Photophysical Properties of a Series of Electron-Donating and

Dec 20, 2010 - Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson Air Force Base, Ohio 45433, United States, UES...
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J. Phys. Chem. A 2011, 115, 265–273

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Photophysical Properties of a Series of Electron-Donating and -Withdrawing Platinum Acetylide Two-Photon Chromophores Joy E. Haley,†,‡ Douglas M. Krein,†,§ Jennifer L. Monahan,†,⊥ Aaron R. Burke,†,§ Daniel G. McLean,†,# Jonathan E. Slagle,†,# Albert Fratini,†,∇ and Thomas M. Cooper*,† Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson Air Force Base, Ohio 45433, United States, UES, Inc., Dayton, Ohio 45432, United States, General Dynamics Information Technology, Dayton, Ohio 45431, United States, SOCHE Student Research Program, Dayton, Ohio 45420, United States, Science Applications International Corporation, Dayton, Ohio 45431, United States, and Department of Chemistry, UniVersity of Dayton, Dayton, Ohio 45469, United States ReceiVed: May 19, 2010; ReVised Manuscript ReceiVed: NoVember 22, 2010

To explore spectroscopic structure-property relationships in platinum acetylides, we synthesized a series of complexes having the molecular formula trans-bis(tributylphosphine)-bis(4-((9,9-diethyl-7-ethynyl-9H-fluoren2-yl)ethynyl)-R)-platinum. The substituent, R ) NH2, OCH3, N(phenyl)2, t-butyl, CH3, H, F, benzothiazole, CF3, CN, and NO2, was chosen for a systematic variation in electron-donating and -withdrawing properties as described by the Hammett parameter σp. UV/vis, fluorescence, and phosphorescence spectra, transient absorption spectra on the fs-ps time scale, and longer time scale flash photolysis on the ns time scale were collected. DFT and TDDFT calculations of the T1 and S1 energies were performed. The ES and ET values measured from linear spectra correlate well with the calculated results, giving evidence for the delocalized MLCT character of the S1 state and confinement of the T1 exciton on one ligand. The calculated T1 state dipole moment ranges from 0.5 to 14 D, showing the polar, charge-transfer character of the T1 state. The ultrafast absorption spectra have broad absorption bands from 575 to 675 nm and long wavelength contribution, which is shown from flash photolysis measurements to be from the T1 state. The T1 energy obtained from phosphorescence, the T1-Tn transition energy obtained from flash photolysis measurements, and the tripletstate radiative rate constant are functions of the calculated spin density distribution on the ligand. The calculations show that the triplet exciton of chromophores with electron-withdrawing substitutents is localized away from the central platinum atom, red-shifting the spectra and increasing the triplet-state lifetime. Electrondonating substituents have the opposite effect on the location of the triplet exciton, the spectra, and the tripletstate lifetime. The relation between the intersystem crossing rate constant and the S1-T1 energy gap shows a Marcus relationship with a reorganization energy of 0.83 eV. The calculations show that intersystem crossing occurs by conversion from a nonpolar, delocalized S1 state to a polar, charge-transfer T1 state confined to one ligand, accompanied by conformation changes and charge transfer, supporting the experimental evidence for Marcus behavior. Introduction Development of two-photon absorbing materials has increased significantly because of their usefulness in various applications.1 There are a host of uses for two-photon absorbers that include use in optical data storage,2 frequency upconverted lasing,3 nonlinear photonics,4 microfabrication,5 fluorescence imaging,6 and photodynamic therapy.7 These materials provide an advantage by exciting in the lower-energy near-IR region, resulting in the inherent higher-energy photophysical properties of the chromophore. This is important to prevent damage to the material as a result of higher-energy photons. Also, the quadratic dependence of two-photon absorption on intensity causes photochemistry to occur in a small focal region, allowing for more control in microfabrication and imaging applications. * To whom correspondence should be addressed. † Air Force Research Laboratory. ‡ UES, Inc. § General Dynamics Information Technology. ⊥ SOCHE Student Research Program. # Science Applications International Corporation. ∇ University of Dayton.

