sunlight photodepolymerization of transient...

Sunlight photodepolymerization of transient polymers

O. Phillips, A. Engler, J. M. Schwartz, J. Jiang, C. Tobin, Y. A. Guta, P. A. KohlSchool of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100Correspondence to: P. A. Kohl (E-mail: [email protected])

ABSTRACT: The study and development of transient devices is an emerging field where the disposal of a device after use is desired to avoidreverse engineering and minimize the environmental impact. Polyaldehydes with phototriggers have been investigated because the radiationwavelength can be adjusted to meet the transient application. Polynuclear aromatic hydrocarbons (PAHs) were used as the optical sensitizerfor photoacid generators (PAGs). Photoinduced electron transfer (PET) with an iodonium-based PAG was used to expand the spectral sensitiv-ity range. Anthracene, tetracene, and pentacene derivatives were synthesized with appended phenylethynyl groups to improve the solubility ofthe sensitizer and adjust the absorption wavelength. Sensitization of the iodonium-based PAG with the PAH derivatives was found to havethermodynamically favorable PET reactions for depolymerization of poly(propylene carbonate) and poly(phthalaldehyde) (PPHA). The Rehm–Weller equation and Stern–Volmer analysis were used to study the electron transfer and the fluorescence quenching rates of the PAHs withthe iodonium salts, respectively. The photosensitivity, efficiency, and byproducts of the PET reactions in the decomposable polymer films arereported. A rapid photoreaction is reported for the depolymerization of PPHA exposed to a sunlight dose of

of the onium salts is limited to the UV portion of the electromag-netic spectrum making them insensitive to sunlight. Sensitizingthe onium salts to longer wavelengths of light makes a more effi-cient photolysis in sunlight applications.38,39

The goal of this study is to extend the spectral sensitivity of PAGto longer wavelengths of light via photoinduced electron transfer(PET).36 A simplified reaction scheme, Scheme 1, uses MtXn—asthe weakly nucleophilic counterion [e.g., BF4

−, PF6−, SbF6

−,(C6F5)4B

−] in the PAG. The PET is initiated by photosensitizer(PS) absorption of light creating the excited state [PS]*. The[PS]* undergoes energy transfer to the onium salt generating anexcited complex state. The onium is reduced by a formal oneelectron-transfer reaction. The electron-transfer reaction is ren-dered irreversible due to the rapid decay of the onium radical asshown in Scheme 1, eq. 4. The PS cation radical can decay in anumber of pathways to produce a strong Bronsted acid.36

A variety of compounds, including carbazoles, phenothiazines,isobenzofurans, cyanines, and conjugated olefins, can be used tophotosensitize onium salts.38–40 Diaryliodonium salts are morefavorable for electron-transfer reactions because the reductionpotential is low compared to triarylsulfonium salts. Polynucleararomatic hydrocarbons are an efficient class of electron-transferPSs.36,40 The absorption characteristics and oxidation potentialsof the acenes are related to the size of their aromatic frameworks.For example, anthracene has an absorption wavelength cut-off at390 nm while pentacene is at 585 nm. However, increasing thenumber of aromatic rings lowers the solvent solubility due to thehigher crystallinity from π–π stacking. Pentacene suffers frompoor solubility in most common organic solvents that makes itdifficult for solvent casting films. The spectral absorption and sol-ubility of acenes can be adjusted through functionalization.Incorporation of other electron-donating groups, such as alkoxygroups, has been shown to improve the photosensitization pro-cess by refining the redox potentials, as well as improving the sol-ubility.36 Electron donating groups can potentially increase theexcited-state lifetime to improve energy transfer with onium salts,especially in dilute solutions.41

The sensitization of iodonium salts has been used in cationicphotopolymerizations to achieve faster rate with anthracenederivatives.36 Near infrared sensitization of iodonium salts hasbeen assessed previously with cyanines; however, the basicfunctionality is likely not compatible with acid-catalyzedprocesses.39,42–46 Wallraff et al. studied the sensitization ofonium salts for acid-catalyzed deprotection of polymethacrylatesas wet-developable photoresist.40 However, sensitization of oniumsalts has not been extensively studied for acid-catalyzed

depolymerization of polymers. The aim of this study is to evalu-ate the sensitization of iodonium PAGs into the entire visiblespectrum with anthracene, tetracene, and pentacene derivatives.It is desirable for transient polymers to have low solid residuesafter depolymerization. Similarly, it is desirable for the decom-posable polymers have a low chemical footprint after depolymeri-zation to avoid detection and reverse engineering. Hence, thebyproducts of the PET reaction are studied to minimize solid res-idue post-depolymerization.

