metastable triplet state.9,21–27 However, because the population in T1 cannot build instantaneously, a purely 1PAinduced RSA is inefficient for nonlinear absorption on time scales shorter than ∼100 ps. In contrast, simultaneous twophoton absorption (2PA) has a practically instantaneous response * To whom correspondence should be addressed. K.S.S.: e-mail [email protected], Tel 352-392-9133. A.R.: e-mail rebane@physics. montana.edu, Tel 406-994-7831. † University of Florida. ‡ Montana State University. § National Institute of Chemical Physics and Biophysics.

Figure 1. Energy level diagram for a chromophore that exhibits both 2PA and triplet-triplet absorption.

time, and there are known chromophores with very large intrinsic 2PA cross section, σ2 ≈ 104 GM (1 GM ) 10-50 cm4 s).28–36 Figure 1 illustrates a Jablonski diagram for a modified RSA chromophore which combines 2PA with ESA arising from a long-lived triplet excited state.19,34,37 In this case, 2PA enhances the fast-time scale nonlinear absorption from the ground state, while the T1 f Tn absorption occurs on longer time scales. This “hybrid” or dual-mode nonlinear absorption scheme offers the

10.1021/jp1005567  2010 American Chemical Society Published on Web 06/10/2010

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CHART 1

possibility of nonlinear absorption over a broad range of pulse durations, potentially encompassing a time domain ranging from subpicosecond to microseconds.20 The outstanding challenge is, however, creating chromophores that simultaneously show sufficiently large ΦISC, large σ2, and large triplet-triplet 1PA cross section, σTT. For such molecules to be effective nonlinear absorbers, the maxima of both σ2 and σTT should coincide within the specified wavelength range of interest. Furthermore, the 1PA cross section from the ground state, σSS, should be small, because otherwise it may obstruct 2PA. We recently reported a series of Pt-acetylide complexes which feature two π-conjugated donor (D) or two acceptor (A) groups that end-cap a Pt-acetylide unit to form D-π-Pt-π-D or A-πPt-π-A chromophore systems.38 These chromophores exhibit efficient 2PA and strong triplet-triplet absorption. In the present work, we address the issue of spectral overlap between the transient triplet state absorption and the 2PA spectra in the series of dimeric platinum acetylide (Pt2) oligomers illustrated in Chart 1. In previous work it has been shown that the lowest triplet state (T1) wave function in Pt-acetylide dimers of the type [Ar′CtC-Pt(PBu3)2-CtC-Ar-CtC-Pt(PBu3)2-CtC-Ar′] is concentrated on the central unit: -[Pt(PBu3)2-CtC-Ar-CtCPt(PBu3)2]-.21,23–25,39 As a result, the transient triplet-triplet absorption is largely determined by the structure of the central arylene linker (Ar) in the Pt2 oligomers. For example, if an oligothiophene is used as Ar, then one could expect a systematic long-wavelength shift of triplet-triplet absorption and an increase in cross section σTT as the length of the oligothiophene unit increases.40–42 Another possibility would be to tailor the triplet state properties by varying the chemical nature of the central Ar unit.39,43 However, changing the electron accepting/ donating ability of the Ar group may also affect the strength of 2PA which, among other factors, depends on the square of the permanent dipole moment change upon excitation.44 The present study is focused on the series of Pt-acetylide dimers shown in Chart 1. These complexes are symmetrically substituted on each end with diphenylamino-2,7-fluorenylene (DPAF)29 chromophores which are linked via trans-Pt(PBu3)2linkages to a core arylene (Ar) unit. In the set of Pt2 complexes the structure of the core arylene unit is varied and includes 1,4phenylene (P1), 2,5-thienylene and 5,5′-(2,2′-bithienylene) (T1 and T2), 2,5-(3,4-ethylenedioxy)thienylene (EDOT), 2,1,3benzothiadiazole (BTD), and 4,7-dithien-2-yl-2,1,3-benzothiadiazole (TBTDT). The photophysical properties of the series are fully characterized, including one- and two-photon absorption spectra, photoluminescence, intersystem crossing, singlet oxygen sensitization yields, and triplet-triplet absorption