Materials that possess good two-photon properties have been found to have either an asymmetrical D-π-A (donor-πconjugated group-acceptor) or a symmetrical D-π-A-π-D or A-π-D-π-A structural motif leading to a large change in polarization upon excitation.8 Using this design, many enhancements in the two-photon cross section values have been observed in the literature. It has been found that there are large differences in the two-photon cross section values dependent on the laser pulse width with the largest values found with a nanosecond laser.9 This enhanced loss in nonlinear transmission when using nanosecond pulses is attributed to contributions from the excited-state absorption.4b,10 We recently confirmed that this effect is indeed caused by absorption from both the singlet and triplet excited states.11 Previously, our efforts were focused on an understanding of structure property relationships of platinum polyynes.12-14 These materials rapidly undergo intersystem crossing to the triplet excited state upon excitation, leading to large nonlinearities due to excited-state population under nanosecond laser pulse excitation. In the literature, the platinum acetylides have been reported to possess two-photon absorption properties, but the cross

10.1021/jp104596v  2011 American Chemical Society Published on Web 12/20/2010

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Haley et al. two-photon absorption properties as well as the linear photophysical properties. The structures are shown in Figure 1. The compounds are named in boldface by their substituent. For example, the compound Pt-Fl-H is designated as H. Complexes with electron-donating or electron-withdrawing groups are designated as EDG or EWG. Substituent effects appear in the spectroscopic properties of these complexes. In this paper, we describe a comprehensive investigation of steady-state and time-resolved spectroscopy of these compounds. We found the T1 state energy and the energy of the T1-Tn transition is a function of the spin density distribution on the ligand. The intersystem crossing rate constant follows a Marcus relationship as the substituent changes from EDG to EWG. We will present two-photon spectroscopy data in a companion paper describing work done in collaboration with Aleks Rebane, Montana State University. Experimental Section Synthesis. We synthesized the compounds according to systematic variation in the Hammett σp parameter listed in Table 1. Synthesis of the ligands and complexes as well as X-ray diffraction data for H is given in the Supporting Information. General Techniques. Ground-state UV-vis absorption spectra were measured in a 1 cm quartz cuvette on a Cary 500 spectrophotometer. Corrected steady-state emission spectra were measured using a Perkin-Elmer model LS 50B fluorometer. Samples were placed in 1 cm quartz cuvettes, and the optical density was adjusted to approximately 0.1 at the excitation wavelength. Time-correlated single-photon counting (Edinburgh Instruments OB920 spectrometer) was used to determine fluorescence lifetimes. The fluorescence was excited using a 70 ps laser diode at 375 nm. The emission was detected using a cooled microchannel plate PMT. Data were analyzed using a deconvolution software package provided by Edinburgh Instruments. When needed, the samples were deoxygenated by three successive freeze-pump-thaw cycles. Ultrafast pump-probe transient absorption measurements were performed using a commercial femtosecond pump-probe UV-vis spectrometer (HELIOS, Ultrafast Systems LLC). Briefly, 1 mJ, 105 fs pulses at 800 nm at a 1 kHz repetition rate were obtained from a Ti:sapphire laser (Spectra Physics Hurricane). The output laser beam was split into pump and probe by a beam splitter. The pump beam was directed into a frequency doubler (400 nm) and then focused into the sample. The probe beam was delayed in a computer-controlled optical delay (Newport) and then focused into a sapphire plate to generate a white light continuum. The white light was then overlapped with the pump beam in a 2 mm quartz cuvette and

Figure 1. Chemical structure of electron-withdrawing and electrondonating platinum containing two-photon absorbing complexes. The R group denotes the name of the complex given above as Pt-Fl-R.

section values measured are rather small.15-18 Some modifications to the ligands have achieved larger two-photon cross section values, including a study of platinum-terminated polyynediyl chains.19 They found a large increase in the twophoton cross section with increasing number of alkynyl groups present between the two terminal platinum groups. In addition, efforts have been made to modify the ligand to include thiophene groups that have also shown some increase in two-photon cross section values.16 In the newer platinum(II)-containing materials, a large enhancement in the two-photon cross section was observed with the addition of ligands containing either an electron-withdrawing benzothiazole or an electron-donating diphenylamine.20 The benzothiazole material yielded the largest two-photon cross section in the region studied. Similarly, recent work in the literature include branched platinum complexes with various electron-withdrawing and electron-donating ligands.21,22 Overall, they found that the two-photon cross section values are found to be slightly dependent on the electron-richness of the peripheral substituents. On the basis of our initial findings and data from the literature, a series of platinum(II)-containing chromophores were synthesized with various strength electron-donating and electronwithdrawing groups to explore this effect of the ligand on the

TABLE 1: Absorbance and Emission Properties of EDG/EWG Pt Dyes in Benzene R

σpa

Absmax (nm)

ε (M-1 cm-1)

Flmax (nm)

Φfl

Es (eV)

Phmaxb (nm)

Φphb

ETc (eV)