EXPERIMENTAL

Materials1-Chloro-4-propoxythioxanthone (CPTX), anthracene, n-buty-llithium (2.5 M in hexane), and anhydrous dimethylformamide(DMF) at 99.8% purity were purchased from Sigma-Aldrich.Phenylacetylene was purchased from Acros Organics. Tetrahy-drofuran (THF) at >99% purity was purchased from British DrugHouse. 3,4,5-Trimethoxyphenylacetylene was prepared from tri-methoxybenzaldehyde via Corey–Fuchs reaction. 1,8-Dimethoxy-anthraquinone was prepared from reaction between iodomethaneand 1,8-dihydroxyanthraquinone. 5,12-Tetracenedione and 6,13-pentacenedione were prepared from literature methods.47,48

PPC was generously supplied by Novomer Inc. The weight-average molecular weight of PPC material was 160 kDa with adispersity (Ð) of 1.16. PPHA was synthesized followingSchwartz et al. procedure with a weight-average molecularweight of 246 kDa and a dispersity (Ð) of 1.57.30 The PAGused in this study was an iodonium salt, 4-isopropyl-40-methyldiphenyliodonium tetrakis(pentafluorophenyl) borate(referred to as FABA-PAG) from Solvay Inc. Materials used inthis study are shown in Figure 1.

General Synthesis Procedure for Acene DerivativesTotal of 2.05 equivalents of phenylacetylene was dissolved at0.91 M in anhydrous THF in a flame-dried round-bottom flaskunder an argon atmosphere. The solution was cooled to −78 �Cfollowed by the slow addition of 2.02 equivalents of n-butyllithium in hexane via syringe. The reaction vessel was stirredat room temperature for 30 min before being cooled to −78 �C.Total of one equivalent of the appropriate quinone was addedwith anyhdrous THF to the reaction mixture. After stirring atroom temperature for 60 min, the reaction was added dropwiseinto a 0.78 M solution of Sn(II) chloride dihydrate in 10% HClsolution. The solution stirred for 90 min before collecting thecrystalline product via vacuum filtration. Products were recrystal-lized from chloroform.

1,8-Dimethoxy-9,10-bis(phenylethynyl)anthracene (DMBA),5,12-bis(phenylethynyl)tetracene (BPET), and 6,13-bis(pheny-lethynyl)pentacene (BPEP) were prepared according to the gen-eral procedure. 6,13-bis(3,4,5-trimethoxyphenylethynyl)pentacene(BTMP) was prepared from 3,4,5-trimethoxyphenylacetlyeneusing the same conditions as the general procedure. Molecularstructure was verified by nuclear magnetic resonance and highresolution mass spectrometry, and matched previous reports onthese compounds.48,49Scheme 1. Mechanism for PET of onium salts.

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47141 (2 of 12) J. APPL. POLYM. SCI. 2019, DOI: 10.1002/APP.47141

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CharacterizationThe UV–visible (vis) absorption spectrum of PSs used in thisstudy was investigated on Hewlett Packard 8543 UV–vis spectro-photometer in solutions of degassed DMF. Formulations of allpolymer solutions with sensitizer and PAG/base generator weremixed on a ball roller for 12 h. The PS-to-PAG molar ratiowas1.2:1 for all formulations. Fluorescence spectra were evaluatedwith a Horiba FL3-2i Fluorometer. Time-correlated single photon

counting method with a Horiba NanoLed excitation source at334, 455, 570, and 625 nm was used to obtain the fluorescencelifetimes. Stern–Volmer analysis was conducted with sensitizersmaintained to approximately 4 μM solutions in degassed DMFwith concentrations of PAG-FABA ranging from 0.008 to 0.3 M.