spectra. The photophysical properties are analyzed across the series and discussed in terms of their dependence on the conjugation length of the core chromophore, extent of charge transfer and the topology of the chromophores. The nonlinear absorption of the chromophores with respect to near-infrared nanosecond pulses is also investigated, and the efficiency is analyzed in terms of the product of the 2PA cross section and triplet-triplet absorptivity (σ2 × TT). The results clearly reveal the promise that this class of chromophores has in application as near-infrared nonlinear absorption and provides insight concerning the relationship of the excited state properties and the nonlinear absorption efficiency to nanosecond laser pulses. Experimental Section Synthesis. The details of the synthesis and structural characterization of the compounds that were the subject of the photophysical studies reported herein are provided in the Supporting Information. A synthetic scheme is provided that outlines the overall routes used to prepare the compounds. Photophysical Measurements. Photophysical studies were carried out with samples contained in 1 × 1 cm quartz or glass spectroscopic cuvettes. The sample solutions were prepared in THF and deoxygenated by bubbling argon, unless otherwise noted. Varian Cary 100 (University of Florida group) and Perkin-Elmer Lambda 900 (Montana State University group) dual-beamspectrophotometerswereusedforrecordingUV-visible absorption spectra. Corrected UV-visible steady-state emission measurements were performed on a SPEX F-112 fluorescence spectrometer equipped with a Hamamatsu R928 PMT detector. Sample concentrations were adjusted to produce optically dilute solutions, ODmax < 0.20. Luminescence emission quantum efficiencies of P1, T1, T2, EDOT and TBTDT were determined relative to a reference compound, Ru(bpy)3Cl2, with known emission quantum yield, ΦL ) 0.038 in air-saturated H2O.45 The emission quantum efficiency of BTD and TBTDT was measured relative to diphenylanthracene in cyclohexane (ΦL ) 0.90).46 The methods and apparatus used for transient triplet-triplet absorption measurements are described in detail elsewhere.47,48 Briefly, the excitation pulse energy was Ep ) 10 mJ, pulse duration τp ) 10 ns fwhm, at wavelength λ ) 355 nm (third harmonic of Continuum Surelite II-10 Nd:YAG laser). Sample concentrations were adjusted to approximately OD ) 0.8 at 355 nm. The sample solutions were placed in a continuously circulating flow cell holding a volume of 10 mL.47 Time-resolved emission measurements were performed using a Princeton Instruments PI-MAX iCCD detector coupled to an Acton SpectraPro 150 spectrograph.

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Intersystem crossing quantum yields (ΦISC) were determined using a home-built photoacoustic calorimetry (PAC) setup that was previously reported.49 The third harmonic of Nd:YAG laser was used as the excitation source. A solution of 2-hydroxybenzophenone in benzene with matched optical density OD ) 0.6 at the excitation wavelength 355 nm was used as a calorimetric reference sample. The beam was focused and passed through a 2.0 mm slit. To minimize spurious nonlinear effects, the data were collected only at low pulse energy, 0-30 µJ/pulse. The acoustic signal produced by the dissipation of the absorbed laser pulse energy was detected using a piezoelectric transducer with 1 MHz high-pass frequency response. The amplitude of the PAC signal was measured as a function of the laser pulse energy, and the linear slope coefficient R of the dependence was determined. The value of ΦISC was then obtained from the energy conservation relation50