NH2 OCH3 NPh2 t-butyl CH3 H F Bt CF3 CN NO2

-0.66 -0.27 -0.22 -0.20 -0.17 0.00 0.06 0.29 0.54 0.66 0.78

385 381 381, 399 381 381 381 380 388, 407 389 399 406

128600 166200 158000, 173300 142600 156900 159000 152600 153900, 148600 151300 136800 89500

397 411 411 394 410 395 394 427 407 418 517

0.014 0.010 0.027 0.007 0.009 0.006 0.005 0.036 0.009 0.019 0.030

3.17 3.19 3.05 3.20 3.18 3.20 3.20 2.97 3.12 3.02 2.76

559 554 564 553 553 553 553 575 559 566 573

0.008 0.022 0.012 0.025 0.024 0.024 0.016 0.009 0.020 0.010 0.006

2.28 2.29 2.26 2.29 2.29 2.29 2.29 2.21 2.27 2.24 2.22

a

Values taken from Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165. b Deoxygenated by freeze-pump-thaw. c Taken from the blue edge of phosphorescence data.

Photophysical Properties of Platinum Acetylide

Figure 2. UV/vis absorbance spectra of the chromophores in airsaturated benzene (25 °C). The top half shows EDG, and the lower half shows EWG. Molar absorbance coefficients are given for each.

then coupled into a CCD detector (Ocean Optics). Data acquisition was controlled by Surface Explorer Pro software (Ultrafast Systems LLC). The chirp effects on the spectra were corrected using the instrument response of benzene. Nanosecond transient absorption measurements were carried out using the third harmonic (355 nm) of a Q-switched Nd:YAG laser (Quantel Brilliant, pulse width of ∼5 ns). Pulse fluences of up to 1 mJ cm-2 at the excitation wavelength were typically used. A detailed description of the laser flash photolysis apparatus was published earlier.12 Fluorescence quantum yields were determined using relative actinometry, as previously described.12 Quinine sulfate was used as an actinometer with a known fluorescence quantum yield of 0.55 in 1.0 N H2SO4.23 Phosphorescence quantum yields were measured at room temperature using deoxygenated samples, as previously described.12 All samples were excited at 355 nm with a matched optical density of 0.1. Computational Chemistry. Calculations on the platinum complexes were done using Gaussian 03W, version 6.1.24 We performed DFT energy minimizations for both the ground state and the T1 state using B3LYP/LANL2DZ. To save computer time, the phosphine portion of the molecule was converted from tributyl phosphine to trimethyl phosphine. We then performed TDDFT calculations PBE1PBE/LANL2DZ(Pt),6-31G(d)(all other atoms). The energy ET was calculated by the ∆DFT method from previously minimized T1 and ground-state conformations using PBE1PBE/LANL2DZ(Pt),6-31G(d)(all other atoms). To analyze the spin distribution in the T1 state, a center of spin, defined to be the spin-weighted average distance from the central platinum atom, was calculated

〈RS-Pt〉 )

∑ SiRi i

∑ Si i

where the sum is over all atoms i having spin Si with a distance Ri from the central platinum atom. Results and Discussion Ground-State Absorption. Quantitative ground-state absorption spectra for all chromophores are shown in Figure 2. The top half represents the EDG and the lower half the EWG.

J. Phys. Chem. A, Vol. 115, No. 3, 2011 267

Figure 3. Normalized fluorescence spectra of EDG and EWG chromophores excited at 355 nm in air-saturated benzene. All data were obtained at room temperature (25 °C).