Contrast curves were generated from sunlight in Atlanta, Georgiabetween the hours of 11 a.m. to 2 p.m. in September 2016.A variable density mask filter (Model 400 F.S.) was used from

Figure 1. Decomposable polymers (PPC and PPHA), iodonium salt (PAG-FABA), and PSs (anthracene, CPTX, DMBA, BPET, and BTMP).




Opto-line. Square band-pass filters of 50.8 × 50.9 mm at wave-lengths of 500, 560, and 650 nm were used. All filters had a10 � 2 nm full-width half-max value. Formulations of PPC films,sensitizer, and PAG were spin-coated onto silicon wafers wherethe variable density mask filter and corresponding band-pass fil-ter were placed on-top. PPC films were developed after exposureon a hotplate at 100 �C for 7 min. The film thickness was mea-sured using a Veeco Dektak profilometer. The intensity of sun-light was recorded with a SM206 Solar Power Meter. UV lightexposures filtered to 248 and 365 nm were exposed using anOriel Instruments flood exposure source with a 1000 WHg(Xe) lamp.

The electrochemical properties of the sensitizers and PAGs wereevaluated by cyclic voltammetry using a Princeton PARSTAT2263 potentiostat. Redox potentials were recorded under drynitrogen atmosphere with a three-Pt-electrode electrochemicalcell in an anhydrous DMF solution of 0.13 M tetrabutylammo-nium perchlorate. Ferrocene was used as an internal standard fordetermining the electrochemical potential.

Characterization of the residual products of the PET reactionswithin PPHA films were conducted with proton nuclear magneticresonance (1H-NMR), matrix-assisted laser desorption/ioniza-tion–time of flight–mass spectrometry (MALDI–TOF–MS), andelectrospray ionization (ESI) triple quadruple analysis. NMRmeasurements were performed using a Varian Mercury Vx400 (400 MHz) tool. Chloroform-D (CDCl3) was used for theNMR solvent and was supplied by Sigma-Aldrich at a 99.8%purity level. The concentration of residue in NMR sample tubeswas held at 46 mg in 0.75 mL CDCl3. ESI measurements wereperformed on a Micromass Quattro LC and MALDI–TOF–MSmeasurements were performed on a Bruker AutoFlex III.MALDI–TOF–MS samples were prepared with a 10 mg mL−1

solution of the matrix trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile in dichloromethane (DCM). Theresidue was prepared in a separate vial containing 10 mg/mLsolution in DCM. The matrix solution and residue solution weremixed at equal parts and spotted 1 μL onto the plate for analysis.

RESULTS AND DISCUSSION

Optical Properties of PSsThe optical properties of the modified acenes were investigatedby UV–vis absorption and fluorescence spectroscopy in degassedDMF. The six sensitizers, anthracene, CPTX, DMBA, BPET,BPEP, and BTMP, exhibited strong absorption bands from312 to 673 nm, as shown in Figure 2 and listed in Table I. Thefluorescence spectrum corresponding to the first excited singletstate for BPET is shown in Figure 3.

The four sensitizers that absorb in the visible region havehigh molar extension coefficients between 24 × 103 and 33 ×103 M−1 cm−1 which is three to four times greater than the molarextinction coefficient CPTX, 6.9 × 103 M−1 cm−1 at 387 nm, andanthracene, 7.1 × 103 M−1 cm−1 at 370 nm. However, all of theabsorption bands are redshifted compared to their unmodifiedacene counterparts (anthracene, tetracene, and pentacene) due tothe halogenation of the acene core or the extension of theπ-conjugated system by the appended phenylethynyl groups.49

The two pentacene derivatives, BPEP and BTMP, show very simi-lar spectra. The addition of methoxy groups to the moleculesresults in a redshift in the absorption spectrum by 5 nm, andmarkedly improved solubility in common organic solvents andpolymer films. Furthermore, DMBA, BPET, and BTMP displaygood solid-state stability from oxidation in ambient atmospherewhere only minimal degradation under fluorescent lighting andair was detected after 6 weeks via 1H-NMR (see SupportingInformation).

Gibbs Free Energy of PETMolecular compounds, such as polynuclear aromatic hydrocar-bon (PAH), can be used as PSs for the iodonium PAGs given thatthe energetics are thermodynamically feasible. PET is theoxidation–reduction process between a donor and its acceptorthrough molecular orbitals. A simplified diagram is shown inScheme 2.

