Driving Force Dependence of Electron Transfer from Electronically Excited [Ir(COD)(μ-Me2pz)]2 to Photo-Acid Generators

Article abstract of DOI:10.1021/acs.jpca.7b07777

We report the rates of electron transfer (ET) reactions of electronically excited [Ir(COD)(μ-Me₂pz)]₂ with onium salt photoacid generators (PAGs). The reduction potentials of the PAGs span a large electrochemical window that allows determination of the driving force dependence of the ET reactions. Rate constants of ET from electronically excited [Ir(COD)(μ-Me₂pz)]₂ to onium PAGs are determined by the reaction driving force until the diffusion limit in acetonitrile is reached.

Full text of DOI:10.1021/acs.jpca.7b07777

Article
pubs.acs.org/JPCA
Driving Force Dependence of Electron Transfer from Electronically
Excited [Ir(COD)(μ-Me2pz)]2 to Photo-Acid Generators
Wesley Sattler,†,‡ Aaron A. Rachford,*,§ Paul J. LaBeaume,§ Suzanne M. Coley,§ James W. Thackeray,§
James F. Cameron,§ Astrid M. Muller,‡ Jay R. Winkler,*,‡ and Harry B. Gray*,‡
̈
†The Dow Chemical Company, Formulation Science, Core R&D, 400 Arcola Road, Collegeville, Pennsylvania 19426, United States
§The Dow Chemical Company, Dow Electronic Materials, 455 Forest Street, Marlborough, Massachusetts 01752, United States
‡Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States
ABSTRACT: We report the rates of electron transfer (ET) reactions of electronically excited [Ir(COD)(μ-Me2pz)]2 with
onium salt photoacid generators (PAGs). The reduction potentials of the PAGs span a large electrochemical window that allows
determination of the driving force dependence of the ET reactions. Rate constants of ET from electronically excited
[Ir(COD)(μ-Me2pz)]2 to onium PAGs are determined by the reaction driving force until the diffusion limit in acetonitrile is
reached.
employing electronically excited [Ir(COD)(μ-Me2pz)]2 (COD
= 1,5-cyclooctadiene; Me2pz = 3,5-dimethylpyrazolyl) as an
electron donor (Figure 2). To facilitate comparisons with
previous work, our investigations were performed in acetoni-
trile solution.18
INTRODUCTION
■
State-of-the-art photolithography utilizes onium salts that
decompose to produce strong acids upon exposure to light.
The strong acid reacts with acid sensitive functional groups in a
polymeric matrix, creating a solubility switch and ultimately
producing a lithographic pattern. Such lithographic patterning
schemes are important as they are used to manufacture
advanced integrated circuits for semiconductor chips.1,2 The
onium salt photoacid generators (PAGs, Figure 1) used in
photolithography have been the subject of much research
owing to the ability to tune their properties to the different
wavelengths typically used in modern optical lithography (248
and 193 nm). Extreme ultraviolet (EUV) lithography at 13.5
nm has been identified as a leading candidate for next-
generation high-resolution patterning technology.3
PAGs have been studied in detail concerning their
mechanism(s) to produce acid upon direct photolysis and
sensitization.4−7 Electron transfer to PAG has been proposed to
be one excited-state pathway that leads to the production of
acid. This pathway may become more important in EUV
lithography, as 13.5 nm light (91.8 eV) is proposed to produce
high energy photoelectrons which in turn thermalize upon
interaction with various components of a photoresist, including
the PAGs.8−17 While a correlation has been proposed for the
acid generation efficiency to the reduction potential of
PAGs,10,11 to date, no studies have been conducted to
understand which factors determine the rates at which PAGs
are reduced. Therefore, we investigated PAG reduction by
RESULTS AND DISCUSSION
■
Absorption and luminescence spectra of [Ir(COD)(μ-Me2pz)]2
are shown in Figure 3. As previously reported, the absorption
spectrum reveals an intense peak at λmax = 525 nm (ε ∼ 8.7 ×
103 M−1 cm−1).18,19 This feature is accompanied by a less
intense absorption at ∼390 nm (ε ∼ 1390 M−1 cm−1) and a
higher energy π−π* system (below 250 nm). Upon excitation
at 530 nm, the iridium complex luminesces in solution (Figure
3). There are two broad components in the emission spectrum:
the less intense feature is at ∼575 nm (Stokes shift ∼1660
cm−1), and the more prominent component is centered near
705 nm (Stokes shift ∼4865 cm−1). Note that the excitation
spectra that give rise to both emission features (λem = 575 and
705 nm) are virtually identical, confirming that both are
attributable to a single molecular species. The 575 nm emission
1
has been assigned to B2 fluorescence and the 705 nm feature
3
to B2 phosphorescence. The quantum efficiencies are 0.0001
Received: August 4, 2017
Revised: September 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.jpca.7b07777
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The Journal of Physical Chemistry A
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Figure 1. PAG structures. The anion (not shown) is perfluorobutanesulfonate (PFBUS) in all cases.