Wavelength maxima and molar absorption coefficients are given in Table 1. Assuming that H acts as a model neutral compound with minimal effect due to an end group, upon adding the electron-donating/withdrawing groups, we observe some rather dramatic changes in the spectral properties. In the EDG spectra, there is very little red shift with increased electron donation, except for NPh2, which shows 18 nm shift to the red. For the EWG spectra, a large red shift is observed with increased withdrawing strength. In addition, a trend is seen that the peak molar absorption coefficient decreases with increased withdrawing strength, which may be, in part, due to broadening of the spectra. There is moderate calculated charge transfer from the central platinum to the ligand upon excitation to the S1 state (Table 3). These results are consistent with our previous findings that the transition between the HOMO and LUMO levels is a mixture of MLCT and ππ* transitions.12,20 Because these are large chromphores, the MLCT character is small, but the S1 exciton is delocalized over the entire molecule. For NO2, the two-vibronic-peak band shape seen in the other complexes is almost completely lost or masked by spectral broadening, suggesting that the HOMO-LUMO transition has taken on more MLCT character. Emission. Figures 3 and 4 are the normalized fluorescence and phosphorescence spectra of all of the chromophores in benzene, respectively. Samples were excited at 355 nm in airsaturated benzene for the fluorescence and deoxygenated benzene for the phosphorescence. Fluorescence and phosphorescence quantum yields are given in Table 1. The fluorescence yield varies, Φfl ) 0.005-0.036, that is, it is much decreased relative to the corresponding value for the ligands taken separately (ligand data to be published separately). The fluorescence quantum yields listed in Table 2 show the trend Bt > NO2 > NPh2 > CN > NH2 > OCH3 ≈ CH3 ≈ CF3 ≈ CH3 > t-Bu ≈ H ≈ F. The phosphorescence quantum yields show a nearly opposite trend t-Bu ≈ CH3 ≈ H > OCH3 > CF3 > F > NPh2 > CN > Bt > NH2 > NO2. The neutral ligands, H, CH3, t-Bu, F, CF3, and OCH3, have average fluorescence and phosphorescence quantum yields of Φf(I) ) 0.008 ( 0.002, Φph(I) ) 0.022 ( 0.003, and 〈RS-Pt〉 < 9.5 Å. The low fluorescence and higher phosphorescence quantum yields result from stronger metal-ligand interaction. The EDG/EWG ligands, NH2, CN, NPh2, NO2, and Bt, have fluorescence and phosphorescence quantum yields of Φf(II) ) 0.025 ( 0.009, Φph(II) ) 0.009 ( 0.002, and 〈RS-Pt〉 > 9.5 Å. The relatively higher

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Figure 4. Phosphorescence spectra of Pt complexes in deoxygenated benzene upon 355 nm excitation. Data were normalized for comparison.

fluorescence and lower phosphorescence quantum yields suggest weaker metal-ligand interaction. A plot of fluorescence quantum yield versus 〈RS-Pt〉 (Figure 5a) shows the importance of spin-orbit coupling leading to intersystem crossing upon excited-state decay. The plot shows that the fluorescence quantum yield increases as the calculated T1 state spin distribution moves away from the central platinum atom. A plot of the phosphorescence quantum yield versus 〈RS-Pt〉 (Figure 5b) shows opposite behavior to Figure 5a, where the phosphorescence quantum yield decreases with increasing 〈RS-Pt〉, showing that strong spin-orbit coupling increases the phosphorescence quantum yield. Time-correlated single-photon counting (TCSPC) was used to measure the singlet-state lifetimes under air-saturated conditions. For all of the platinum complexes, the emission decays well within the instrument response function of less than 50 ps, except for NO2, which has a singlet lifetime of 73 ps. The TCSPC signal monitors the emission of the S1 state as it undergoes both radiative and nonradiative decay. The shortening of the lifetime is consistent with strong spin-orbit coupling of the platinum, which effectively promotes intersystem crossing to the triplet excited state. Transient Absorption. Because the S1 state lifetime is shorter than what could be measured with TCSPC, we used ultrafast transient absorption spectroscopy to measure the conversion of the S1 state to the T1 state. Figure 6 shows femtosecond transient absorption spectra immediately following the pump laser pulse. The data have been normalized at the peak for comparison. Most

Figure 5. (a) Plot of fluorescence quantum yield versus 〈RS-Pt〉 (Å). (b) Plot of phosphorescence quantum yield versus 〈RS-Pt〉 (Å).

EDG show a similar transient absorption, except for NH2 and NPh2, which have red-shifted bands. The data for the EWG show that there is a predominant peak at around 575 nm. Bt shows the most different spectrum with a broad peak at 645 nm. All chromophores show multiexponential decay of the excited state. The shorter lifetime (τS1) is ∼1 ps and corresponds to intramolecular vibrational relaxation (IVR) from upper excited electronic and vibrational levels. The longer lifetime (τS2) corresponds to decay from the singlet excited state via fluorescence, internal conversion, and intersystem crossing. The τS2 values are given in Table 2. As anticipated from the TCSPC data, the lifetimes are all less than 50 ps, except for NO2, which has a longer lifetime under air-saturated conditions. In the femtosecond transient absorption experiment, a longlived absorption tail with a lifetime greater than the 6 ns limit

TABLE 2: Summary of Transient Absorbance Data of EDG/EWG Pt Dyes in Benzene

a

R

S1-Sn,max (nm)

τS1(air) (ps)

τS2(air) (ps)

T1-Tn,max (nm)

τT(deoxy)a (µs)