Rs × hν ) (hν - ES) + (1 - ΦISC - ΦF)ES + Rref ΦISC(ES - ET) (1) where Rs and Rref are, respectively, the linear slope coefficients of the sample and the reference, hV is the photon energy (80.5 kcal, corresponding to 355 nm), ES is the energy of singlet excited state of the sample molecule, ET is the energy of the lowest triplet excited state, and ΦF is the fluorescence quantum yield. Note that eq 1 does not include the energy dissipated due to nonradiative decay of the T1 state. This is because the transducer in our experiment does not detect low frequencies 400 nm), because in these systems the contribution from the core is resolved in the spectra. The near-UV region of spectra of the BDT and TBTDT complexes are similar to those for the other set, indicating again that the transition is this region is dominated by the DPAF chromophores. These complexes are unique in that they also feature a lower-energy and weaker transition in the mid-visible region, at λ ≈ 490 and 551 nm for BTD and TBTDT, respectively.54 These lower energy transitions are charge transfer in nature. For the BTD complex the transition may have metal-to-ligand (Pt f BTD) charge transfer character.55 However, for the TBTDT complex the transition is likely a donor-acceptor transition localized on the TBTDT arylene unit.56 The one-photon excited photoluminescence spectra of the Pt2 complexes, along with those for L1 and L2 are also shown in Figure 2. The spectra of the Pt2 complexes are shown for airsaturated (black dashed lines) and argon purged (black solid line) solutions in THF. In addition, Table 1 provides a summary of the photophysical parameters. For the Pt2 complexes the fluorescence and phosphorescence is distinguished by comparing the photoemission in air-saturated and deoxygenated solutions because the phosphorescence is quenched in the presence of oxygen. For P1, T1 and EDOT the total emission is dominated by phosphorescence at λ > 500 nm; however, in each case some fluorescence is observed in region λ ≈ 400 - 450 nm. Interestingly, for T1 and EDOT two distinct phosphorescence bands are seen: one at λ ≈ 530 nm and the second appears at λ > 600 nm. (The longer wavelength emission band appears as two maxima due to vibronic coupling). This observation is consistent with triplet states that are localized on the DPAF chromophore (λ ≈ 530 nm) and the core chromophore.24 The notion that the higher energy band arises from the triplet on the DPAF chromophore is supported by the observation of phosphorescence from L2 at λ ≈ 530 nm. The emission of T2 is dominated by fluorescence from the core bithiophene chromophore, with only a weak phosphorescence emission at λ ≈ 700 nm. For the BTD and TBTDT complexes, only fluorescence emission is observed in the visible region, as the phosphorescence are expected well into the near-infrared region.39 The photoluminescence excitation spectra of L1, L2, P1, T1, EDOT, and T2 are independent of the emission detection wavelength, and generally follow the absorption spectral profile. In contrast, in air-saturated solution of BTD and TBTDT, we observed dual fluorescence, similar to that reported earlier in related Pt-acetylide compounds.38 In these two compounds, the emission spectrum changed as a function of the excitation wavelength in the λex ) 300-370 nm range. The short wavelength emission component occurred at λem ) 400-430 nm (thin line in Figure 2a) and corresponds with the fluorescence peak of L2 at 404 nm (Figure 2b). We conclude that the short wavelength component corresponds most probably to the excitation localized on the DPAF chromophore. The longwavelength emission component occurs in the range λem ) 530-750 nm and corresponds to the Ar core chromophore.54 The observation that the ligand and the core can be excited independently implies that in BTD and TBTDT the corresponding electronic systems are only weakly coupled in the ground state.

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Figure 2. Absorption (blue solid line) and emission spectra of air saturated (black dashed line) and argon deoxygenated (black solid line) solutions. (a) Pt2 series in THF solutions. Thin black line, short wavelength component of dual fluorescence in BTD and TBTDT air saturated solutions. (b) L1 in toluene and L2 in THF. Absorption spectra are simulated (dotted green line) as sum of Gaussians (thin solid green lines). Fluorescence excitation spectrum (tan color solid line). Emission spectra in air saturated (black dashed line) and deoxygenated (black solid line) solutions.

The phosphorescence emission quantum yields and phosphorescence lifetimes are presented in Table 1. The overall luminescence quantum yield in deoxygenated solutions decreases in the order of P1 > T1 > EDOT > T2. This trend is due to a decrease of the phosphorescence quantum yield, as the fluorescence is relatively weak in all cases. This trend is also supported by phosphorescence emission lifetimes which decrease in the same order: P1 (40 µs) > T1 (10 µs) > EDOT (4.6 µs) > T2 (4.5 µs). The correlation between the phosphorescence yields and lifetimes is consistent with the energy gap law, which states that the nonradiative decay rate generally increases as the excited state energy decreases for a series of

structurally related compounds.57 This trend has been observed in a variety of transition metal and organometallic systems, and more recently has been reported for a series of platinum acetylide chromophores.58 Taken together, the results indicate that for the P1, T1, EDOT, and T2 complexes, photoexcitation produces a triplet state with a comparatively high quantum yield, as the fluorescence is relatively weak. The triplet is localized mainly on the core Ar chromophore, yet the observation of weak DPAF phosphorescence suggests that the (higher energy) triplet state localized on this unit may be populated to a small extent in a thermal equilibrium. For BTD and TBTDT the fluorescence