The donor (sensitizer) is at a ground state with two paired elec-trons in the highest occupied molecular orbital. The oxidationpotential of the donor (sensitizer) is increased, compared to theground state value by absorption of a photon thereby promotingelectron to the lowest unoccupied molecular orbital (LUMO)resulting in the formation of the first excited singlet state. Theexcited singlet state of the sensitizer can lose energy by fluores-cence or nonradiative processes before reduction of the oniumsalt. The PS and onium salt can create an excited complex, asshown in Scheme 1, eq. 3, where an electron is transferred fromthe sensitizer to the LUMO of the PAG. A reactive cationic radi-cal is created as a result of this electron transfer that can furtherreact to produce a Bronsted acid. The favorability of electrontransfer is determined by the oxidation and reduction potentialsof the states of the compounds. In this case, the oxidation poten-tial of sensitizer was compared to the reduction potential of thePAG to determine if the reaction is favorable. The thermody-namic favorability for the electron-transfer photosensitization ofthe modified PAHs with PAG/base generators can be evaluatedby the Rehm–Weller equation.38–40,50,51

ΔG eVð Þ¼ Eox1=2−Ered1=2� �

−Eoo ð6ÞThe half-wave oxidation potential (Eox1=2) and the optical band gap

(Eoo) of the sensitizer was compared with the half-wave reductionpotential (Ered1=2Þof the PAG/base generator. If the Gibbs freeenergy is less than zero (ΔG < 0) then the PET is favorable.Another requirement is that there is no ground-state electron-

Figure 2. Molar absorptivity of PSs in DMF solution. [Color figure can be

viewed at wileyonlinelibrary.com]



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transfer between PS and PAG. In this regard, the oxidationpotential of the sensitizer must be unfavorable for electron trans-fer until the excited state is photocreated so that the sensitizercan act as an “optical trigger.” Cyclic voltammetry was used tomeasure the redox potentials of the sensitizers and PAG/basegenerators using ferrocene (Fc/Fc+) as an internal referencepotential. UV–vis was used to evaluate the optical band gap ofthe sensitizers using the onset wavelength of absorbance. Redoxpotentials and band gaps are reported in Table II. The corre-sponding molecular orbitals of PAG and sensitizers are plotted inFigure 4.

PET of the acenes in this study with the iodonium PAG are ther-modynamically favorable, as shown in Table II. The low reduc-tion potential of iodonium PAG is a contributing factor. Thereduction potential of iodonium PAG in this case was −1.33 Vversus Fc/Fc+. However, electron transfer becomes less favorableas the modified acenes become more redshifted. The change inGibbs free energy value for PET is an order of magnitude lowerfor BTMP (ΔG = −2.36 eV), which has an absorption onset at700 nm, compared to anthracene (ΔG = −26.57 eV) which hasan absorption onset at 385 nm. Ground-state electron transferfrom all sensitizers to the PAG is thermodynamicallyunfavorable.

PET reactions are governed by collisional quenching where thefluorescence lifetime decay of the fluorophore (sensitizer)decreases with increasing concentration of the quencher (PAG).Thus, the effectiveness of the modified PAHs for PET reactionswith iodonium salts was investigated by this method. Quenchingrate constants (a measure of PET efficiency) for reducing thefluorescence lifetime of the sensitizers can be obtained from theStern–Volmer relationship50:

τoτ¼ 1 +Ksv PAG½ �,Ksv ¼ kqτo ð7Þ

In eq. (7), τo and τ are the fluorescence lifetimes in the absenceand the presence of the quencher PAG-FABA, respectively; KSVis the Stern–Volmer quenching constant; (PAG) is the concentra-tion of the onium salt; and kq is the bimolecular quenching rateconstant. Rate constants and fluorescence lifetimes are listed inTable II. The Stern–Volmer plots of each PS with various con-centrations of FABA-PAG are shown in Figure 5. The fluores-cence lifetime decay of PET with quencher is also shown inFigure 6.