NH2 OCH3 NPh2 t-Butyl CH3 H F Bt CF3 CN NO2

611 575 670 575 575 575 570 645 567 578 578

0.8 ( 0.3 2.2 ( 1.8 0.7 ( 0.3 0.8 ( 0.4 0.8 ( 0.5 0.5 ( 0.1 1.2 ( 0.7 3.0 ( 2.3 0.8 ( 0.4 1.0 ( 0.6 1.2 ( 1.2

20.9 ( 18.9 42.5 ( 19.4 23.4 ( 9.2 25.3 ( 19.3 28.5 ( 22.0 48.5 ( 17.7 30.3 ( 22.3 34.4 ( 12.7 12.1 ( 8.5 17.1 ( 3.7 109 ( 53

610 600 700 600 600 610 610 >800 670 750 510, >800

194 ( 35 172 ( 5 199 ( 9 197 ( 3 220 ( 13 185 ( 8 178 ( 7 199 ( 7 191 ( 2 199 ( 6 159 ( 9

Deoxygenated by freeze-pump-thaw.

Photophysical Properties of Platinum Acetylide

Figure 6. Femtosecond transient absorption spectra of EDG/EWG complexes in air-saturated benzene upon 400 nm excitation. Times shown are approximately time zero after the 100 fs laser pulse. Data have been corrected for chirp effects and normalized at the peak for comparison.

Figure 7. T1-Tn difference absorbance data observed immediately following nanosecond pulsed 355 nm excitation of each chromophore in deoxygenated benzene. Data have been normalized at the bleaching peak for comparison.

of the experimental technique appears for each chromophore that is due to triplet excited-state absorption (T1 f Tn). To better characterize the triplet excited-state properties, nanosecond laser flash photolysis was done on each chromophore, exciting at 355 nm with a 5 ns pulse. These data collected immediately following the laser pulse are shown in Figure 7 under deoxygenated conditions. The nanosecond transient absorption spectra coincide within experimental error with the long time data from the femtosecond pump-probe experiment. Oxygen quenching further confirms the assignment of this spectra to the triplet excited state. The triplet lifetimes obtained with nanosecond laser flash photolysis under deoxygenated conditions are presented in Table 2. The values fall around 200 µs with only little variation. It is interesting to note that there is a small red shift in the triplet spectra for the NPh2 but very little change for the other EDG. In the EWG, we see a very steady red shift with increasing strength of the electron-withdrawing group. For NO2, the peak maximum is well beyond 800 nm. Computational Chemistry. The series of chromophores synthesized in this study and given in Figure 1 represent a decreasing electron-donating strength moving into an increasing electron-withdrawing strength of the R group attached to the phenyl ring in the para position. They were uniquely designed to understand the effect of the substituent on both the one-photon

J. Phys. Chem. A, Vol. 115, No. 3, 2011 269 inherent photophysical properties as well as the two-photon nonlinear absorption properties. The literature has shown that molecules possessing charge transfer through D-π-A-π-D and A-π-D-π-A design exhibit exceptionally large twophoton absorption cross section values.25 On the basis of previous experimental findings and TDDFT calculations on similar platinum-containing two-photon absorbing chromophores, we believe that the central platinum is acting as an electron donor leading to more charge-transfer character in the EWG molecules.20,26,27 This is further confirmed by cyclic voltammetry data showing that the oxidation potential at the platinum center in a trans-bis(tri-n-butylphosphine)bis(phenylethynyl)platinum(II) molecule is 1.2 V versus Ag/AgCl.28 No reductive electrochemistry was observed. Therefore, the platinum complexes presented in this article either possess a D-π-D-π-D or an A-π-D-π-A structure dependent on the substituent. On the basis of this knowledge alone, we expect the moreelectron-withdrawing platinum complexes to possess larger twophoton cross section values. Previous investigations have shown the S0-S1 transition to have some MLCT character with the singlet exciton delocalized over the entire chromophore.14 Assuming that the chromophores have Ci symmetry, the ground state has Ag symmetry, and the S1 state has Au symmetry. A transition from the HOMO to the LUMO will have MLCT character due to the g symmetry of the platinum d orbital. From these data, the energies of the S0 f S1 (ES) and T1 f S0 (ET) transitions were determined and are given in Table 1. Table 3 lists calculated ES and ET values. Listed in Table 3 are the calculated changes in charge (∆Q) on the central platinum atom of the S1 and T1 states referenced to the ground state and the T1 state dipole moment. For all of the chromophores, ∆Q > 0, giving evidence for the MLCT character of the S1 state. There is considerably more charge-transfer character in the T1 state along with formation of a dipole. The result gives evidence for electron transfer from the platinum to a ligand occurring during intersystem crossing. We originally did the TDDFT calculations with the B3LYP functional. Although there is good agreement between calculated and experimental values for complexes having EDG, B3LYP gives large underestimates of the singlet-state energy for EWG complexes. We find that using the functional PBE1PBE improves the agreement between theory and experiment, although the results are not optimal. The TDDFT results show that for all of the complexes, the S1 state has Au symmetry and results from a HOMO to LUMO transition. Figure S2 in the Supporting Information shows a plot of calculated versus experimental ES and ET values. There is good correlation between the calculated and experimental values. The regressions are ES(expt) (eV) ) 1.5402 + 0.51282ES(calc)