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TABLE 1: One Photon Photophysical Propertiesa absorption

photoluminescence ΦLb

λmax, nm molecule L1 L2 P1 T1 EDOT T2 BTD TBTDT

λmax, nm 373 376 382 386 387 407 381 375 490 380 551

, 104 M-1 cm-1 3.81 ( 0.34 6.15 ( 0.28 16.8 ( 0.26 17.6 ( 0.11 12.6 ( 0.24 12.4 ( 0.21 10.9 ( 0.11 15.3 ( 0.12 3.63 ( 0.035 18.6 ( 0.13 4.6 ( 0.028

fluor.

phosp.

deoxy

τem, µs -

0.005 0.004 0.002

0.01 0.16 0.021 0.008

-

0.005 0.05

0.006 0.05

-

0.29

0.29

417 404 401 414 433

531 537 607 637

459 584

729

(434) 698 (430)

air -

40 10 4.6 4.5

-

All measurements carried out in THF solution unless otherwise noted. b Emission quantum efficiency was measured relative to Ru(bpy)32+ (ΦL ) 0.038 in air saturated water)45 and diphenylanthracene (ΦL ) 0.90 in cyclohexane).46 a

TABLE 2: Quantitative 2PA Data 2PA

a

molecule

λmax, nm

L1 L2 P1 T1 EDOT T2 BTD TBTDT

748 740 724 732 730 734 756 760

σ2(λmax), GM 125 ( 15 44 ( 4 88 ( 10 260 ( 30 190 ( 20 200 ( 20 230 ( 30 230 ( 40

(λ2PA/2), 104 M-1 cm-1

|∆µ b|, Debye

ELUMO, eVa

2.5 4.5 9.3 9.75 6.4 7.5 7.0 8.2

12 6.3 6.1 9.8 11 10.6 9.4 10.5

-2.15 -2.39 -2.51 -2.72 -3.06

LUMO energies obtained for a series of diphenylamine-end-capped oligoarylenes possessing the same Ar groups.56

emission is more efficient, suggesting that for these complexes intersystem crossing may be somewhat less efficient, especially for the latter. Nonetheless, in both cases the fluorescence yield is considerably less than unity, suggesting that intersystem crossing is still relatively efficient in these complexes as well. Two-Photon Absorption Spectra and Cross Sections. The ability of the Pt2 chromophores to absorb near-infrared light in an instantaneous 2PA process (Figure 1) was assessed by exciting solutions of the complexes with near-infrared pulses from a Ti:Sapphire pumped OPA system (pulse duration ≈ 100 fs). Quantitative 2PA excitation spectra were constructed by monitoring the intensity of the two photon excited fluorescence. Figure 3a shows the 2PA spectra of the Pt2 complexes and Figure 3b presents the 2PA spectra of L1 and L2 for comparison. All 2PA measurements were performed in airsaturated solutions. For BTD and TBTDT, which exhibit dual fluorescence, the 2PA excitation of both short wavelength components (circles) as well as long wavelength component (triangles) are presented. The quantitative 2PA data are collected in Table 2. It is practical to start by discussing the 2PA properties of the corresponding monomers. Dipolar molecules L1 and L2 show a well-defined 2PA peak at ∼740-750 nm and another, shorter wavelength transition at λ < 550 nm (see Figure 3b). The long-wavelength transition occurs in both L1 and L2 very close to twice the corresponding 1PA S0 f S1 band maximum wavelength (shown in blue plotted with wavelength ) 2λ1PA). The maximum 2PA cross section is σ2 ) 125 GM and 44 GM for L1 and L2, respectively. Similar 2PA spectra and cross sections were previously observed for other dipolar D-π-A molecules that also contain the diphenylamino group as a donor (D) and the fluorene unit as a π -linker.29,59 Specifically, compound AF-50, which contains the 2(4-vinyl)pyridyl electron

accepting group was measured to have the peak value, σ2 ) 30 GM at 796 nm,60 and AF-69 with the 2-benzothiazolyl acceptor features σ2 ) 72 GM at 770 nm.61 The fact that the 2PA excitation wavelength was far removed from any one-photon transitions, as well as the correspondence between the longest-wavelength peaks in 1PA and 2PA spectra serve as an indication that the 2PA in the S0 f S1 transition region can be quantitatively described with the two-level model.44 The expression for the corresponding maximum 2PA cross section is

σ2(νmax) )