Table I. Absorption and Fluorescence Maxima of Modified PAH PSs

PSAbs. λmax(nm)

Abs. ε(103 M−1 cm−1)

Emissionλmax (nm)

Anthracene 326 2.8 381

342 5.3 403

360 7.6 427

370 7.1 453

CPTX 315 12 ---

387 6.9 ---

DMBA 320 20 536

476 23 563

504 24

BPET 341 21 411

355 24 433

410 3.6 568

485 9.8 611

517 24 660

556 33

BPEP 312 254 467

371 42 492

569 6.2 684

612 15.7 738

665 27

BTMP 314 274 477

382 33 503

441 5 690

576 6 750

619 16

673 27

BPEP, 6,13-bis(phenylethynyl)pentacene; BPET, 5,12-bis(phenylethynyl)tetracene; BTMP, 6,13-bis(3,4,5-trimethoxyphenylethynyl)pentacene;CPTX, 1-chloro-4-propoxythioxanthone; DMBA, 1,8-dimethoxy-9,10-bis(phenylethynyl)anthracene; PAH, polynuclear aromatic hydrocarbon; PS,photosensitizer.

Scheme 2. PET of iodonium salts as expressed by molecular orbitals to gen-erate super acid. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3. Normalized absorbance and fluorescence emission spectra ofBPET PS after excitation at 514 nm in DMF solution. [Color figure can beviewed at wileyonlinelibrary.com]



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The theoretical diffusion-controlled rate constant (kd) in DMF isapproximately 7.06 × 109 M−1 s−1, as estimated from a deriva-tion from the Stokes–Einstein equation.52 In PET systems wherethe electron transfer is sufficiently exergonic, kd ≈ kq.51 Anthra-cene and DMBA are practically diffusion controlled; however, thekq values of tetracene and pentacene derivatives (BPET, BPEP,and BTMP) are an order of magnitude lower. As expected, thedecrease in kq values can be rationalized by following the thermo-dynamic considerations (ΔGPET) as listed in Table II.

The Rehm–Weller equation only considers the thermodynamicfavorability of the first excited singlet state for electron transfer.It is possible that upper excited state energy levels (S2) of the sen-sitizers could contribute to more exergonic electron transfer reac-tions. The PSs have high molar absorptivity peaks in the near-UV region which can activate upper excited state levels. In manycases, the internal conversion from S2 to S1 is rapid, for example,picroseconds.50 However, it was found that the pentacene deriva-tives had long-lived second excited singlet states. The fluores-cence spectrum of BTMP under excitation at 334 nm showsstrong S2 to S0 fluorescence λmax values at 477 and 503 nm, aswell as the S1 to S0 fluorescence λmax at 690 and 750 nm, shownin Figure 7. The response between 642 and 677 nm was removeddue to emission peaks from second-order diffraction of the exci-tation wavelength that is unrelated to BTMP.50

The BTMP sensitizer at the S2 fluorescence region recordednanoscale lifetimes as well as an order of magnitude increase inthe bimolecular quenching rate as shown in Table II andFigure S14. BPET and BTMP showed similar increases in theirbiomolecular quenching rate of their S2 fluorescence. The S2quenching rate of DMBA was not recorded due to the rapidinternal conversion of S2 to S1 giving fluorescence lifetimes lessthan 0.0013 ns.

Photosensitivity of Sensitized Polymer FilmsThe performance of the sensitizers were evaluated in solvent castfilms consisting of PPC with 3 parts per hundred resin (pphr)PAG and exposed to sunlight under a variable neutral densitymask and band-pass filter for wavelength selection. Sensitizerswere exposed at selected wavelengths that correspond to regionsnear the long wavelength end of the absorption spectrum foreach. Films with DMBA were exposed at 500 nm, BPET filmswere exposed at 560 nm, and BTMP films were exposed at650 nm wavelength. Anthracene and CPTX were evaluated at365 nm under a UV exposure tool. Exposed PPC films were sub-sequently developed at 100 �C on a hot plate for 7 min. Theacid-catalyzed decomposition of PPC requires higher tempera-tures (100 �C) which makes characterization of photosensitivityat room-temperature possible after exposure. The normalized

Table II. Thermodynamic and Photophysical Properties of PSs for Photoinduced/Ground State Electron Transfer with FABA-PAG