ET(expt) (eV) ) 0.7196 + 0.6822ET(calc)

r ) 0.9877

r ) 0.9868

Even though the regressions reproduce the substituent effect in the singlet- and triplet-state energies, the slopes are less than unity, and there is a large intercept. The agreement between calculations and experiment is best for EDG. The calculations tend to underestimate the energies for EWG, but they reproduce the substituent effect on ordering of EWG S1 and T1 state energies. In particular, the calculations reproduce the S1 versus T1 order difference between NO2 and Bt, where ES(NO2) < ES(Bt) and ET(Bt) < ET(NO2). For the ES data, the trend

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TABLE 3: Results from DFT Calculations R

E(S1)a

CHfLb

∆QS1c

∆QT1d

µT1e

ETf

〈RS-Pt〉g

SDproxh

SDdisi

sumj

NH2 OCH3 NPh2 t-butyl CH3 H F Bt CF3 CN NO2

3.20 3.21 3.01 3.20 3.20 3.30 3.19 2.78 3.01 2.89 2.39

0.6105 0.6254 0.5725 0.6305 0.6317 0.6334 0.6385 0.6177 0.6431 0.6461 0.6796

0.011 0.010 0.008 0.015 0.016 0.013 0.018 0.009 0.018 0.014 0.015

0.024 0.030 0.027 0.033 0.033 0.036 0.038 0.017 0.041 0.039 0.043

0.51 2.56 1.32 3.60 3.56 4.01 4.33 6.54 7.14 8.06 14.3

2.28 2.30 2.25 2.29 2.30 2.30 2.30 2.19 2.29 2.22 2.20

9.89 9.27 10.0 9.14 9.05 9.07 8.98 14.6 9.57 10.3 12.3

1.497 1.560 1.472 1.571 1.574 1.583 1.587 0.536 1.496 1.387 1.016

0.450 0.363 0.472 0.350 0.335 0.345 0.332 1.479 0.434 0.556 0.949

1.947 1.923 1.944 1.921 1.909 1.928 1.919 2.015 1.930 1.943 1.965

a S1 state energy (eV). The S1 state has Au symmetry in all cases. b The largest CI coefficient (multiplied by 21/2) corresponds to the HOMO f LUMO transition. The remainder is a HOMO-1 f LUMO+1 transition. c Calculated change in charge of the central platinum atom upon excitation from the ground state to the S1 state. d Calculated change in charge of the central platinum atom of the minimum-energy T1 state conformation referenced to the ground state. e Calculated dipole moment (D) obtained from minimum-energy T1 state conformation. f T1 state energy (eV). g Spin-weighted average distance defined as 〈RS-Pt〉 ) ∑ SiRi/∑ Si, where the sum is over all atoms and Ri is the distance between atom i and the platinum atom in Ångstroms. h Sum of calculated spin densities on the CtC-fluorenyl-CtC-phenyl-X portion of the ligand. i Sum of calculated spin densities on the CtC-fluorenyl-CtC-phenyl-X portion of the ligand. j Sum (SDprox + SDdist) of calculated spin densities on the entire CtC-fluorenyl-CtC-phenyl-X ligand.

observed shows that there is a significant decrease in the S1 state energy with the addition of the electron-withdrawing groups. The range between maximum and minimum ES values for all of the complexes is 0.44 eV, while the range of ET values is 0.07 eV. The lower sensitivity of ET to change in substituent parallels the conclusions from previous studies of the platinum phenylacetylene complexes that show exciton delocalization of the singlet state throughout the molecule and confinement of the triplet exciton to one ligand.14,29 To increase understanding of the S1 and T1 states, various descriptors were calculated and evaluated. The values for the descriptors are listed in Tables S1 and S2 (Supporting Information) and Table 3. Using the data from Table S1 and that plotted in Figure S3 (Supporting Information), a regression of ES versus the calculated HOMO-LUMO gap gives the regression