Figure 4. Modified Latimer diagram (vs SSCE) for [Ir(COD)(μ-
Me2pz)]2.
Figure 2. Structure of [Ir(COD)(μ-Me2pz)]2.
Figure 5. Cyclic voltammograms of 1−4 in 0.1 M tetrabutylammo-
nium perchlorate acetonitrile solution.
shown in Figure 6; the quenching rate constant from Stern−
Volmer analysis is 6.7 × 109 M−1 s−1. Dynamic quenching
constants of 2.0 × 106, 4.2 × 108, and 3.3 × 109 M−1 s−1 were
determined for PAGs 1−3, respectively. Importantly, sulfonium
cations have been shown to efficiently quench triplet excited
states through triplet-to-triplet energy transfer.22 However,
energy transfer is ruled out as a quenching mechanism, as the
PAG excited triplets are energetically well above 3[Ir(COD)(μ-
Me2pz)]2*. Thus, the observed quenching processes are
attributable to reduction of the PAGs.
Quenching rate constants vs PAG cathodic peak potentials
are plotted in Figure 7. Data in blue circles are from this work;
the data in red squares are for reactions with pyridinium
acceptors.18 Notably, the two data sets are in strikingly good
agreement: both establish that ET rates at higher driving forces
are near the diffusion limit.
Figure 3. Room temperature absorption and luminescence spectra of
[Ir(COD)(μ-Me2pz)]2 in acetonitrile solution.
and 0.0038, respectively (relative to [Ru(bpy)3][PF6]2 in
CH3CN: ΦPL = 0.062).20
Cyclic voltammetry of [Ir(COD)(μ-Me2pz)]2 in CH3CN
solution has been reported previously.21 One-electron oxidation
occurs at 0.28 V, and the first one-electron reduction is at
−2.51 V vs SCE (saturated calomel electrode).
Combining luminescence data with electrochemical data
reveals the reduction potential of the iridium excited state
(Figure 4). The (cation/excited state) reduction potential is ∼
−1.6 V vs SSCE (saturated standard calomel electrode).
Although the methylated complex may not produce enough
driving force for reduction of the most negative onium PAG, it
provides a good trade-off between reducing power and
molecular stability for our work.
Cyclic voltammetry of the onium PAGs reveals irreversibility
upon one-electron reduction (Figure 5). The electrochemical
irreversibility of all PAGs is maintained based on the scan rate
dependence (up to 6 V/s). This indicates that the chemistry
ensuing reduction is fast on the electrochemical time scale. The
resultant decomposition following electrochemical reduction
affords the strong acid required for resist patterning. The four
PAGs utilized in this study have the following reductive peak
potentials: (1) Epc = −1.83; (2) Epc = −1.50; (3) Epc = −1.14;
(4) Epc = −0.52 V. All electrochemical values were adjusted to
an SSCE reference for comparison with previous work.
Luminescence lifetime quenching of 3[Ir(COD)(μ-
Me2pz)]2* by DBI-PFBUS (4) in acetonitrile solution is
CONCLUSIONS AND OUTLOOK
■
We established that a series of onium PAGs employed in
lithographic technologies can be reduced by electronically
excited [Ir(COD)(μ-Me2pz)]2. ET rate constants are deter-
mined by the reaction driving force until the diffusion limit in
acetonitrile is reached. Future work will help determine if the
rate of PAG reduction is a crucial component in EUV
lithography.