4(1 + 2cos2 R) π103ln 10 f2 f 2 ε(νmax) |∆µ | 5 νmax pc2N n A

(4) where ∆µ b is the difference between the permanent dipole b and the moments in S1 and S0, R is the angle between ∆µ transition dipole moment vector, b µ, f is the Lorentz local field factor, f ) (n2 + 2)/3, n is the refractive index of the medium, p is the Planck constant, c is the speed of light, νmaxis the transition maximum frequency (in cm-1), and NA is Avogadro’s number. Equation 4 was previously verified in a number of different dipolar chromophores, including D-A linear systems,44 corroles62 and nonsymmetrical porphyrins,63 for the lowest energy dipole-allowed transition. This approach is also qualitatively confirmed by quantum-chemical calculations64 of related dipolar molecules. In the following, we will reverse this relation, i.e. we will determine |∆µ b| from the experimentally measured 2PA cross section and extinction coefficient values. It is interesting to note that even though the maximum extinction coefficient of monomer L2 is 1.6 times larger than that of L1, the latter exhibits ∼3 times larger maximum value

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Figure 3. Two photon absorption spectra of Pt2 complexes (a) and L1, L2 monomers (b). The 2PA-excited fluorescence was detected at 400-500 nm (circles) and, in case of dual fluorescence compounds BTD and TBTDT, at 500-700 nm (triangles). 2PA spectrum is simulated (dotted orange and magenta line) as sum of Gaussians (thin orange and magenta lines). 1PA transition spectrum in 2λ units (solid blue line) is shown for comparison.

for σ2. In the two-level approximation (4), this difference in σ2 can be explained if L1 has a larger |∆µ b|. In chemical terms, this would be consistent with the Si(CH3)3 (TMS) group being a better acceptor (or a poorer donor) compared to Pt(PBu3)2Cl. Indeed, although TMS is rather “neutral” with a Hammett constant σp ) 0.05, the Pt(PBu3)2Cl group is a moderate electron donor with σp ≈ -0.62.65 The moderate donor nature of the Pt(PBu3)2Cl unit likely arises due to d-π* backbonding. Thus, the fact that the Pt(PBu3)2Cl unit is a stronger electron donor relative to TMS is the likely origin of the lower σ2 value for L2. The 2PA spectra of the Pt2 complexes show generally similar features as the spectra of L1 and L2. The long-wavelength 2PA peak occurs in P1 (724 nm), T1 (732 nm), EDOT (730 nm) and T2 (734 nm) at wavelengths close to the corresponding S0 f S1 transition region (when expressed as 2λ1PA). In TBTDT, there are two distinct peaks (760 and 880 nm), depending on

which of the two fluorescence components is monitored. In the second dual-fluorescence compound BTD, these two peaks may be also present but are largely overlapping. In all cases, we identify the shorter wavelength peak to be similar to that of L2 (740 nm). These observations, together with the fact that the maximum σ2 value of P1 is almost exactly twice that of the monomer complex L2, suggest that the individual π-conjugated chromophores within the Pt2 complexes act independently in 2PA. Conformational disorder is intrinsic in Pt-acetylides,38 and may induce localization of the excited state wave function. Unlike L1 and L2, the 1PA S0 f S1 transition of Pt2 dimers is shifted to the red with respect to the 2PA maximum (when comparing the 2λ1PA spectrum with the 2PA spectrum, see blue lines in Figure 3a). However, the next higher energy 1PA transition, which is observed as a short-wavelength shoulder 1000-3500 cm-1 above the primary absorption band, coincides with the