ES(expt) (eV) ) 2.1211 + 0.3367EHOMO-LUMO (eV) r ) 0.9623 This result follows from the TDDFT calculation results in Table 3, where the S1 state is primarily a HOMO f LUMO transition. Regressions of ES versus EHOMO or ELUMO gave poor correlations. An alternative descriptor for the triplet state is the spin-philicity index ωS+.30 The spin-philicity index estimates the energy change upon increasing the spin multiplicity by 2, for example, the energy change upon conversion of the S0 state to the T1 state. A regression of the ET versus ωS+ plotted in Figure S3 (Supporting Information) gives the result

ET(expt) (eV) ) 1.636 + 0.2775ωS+ (eV)

r ) 0.9624

The spin-philicity descriptor is an alternate way to calculate E T. The platinum acetylide complex L1-Pt(PBu3)2-L2 has two ligands. Although L1 and L2 have the same molecular formula, previous work has shown that the singlet exciton is delocalized throughout the chromophore, and the triplet exciton resides on L2, with L1 being in the ground state.12-14 Shown in Table 3 is the calculated spin density on the CtC-fluorenylCtC-phenyl-X portion of the ligand, defined as SDprox, and the CtC-fluorenyl-CtC-phenyl-X portion of the ligand,

defined as SDdist. As there is a linear relationship between 〈RS-Pt〉 and SDdist

〈RS-Pt〉 ) 7.72 + 4.86SDdist

r ) 0.9968

either quantity can be used to describe the spin distribution in these complexes. For all of the complexes, the sum is SDprox + SDdist ≈ 2, giving evidence that the triplet exciton is confined to one ligand. By dividing the ligand into two portions, it is possible to determine substituent effects on the spin distribution throughout the ligand. For most of the complexes, the triplet exciton resides on the CtC-fluorenyl portion of the ligand. As the electron-withdrawing character of the substituent increases, SDprox decreases, and SDdist increases. In Bt, most of the spin resides almost entirely on the CtC-phenyl-X portion of the ligand, showing it to be a spin trap. Figure 8A and B shows plots of ET and E(T1fTn) versus SDdist. For both plots, EWG chromophores have increasing spin density on the CtC-phenyl-X portion of the ligand, resulting in a lower transition energy. As the intersystem crossing quantum yield φisc was not measured, Figure 9 shows a plot of the weighted triplet-state radiative decay rate constant, φisckT versus SDdist. This rate constant was calculated from the relation between the phosphorescence quantum yield and the triplet-state lifetime.

Φph ) ΦisckTτT The plot shows that the weighted rate constant decreases as SDdist increases. This is especially true for NO2 and Bt, where the strong electron-withdrawing character of these groups places the triplet exciton further away from the central platinum atom, decreasing the heavy atom effect on spin-orbit coupling, thereby decreasing the relative phosphorescence radiative rate constant. Complexes with EWG ligands all have a radiative decay rate less than 80 s-1 and tend to have a larger range of SDdist values from 0.4 to 1.5. Complexes with EDG ligands all have a radiative decay rate greater than 80 s-1 and SDdist values around 0.4. The intersystem crossing quantum yield has been shown to be nearly unity in smaller platinum acetylides PE1Pt, PE2-Pt, and PE3-Pt.12 The strong influence of the central platinum atom in the EDG complexes will most likely result in φisc approaching unity.

Photophysical Properties of Platinum Acetylide

J. Phys. Chem. A, Vol. 115, No. 3, 2011 271 applied the theory to describe electron-transfer processes in platinum complexes that occur during intersystem crossing. We used the ultrafast transient absorption results to measure the intersystem crossing rates. Table 2 lists S1 state lifetimes obtained from these experiments. The faster component τ1 is associated with vibrational cooling and solvent relaxation, while the slower component τ2 is associated with fluorescence, intersystem crossing, and internal conversion. From Table 1, the fluorescence quantum yield, given by

φfl )

kr kr + knr

is measured to be no more than 0.036, allowing the simplification knr . kr. From this result, τ2 primarily measures nonradiative processes. The nonradiative decay rate constant knr has contributions from intersystem crossing and internal conversion.

knr ) kic + kisc

Figure 8. (A) Plot of ET versus SDdist; (B) plot of E(T1-Tn) versus SDdist.

Figure 9. Plot of the weighted T1 state radiative decay rate constant versus SDdist.