EXPERIMENTAL SECTION
■
[Ir(COD)(μ-Me2pz)]2 was prepared according to a literature
method and gave satisfactory analytical data.23 All chemicals
were used directly from the supplier. Nuclear magnetic
B
DOI: 10.1021/acs.jpca.7b07777
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The Journal of Physical Chemistry A
Article
−122.53 (2F), −126.90 (2F). HRMS cation (ESI), m/z
C30H39S (M)+ calcd 263.1, obsd 263.4. HRMS anion (ESI),
m/z C4F9O3S− calcd 299.0, obsd 299.1.
1
DTBPI-PFBUS (3): H NMR (500 MHz, (CD3)2CO) δ:
8.27 (d, J = 8 Hz, 4H), 7.61 (d, J = 8 Hz, 4H), 1.30 (s, 18H).
13C{1H} NMR cation (500 MHz, (CD3)2CO) δ: 156.51,
135.74, 129.59, 110.97, 35.15, 30.57. 19F NMR (300 MHz,
(CD3)2CO) δ: −82.32 (3F), −115.79 (2F), −122.53 (2F),
−126.94 (2F). HRMS cation (ESI), m/z C30H39S (M)+ calcd
393.1, obsd 393.7. HRMS anion (ESI), m/z C4F9O3S− calcd
299.0, obsd 299.1.
1
DBI-PFBUS (4): H NMR (500 MHz, (CD3)2CO) δ: 8.35
3
Figure 6. Luminescence quenching of [Ir(COD)(μ-Me2pz)]2* by 4
(dd, J = 1, 8 Hz, 2H), 8.30 (d, J = 8 Hz, 2H), 7.84 (t, J = 8 Hz,
2H), 7.69 (dt, J = 1, 8 Hz, 2H). 13C{1H} NMR cation (500
MHz, (CD3)2CO) δ: 142.40, 131.57, 131.37, 131.17, 127.43,
121.10. 19F NMR (300 MHz, (CD3)2CO) δ: −82.36 (3F),
−115.48 (2F), −122.49 (2F), −126.94. HRMS cation (ESI),
m/z C30H39S (M)+ calcd 279.0, obsd 279.2. HRMS anion
(ESI), m/z C4F9O3S− calcd 299.0, obsd 299.1.
(black = 0.00 mM; red = 0.37 mM; blue = 0.46 mM; cyan = 0.88
mM). Inset: Stern−Volmer plot.
Photochemistry. UV−visible absorption measurements
employed a Cary 50 UV−vis spectrophotometer with 1 cm
path length quartz cuvettes. Steady-state and time-resolved
spectroscopic data were collected in the Beckman Institute
Laser Resource Center (California Institute of Technology).
Emission and excitation spectra were recorded on a Jobin Yvon
Spec Fluorolog-3-11. Sample excitation was achieved via a
xenon arc lamp with wavelength selection provided by a
monochromator. Right angle luminescence was sorted using a
monochromator and detected with a Hamamatsu photo-
multiplier tube (PMT) with photon counting (PMT model
R928P).
Figure 7. Rate constant vs quencher cathodic peak potential (blue
circles, this work; red circles, data from refs 18 and 19). Dotted line is
the diffusion limit in acetonitrile solution. The [Ir(COD)(μ-
+
Me2pz)]2 /3[Ir(COD)(μ-Me2pz)]2* potential is −1.6 V vs SSCE.
For time-resolved measurements, laser excitation was
provided by 8 ns pulses from a Q-switched Nd:YAG laser
(Spectra-Physics Quanta-Ray PRO-Series) operating at 10 Hz.
The third harmonic was used to pump an optical parametric
oscillator (OPO, Spectra-Physics Quanta-Ray MOPO-700)
tunable in the visible region to provide laser pulses at 525 nm.