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2PA transition maximum. The shift to higher energy for the 2PA transition can be explained if we consider that the strongest 1PA-allowed transition is to an excited state having odd symmetry which is delocalized symmetrically over the dimer. Such an odd symmetry transition, however, is forbidden for 2PA. Only the (doubly degenerate) state localized on one-half of the molecule (possibly on one DPAF chromophore), and thus lying higher in energy, will be allowed for 2PA. An alternative explanation could be that the 2PA peak corresponds to a vibronic replica of the pure electronic S0 f S1 transition and may become 2PA-allowed due to coupling to a low-symmetry vibration.9 If we assume, as described above, that the system is indeed due to two independently absorbing halves, then the measured 2PA cross section should be a sum of two equivalent dipolar molecules, each described by eq 4. On the other hand, the role of the core can be elucidated if we compare P1 and T1 (e.g., 1,4-phenylene vs 2,5-thienylene) and note that the peak cross section of the latter is ∼3-fold larger than that of P1. These circumstances can be used to clarify the relation between the σ2 and the electron donating/accepting abilities of the core Ar unit. Figure 3b shows that the 1PA spectrum of L1 and L2 can be simulated with a sum of a several Gaussian-shaped bands, with the lowest-frequency Gaussian peak coinciding with the lowest peak of the 2PA transition spectrum. We have applied a similar simulation to the 2PA spectra of the other complexes, and this allows us to determine the extinction coefficient in the 1PA spectrum at the wavelength coinciding to the transition frequency of the 2PA band maximum (column 4 in Table 2). Column 5 of Table 2 shows calculated ∆µ that are obtained by inserting the experimental values in (4) and assuming R ) 0. Li and co-workers recently reported a study of the optical and electrochemical properties of a series of organic conjugated oligomers that are remarkably similar in structure to the Pt2 series that is the subject of the present investigation. The reported series of organic oligomers feature an aromatic core unit (Ar ) 1,4-phenylene-, 2,5-thienylene, bithienylene, 2,1,3benzothiadiazole or 4,7-dithien-2-yl-2,1,3-benzothiadiazole) symmetrically end-capped with diphenylaminofluorene (DPAF) units.56 The electrochemical results on this series indicate that the energy of the LUMO decreases with the electron-accepting ability of the central core in the following sequence: P1 > T1 > T2 > BTD > TBTDT (values listed in column 6 of Table 2). Given the similarity in the structures of this series and the Pt2 complexes, we anticipate that the LUMO energies will follow the same trend. Interestingly, inspection of the ∆µ values in Table 2 shows the trend that the transition dipole generally decreases with ELUMO. This correlation is illustrated by the plot of ∆µ vs -ELUMO for five of the Pt2 complexes shown in Figure 4. Note that although ELUMO varies by as much as ∼1 eV across the series, the effect of ELUMO on ∆µ is muted, as the variation in the latter is relatively small across the series. This is likely due to a “shielding effect” due to the Pt center; in effect, the Pt complex is comparatively electron rich, and reduces the electron accepting ability of the core Ar units. With respect to the 2PA properties, the “shielding” effect of the Pt center means that despite the difference in electron donor strength of the core Ar units, the maximum σ2 values for P1, T1, T2 and EDOT are similar, ∼190-260 GM in the 730-760 nm region. Finally, it is interesting to note that in BTD and TBTDT 2PA also excites the longer-wavelength fluorescence corresponding to the CT-transition in the Ar core (triangles in Figure 3a). For BTD, the corresponding transition occurs at ∼870 nm as a shoulder on the long-wavelength side of the main 2PA band (750 nm), which, at least in part, can be due to excitation

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Figure 4. Correlation between ELUMO and permanent dipole moment difference, ∆µ (see text for explanation).

of the dipolar side group with subsequent energy transfer to the core. In TBTDT the core chromophore has a strong (300 GM) 2PA band centered at λ ≈ 880 nm. The existence of two separate 2PA bands is evidence for independent two-photon excitation of chromophores within the complex, followed by localization of the excitation on the core chromophore. Properties of the Triplet States in Pt2 Series. Nanosecond transient absorption spectroscopy was carried out for the Pt2 complexes in deoxygenated THF solution and the results are presented in Figure 5 and Table 3. The ground state 1PA and 2PA absorption spectra (black dashed lines and insets, respectively) are also displayed alongside the transient spectra in Figure 5 for comparison. In general, all of the complexes feature broad transient absorption in the visible region, peaking in the 600-700 nm region, with bleaching in the region corresponding to the singlet-singlet ground state absorption. The transient absorption decay lifetimes are listed in Table 3, and for most cases they correspond closely to the emission decay lifetimes (Table 1) supporting assignment of the transient absorption to a triplet-triplet (TT) transition from the lowest triplet excited state. The absolute absorptivity of the TT transition (TT) was determined for the series from the intersystem crossing yields (ΦISC) and transient absorption/relative actinometry and the values are also listed in Table 3. Comparison of the TT spectra for P1, T1, EDOT and T2 reveals subtle but significant differences in the spectral band shape and intensity which indicate that the lowest energy triplet state is localized on the core Ar chromophore, as opposed to the peripheral DPAF units. For example, it is notable that the position of the ground state bleach is red-shifted in EDOT and T2 relative to P1 and T1. (Recall that for these complexes the S0-S1 transition concentrated on the core Ar chromophore absorbs at longer wavelength compared to that for the DPAF units.) In addition, the oscillator strength of the TT transition is enhanced in T2 relative to that of the other complexes. This observation is in accord with previous studies, which show that the oscillator strength for the TT transition in thiophene oligomers increases with oligomer length.40,41 The TT spectra for BTD and TBTDT differ substantially from the other members of the series. In these cases, TT absorption extends into the near-infrared beyond 800 nm, and the bleach coincides with the ground state charge transfer transition in the mid-visible region. Again, the substantial difference in the TT spectra for