Rehm–Weller equation Stern–Volmer (S1–So) Stern–Volmer (S2–So)

Sensitizer Eoo (nm)Eox (V) versusFc/Fc+

ΔGPET(kcal mol−1)

ΔGGET(kcal mol−1)

KSV(M−1) τ (ns)

kq × 109

(M−1 s−1) KSV (M−1) τ (ns)kq × 109

(M−1 s−1)

Anthracene 385 0.82 −24.59 49.72 16.27 2.92 5.58 --- --- ---

CPTX 417 1.12 −12.17 56.44 --- --- --- --- --- ---

DMBA 531 0.49 −11.80 42.08 5.48 5.35 1.03 --- --- ---

BPET 574 0.54 −6.72 43.13 5.58 7.59 0.74 5.99 1.16 5.13

BPEP 690 0.31 −3.71 37.75 1.17 3.92 0.3 9.28 3.11 3

BTMP 700 0.31 −2.94 37.94 0.58 2.71 0.21 4.24 2.39 1.77

BPEP, 6,13-bis(phenylethynyl)pentacene; BPET, 5,12-bis(phenylethynyl)tetracene; BTMP, 6,13-bis(3,4,5-trimethoxyphenylethynyl)pentacene; CPTX,1-chloro-4-propoxythioxanthone; DMBA, 1,8-dimethoxy-9,10-bis(phenylethynyl)anthracene, PAG, photoacid generator; PS, photosensitizer.

Figure 4. Energy levels of the FABA-PAG and all PS based on their redox potentials and band gaps. [Color figure can be viewed at wileyonlinelibrary.com]




thickness after development versus log exposure dose (i.e., con-trast curve) was generated, Figure 8.53

The normalized thickness of the film was plotted where eachpoint corresponds to the transmission of light from 1 to 100%through the variable neutral density mask. The full intensity oflight (100% transmission) through the band-pass filters at500, 560, and 650 nm were recorded: 1.38, 1.40, and1.45 mW cm−2, respectively. The linear region was extrapolatedto zero on the x axis where the dosage value was taken as theminimum dose required for exposure (D100). Films of DMBAand BPET had a similar D100 of 284 and 270 mJ cm

−2; however,BTMP had a minimum dose of 6180 mJ cm−2. Thus, the mostredshifted BTMP peak was 22 times less sensitive than films sen-sitized with DMBA or BPET. The photosensitivity of all sensi-tized PPC films is shown in Table III.

A method to normalize the sensitivity in terms of the absorbedlight versus the incident light stated above was used to comparethe efficiency of the PET reactions within films. The contrast-curve efficiency (Ecc) of the PET reactions in the PPC films wascalculated from the ratio of the amount of photoactive com-pounds (PAG or PS) to the number of absorbed photons fromthe D100. Quantum efficiencies approaching unity are desired. As

shown in Table III, direct photolysis of the iodonium PAG at248 nm gave a quantum efficiency of 0.42. The result is in agree-ment with previous measurements of iodonium salts, between0.42 and 0.54.54 Efficiency dropped as the wavelength of the sen-sitizer was extended to higher values. The BTMP sensitizer hadthe lowest efficiency of 0.004. This is due to the less favorablethermodynamics of the electron transfer from BTMP to PAGcompared to the other sensitizers, as discussed above.

The Ecc values show excellent agreement with the Stern–Volmerquenching rates (Ksv) as shown in Figure 9.

The Ecc and Ksv values increase with more negative value of theGibbs energy. It is noted that the Ecc and Ksv values for DMBAand BPET are nearly equal even though the electron transfer ofiodonium salts with DMBA is 5.08 kcal mol−1 more exergonicthan with BPET. Stern–Volmer analysis shows that that BPEThas a longer excited-state lifetime which allows it greater oppor-tunity to interact with and undergo electron transfer with theiodonium salt. The bimolecular quenching rate (kq) of BPET is1.39 times lower than DMBA which also follows the relationshipbetween the PET reaction rate efficiency and the thermodynamicfavorability in Table II.