Scattered excitation light was rejected by suitable long pass and
short pass filters, and luminescence wavelengths were selected
for detection by a double monochromator (Instruments SA
DH-10) with 1 mm slits. Light was detected with a PMT
(Hamamatsu R928). The PMT current was amplified and
recorded with a GageScope transient digitizer. Data were
averaged over approximately 100 shots. All instruments and
electronics in these systems were controlled by software written
in LabVIEW (National Instruments). Data manipulation was
performed with either MATLAB R2012a or MATLAB R2013a
(Mathworks, Inc.).
Electrochemistry. Cyclic voltammograms were collected in
a one compartment cell with a Pt working electrode (BASi,
MF-2013), Pt wire auxiliary electrode (BASi, MW-4130), and
Ag/AgCl reference electrode (BASi, MF-2052). Tetrabutyl
ammonium perchlorate (0.1 M, >99%, Sigma-Aldrich) in
acetonitrile (HPLC grade, Sigma-Aldrich) solution was the
electrolyte for all electrochemical experiments. Caution!
Perchlorate salts are potentially explosive and should be
handled with care.
resonance (NMR) spectra for all compounds were obtained
either on a 300 or 500 MHz spectrometer. The chemical shifts
are reported in δ (ppm) values relative to internal TMS
(tetramethylsilane) for 1H NMR, and the solvent for 13C NMR.
19F NMR chemical sifts are reported in ppm relative to CFCl3
(δ = 0) and were referenced using C6F6 (δ = −164.9) as an
external standard. Multiplicities are indicated by s (singlet), d
(doublet), m (multiplet). Coupling constants, J, are reported in
Hertz (Hz).
All onium−PFBUS salts were prepared similarly to that given
for 1 (below) using the appropriate onium halide.
TBTPS-PFBUS (1): Tris(4-(tert-butyl)phenyl)sulfonium
bromide (5.00 g, 9.77 mmol, 1.0 equiv) and potassium
perfluorobutanesulfonate (3.64 g, 10.8 mmol, 1.1 equiv) were
dissolved in dichloromethane (100 mL) and water (100 mL)
and vigorously stirred at room temperature for 18 h. The layers
were separated, and the organic phase was washed with water
(5 × 50 mL, 250 mL total) and concentrated under reduced
pressure to afford the title compound (6.71 g, 85%) as a white
1
solid. H NMR (500 MHz, (CD3)2CO) δ: 7.84−7.88 (m,
12H), 1.35 (s, 27H). 13C{1H} NMR cation (500 MHz,
(CD3)2CO) δ: 158.62, 131.33, 128.93, 122.33, 35.38, 30.52. 19
F
NMR (300 MHz, (CD3)2CO) δ: −82.28 (3F), −115.66 (2F),
−122.45 (2F), −126.86 (2F). HRMS cation (ESI), m/z
C30H39S (M)+ calcd 431.3, obsd 432.0. HRMS anion (ESI),
m/z C4F9O3S− calcd 299.0, obsd 299.1.
Prior to each experiment, the Pt working electrode was
thoroughly cleaned and polished with an alumina slurry, rinsed
with distilled water, and dried. The electrolyte solution was
checked for contamination of electrochemically active species
by conducting a cyclic voltammetry experiment prior to
addition of the PAG analyte, sweeping across an electro-
TPS-PFBUS (2): 1H NMR (500 MHz, (CD3)2CO) δ: 7.89−
7.96 (m, 9H), 7.83 (t, J = 8 Hz, 6H). 13C{1H} NMR cation
(500 MHz, (CD3)2CO) δ: 134.95, 131.87, 131.62, 125.27. 19F
NMR (300 MHz, (CD3)2CO) δ: −82.36 (3F), −115.70 (2F),
C
DOI: 10.1021/acs.jpca.7b07777
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: arachford@dow.com.
*E-mail: winklerj@caltech.edu.
*E-mail: hbgray@caltech.edu.
■
ORCID
Wesley Sattler: 0000-0001-5316-260X
Jay R. Winkler: 0000-0002-4453-9716
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was supported in part by the National Science
Foundation Center for Chemical Innovation in Solar Fuels
(Grant CHE-1305124), Center for Chemical Innovation
postdoctoral fellowship to W.S. This work was also funded in
part by The Dow Chemical Company through a university
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