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Figure 5. Transient absorption difference spectra of Pt2 complexes in deoxygenated THF solutions following excitation at 355 nm (solid). Dashed lines show the ground state absorption spectra of the corresponding compounds. (Insets) Corresponding 2PA spectra in the same laser wavelength units.

TABLE 3: Summary of Transient Absorption Dataa triplet state absorption compound P1 T1 EDOT T2 BTD TBTDT

λmax, nmb 627 623 567 634 700 679

∆, 104 M-1cm-1c 6.07 ( 0.09 4.08 ( 0.23 4.67 ( 0.51 10.1 ( 0.9 1.87 5.08

ΦISCd

τT (deoxy)e

ΦSOg

0.91 ( 0.02 0.91 ( 0.02 0.87 ( 0.05 0.90 ( 0.03 0.82 ( 0.07f 0.74 ( 0.03f

17 µs 8.4 µs 3.6 µs 4.3 µs 3.2 µs 2.1 µs

0.049 ( 0.005 0.067 ( 0.005 0.052 ( 0.001 0.28 ( 0.03 0.49 ( 0.08 0.107 ( 0.005

a ΦISC was measured in deoxygenated benzene. All other measurements were in deoxygenated THF. b Absorption maximum of T1 f Tn transition. c Extinction coefficient of T1 f Tn transition measured relative to PE2.9 d Intersystem crossing yield measured by PAC relative to 2-hydroxybenzophenone. e T1 state lifetime in deoxygenated solution. f ΦISC of BTD and TBTDT were calculated using the estimated triplet energy (ET). For BTD, this energy was assumed to be equal to that of Pt-acetylide-BTD polymer, ET ≈ 1.49 eV.39 For TBTDT, ET was obtained by subtracting 0.8 eV from the measured ES energy (1.78 eV), resulting in ET ≈ 0.98 eV. g Quantum yield of singlet oxygen generation was measured in CDCl3 relative to acridine (ΦSO ) 0.95 in CDCl3).52

BTD and TBTDT indicates that the lowest triplet state is also localized on the core Ar chromophore in these systems as well. In order to further characterize the triplet state of the Pt2 complexes, the ability of the chromophores to sensitize the production of singlet oxygen (1O2) was determined. These experiments were carried out by monitoring the 1O2 emission at 1270 nm sensitized by the Pt2 complexes relative to that produced by using acridine as a sensitizer, and the resulting quantum yields are listed in Table 3. The yields increase in the order P1 < EDOT < T1 1.9, whereas those that are efficient sensitizers have triplet energies 100 µJ) across the wavelength region 550-800 nm. Detailed experiments were carried out at 600 nm for each of the four complexes, and the results are shown in Figure 6. As can be seen, each member of the series displays a marked nonlinear response for input pulse energies >100 µJ, with the transmittance decreasing with increasing input pulse energy. Interestingly, the nonlinear response varies in the order P1 < T1 ≈ EDOT < T2, with the latter displaying a markedly stronger nonlinear response at higher pulse energies (>1 mJ). The nonlinear absorption experiments were carried out at wavelengths which are well outside of the region where even weak 1PA is observed (due to S0-S1 or S0-T1 transitions). In this situation the nonlinear response of a given chromophore to nanosecond laser pulses is expected to be proportional to the product of the 2PA cross section (σ2) and extinction coefficient for absorption by any long-lived excited states (for the present systems this is TT). As can be seen by comparing the TT and 2PA spectra in Figure 5, the product σ2 × TT is considerable for each of the Pt2 complexes throughout the 550-800 nm region. In particular at 600 nm, the σ2 × TT product varies in the sequence P1 < EDOT < T1 < T2, which is consistent with the experimentally observed trend in nonlinear response for the series. It is important to note that the dominant factor which gives rise to the stronger nonlinear response for T2 is that the TT absorption has a 2-fold greater absorptivity compared to the other chromophores. This underscores the importance of the strength of the TT absorption in contributing to the nonlinear response of a chromophore system with respect to long laser pulses. Summary and Conclusions This study provides a detailed comparison of the photophysical properties of the series of Pt2 complexes, along with the