Our results give evidence for electron transfer from a nonpolar S1 state to a charge-separated T1 state during intersystem crossing. Previously published work has shown that the S1 state is delocalized through the central platinum atom while the T1 state is confined to one ligand.12-14 These chromophores are centrosymmetric, with the nonpolar S1 state having Au symmetry. In contrast, the triplet exciton is confined to one ligand and possesses a dipole moment, and there is charge transfer from the central platinum atom to the ligand (Table 3). The evidence suggests that Marcus theory may be useful for understanding nonradiative processes in these compounds.31 We

From the measurement of τ2 and φfl given in Table 1, knr was calculated. From consideration of the energy gap law for internal conversion,32 kic is considered to be negligible for these compounds; therefore, knr ≈ kisc. The intersystem crossing times for these compounds range from 12.1 to 109 ps. In comparison, these intersystem crossing times are considerably longer than those published for smaller platinum acetylides PE1-Pt, PE2-Pt, and PE3-Pt.33 The tripletstate relaxation process in these compounds follows a two-step mechanism with a rapid intersystem crossing followed by a slower thermal relaxation to the equilibrated T1 state. Upon increasing the ligand length of these three compounds, τisc follows the trend 70 fs, 480 fs, and 2.1 ps, while the thermal relaxation time τth follows the trend 4.1, 4.8, and 9.5 ps. PE1Pt has such a rapid intersystem crossing rate that no conformation changes occur during conversion to the T1 state. The longer intersystem crossing times in the larger chromophores PE2-Pt and PE3-Pt allow for conformation changes to occur during conversion to the T1 state. In contrast, intersystem crossing in the current compounds occurs more slowly and by one step, showing that intersystem crossing occurs along with significant conformation changes and solvent reorganization. The differences are attributed to the larger ligand size leading to weaker spin-orbit coupling effects from the central platinum atom. The Marcus theory of electron transfer describes mechanisms governed by the parameter Q ) HDA/λ.34 The parameter HAD is an electronic coupling element between the donor and the acceptor. In this work, the donor is the S1 state, the acceptor is the T1 state, and HAD is the spin-orbit coupling Hamiltonian. The reorganization energy parameter λ ) λo + λi is the barrier for electron transfer consisting of the intramolecular inner-sphere component λi and outer sphere component λo. The theory describes three domains. Under weak coupling conditions, Q , 1, and the rate of electron transfer is dominated by the reorganization energy. Under strong coupling conditions, Q .1, and the rate of electron transfer is dominated by HAD. In PE1Pt and PE2-Pt, intersystem crossing is rapid, and strong coupling governs the rate. Under intermediate coupling conditions Q ≈ 1, Marcus theory predicts a Gaussian dependence of kisc on ∆EST · 34d,e

kisc ) Ae-(λ

- ∆EST)2/4λRT

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Figure 10. Plot of knr versus ∆EST. The data were fitted to a Gaussian function.

Figure 10 is a plot of knr versus ∆EST. The data show a Gaussian dependence with the reorganization energy λ calculated to be 0.83 eV and gives evidence that intersystem crossing in these compounds behaves according to Marcus theory under intermediate coupling conditions. Summary and Conclusions In this paper, we describe the synthesis of a series of platinum acetylide complexes having a systematic variation in electrondonating and -withdrawing strength. The complexes have been characterized by linear and time-resolved spectroscopy. The TDDFT calculations reproduce the substituent effect of the ground-state absorption and phosphorescence. There is a strong dependence of triplet-state spectroscopic behavior on the spin density distribution of the triplet exciton. EDG chromophores have triplet excitons confined to the CtC-fluorenyl portion of the ligand, resulting in blue-shifted spectra and more rapid radiative decay. EWG chromophores have more spin density on the CtC-phenyl-X portion of the ligand, resulting in redshifted spectra and slower radiative decay. Analysis of the dependence of the intersystem crossing rate constant on ∆EST reveals a Marcus-type behavior. The behavior is consistent with the nonpolar S1 state being delocalized through the central platinum atom, while the polar T1 state is confined to one ligand. This work is a baseline study to assist in interpreting the twophoton spectra of these complexes. A detailed study of the twophoton spectra of these complexes will be presented in a future publication. Acknowledgment. We acknowledge the support of this work by AFRL/RX Contracts F33615-99-C-5415 for D.G.M., F3361503-D-5408 for D.M.K. and A.R.B., and F33615-03-D-5421 for J.E.H. and J.E.S. The authors thank Abby Shelton from Kirk Schanze’s group, University of Florida, for collection of phosphorescence spectra. Supporting Information Available: Complete synthesis procedures, analytical data, NMR data as well as an X-ray crystallographic file (CIF) of H. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) He, G. S.; Tan, L.-S.; Zheng, Z.; Prasad, P. N. Chem. ReV 2008, 108, 1245. (b) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Angew. Chem. 2009, 48, 3244.

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