Kim et al. free DPAF chromophore (L1) and the monomeric Pt-DPAF complex (L2). The comparison provides insight into the relationship between the structure of the organometallic complexes and their photophysical properties under one- and twophoton excitation conditions. Each of the Pt2 complexes displays an intense absorption in the near-UV region due to a superposition of the π,π* transitions of the DPAF chromophore and the core Ar chromophore. In specific cases (T2, BDT and TBTDT) the absorption of the core Ar chromophore extends into the visible region. One-photon excitation of the Pt2 complexes initially produces a singlet state which rapidly undergoes intersystem crossing to afford a triplet state. Phosphorescence is observed from P1, T1, EDOT and T2 and the triplet energy varies in the order P1 > T1 > EDOT > T2. Phosphorescence is not observed from BDT and TBTDT, but given that these complexes absorb at considerably longer wavelength, their triplet energies are believed to be substantially lower than T2. Femtosecond 2PA spectra of the Pt2 series have maxima in the near-infrared region (600-1000 nm) with peak cross section values in the range, σ2 ) 88-260 GM. The 2PA absorption is believed to arise mainly due to the DPAF chromophores, and as expected the σ2 values for the Pt2 complexes are two or more times greater than the cross section of the reference compounds that contain only one DPAF moiety (L1 and L2). The 2PA in the lowest-energy transition was used to evaluate the permanent dipole moment difference between the ground and excited singlet state. The estimated values vary in the range, ∆µ ) 6.1-11 D, and they correlate well with the LUMO energy in this series. Nanosecond transient absorption spectroscopy reveals that all of the Pt2 complexes display broad and intense TT absorption that spans the visible and near-infrared regions of the spectrum. As expected, the complexes also display significant nonlinear absorption to nanosecond pulses in the 600-800 nm region. The nonlinear absorption arises due a combination of instantaneous 2PA combined with absorption by the long-lived triplet state. The nonlinear absorption response of the Pt2 complexes is correlated with the 2PA and TT absorption, and the results suggest that nonlinear absorption of nanosecond pulses is most efficient if the 2PA cross section maxima coincides spectrally with the excited triplet state absorption. Acknowledgment. This work was supported by the Air Force Office of Scientific Research (Rebane Grant FA9550-09-1-0219 and Schanze Grant FA9550-09-1-0186). Supporting Information Available: Detailed description of synthetic procedures and 1H, 13C, and 31P NMR spectra for key intermediates and final complexes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Organic Molecules for Nonlinear Optics and Photonics; Wei, H., Sence, M. J., van Stryland, E. W., Hagan, D. J., Eds., 1991; Vol. 194. (2) Bhawalkar, J. D.; He, G. S.; Prasad, P. N. Rep. Prog. Phys. 1996, 59, 1041–1070. (3) Spangler, C. W. J. Mater. Chem. 1999, 9, 2013–2020. (4) Shirk, J. S.; Pong, R. G. S.; Bartoli, F. J.; Snow, A. W. App. Phys. Lett. 1993, 63, 1880–1882. (5) Perry, J. W.; Mansour, K.; Marder, S. R.; Perry, K. J.; Alvarez, D.; Choong, I. Opt. Lett. 1994, 19, 625–627. (6) McKay, T. J.; Bolger, J. A.; Staromlynska, J.; Davy, J. R. J. Chem. Phys. 1998, 108, 5537–5541. (7) Staromlynska, J.; McKay, T. J.; Bolger, J. A.; Davy, J. R. J. Opt. Soc. Am. B, Opt. Phys. 1998, 15, 1731–1736. (8) McKay, T. J.; Staromlynska, J.; Davy, J. R.; Bolger, J. A. J. Opt. Soc. Am. B 2001, 18, 358–362.

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