Figure 7. The fluorescence spectrum of BTMP at 334 nm excitation inDMF solution. The emission between 642 and 677 nm was removed due topeaks from second-order diffraction of the excitation wavelength. [Colorfigure can be viewed at wileyonlinelibrary.com]

Figure 6. The effect of various concentrations of FABA-PAG on the fluo-rescence lifetime decay of DMBA. Fitted to a monoexponential decay with2.61 < χ2 < 2.86. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5. Stern–Volmer plots of all PSs in DMF of their first excited singletstate with FABA-PAG as the quencher. [Color figure can be viewed atwileyonlinelibrary.com]

Figure 8. Contrast curves of PPC films with 3 pphr PAG with the followingsensitizers: DMBA, BPET, and BTMP at 1.2 PS to 1 PAG molar ratio.[Color figure can be viewed at wileyonlinelibrary.com]



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The effect of path length within the anthracene or DMBA inPPHA films was investigated, as shown in Figure 10. Light trans-mission versus film thickness were plotted for DMBA andanthracene at various loadings within PPHA films. The opticalpath length of anthracene and DMBA loaded PPHA films wasinvestigated near the λmax absorption value.

In Figure 10, the optical path length decreased with higher load-ings of DMBA within the PPHA film. The 0.52 pphr DMBA filmhad

BPET’s chromophoric is related to the Π-conjugation network.Formation of the highly reactive BPET radical cation canundergo a number of reactions such as photodimerization, whichwill disrupts its Π-conjugation network and decreases the onsetabsorption wavelength.55

Cationic and/or radical-based photopolymerization is the com-mon use of sensitizers with onium salts. Formation of nonvolatileresidue after decomposition is not desirable for transient polymerapplications. Transient polymers rely on monomer evaporationfollowing depolymerization. However, the slow evaporation ofphthalaldehyde following PPHA depolymerization gives ampleopportunity for side reactions with the sensitizer byproductsbecause the phthalaldehyde monomer has a very low vapor pres-sure and may take an extended time to evaporate.11

PPHA films with various PAG and DMBA loadings were formu-lated and cast onto glass substrates to evaluate the amount of res-idue formed during photoexposure, Figure 12.

Films were exposed to midday sunlight, 1034 W m−2. PPHA candepolymerize by protonation of the PPHA ether linkage creatingcationic hydroxyl groups that lead to an unzipping decomposi-tion reaction producing phthalaldehyde.56 Acid-catalyzed depoly-merization of PPHA can occur at temperatures above its ceilingtemperature (−43 �C), which makes it suitable for transienceapplications at ambient conditions. The polymer transformsinto liquid during depolymerization followed by evaporation or

sublimation of the monomer.33 The three PPHA films inFigure 12 contain 1.5, 3, or 10 pphr PAG with DMBA at a1.2:1 mol ratio of PAG to DMBA. Photoresponse time for PPHAdepolymerization was dependent on PAG loading. PPHA sam-ples with 10, 3, and 1.5 pphr PAG with DMBA began to liquefyafter 10, 60, and 150 s, respectively. Yellow phthalaldehyde is eas-ily observed upon depolymerization. Previous studies have con-firmed the acid-catalyzed depolymerization of PPHA by NMRand infrared spectroscopy.9,57,58 A dark discoloration and pres-ence of solid residue was visible at higher PAG/DMBA loadings.The higher PAG/DMBA loading increased the likelihood forside-reactions with the phthalaldehyde forming black, nonvolatileresidue.

Crivello and Jang previously proposed multiple decompositionpathways for the PS radical cation after the PET reaction, asshown in Scheme 3.36 Side reactions with phthalaldehyde mayoccur through similar chemical pathways when PPHA depoly-merizes. The sensitizer radical cation can react with the iodoniumneutral radical products forming dimers, or they may directlyreact with phthalaldehyde during or after depolymerization.The formation of the black-colored residue may indicate that theDMBA conjugation has been extended.

It is known that the anthracene radical cation can undergo pheny-lation with the aryl radical from the iodonium to produce9-phenylanthracene and a proton.54 The 9-phenylanthracene canabsorb another photon and act as an electron donor with anotheriodonium salt. It was proposed that poly(acenes) can be generatedin this manner.54 The modified PAHs used in this study may alsoreact with aryl radicals or phthalaldehyde in a similar way. Themodified PAH appears to induce a larger amount of solid residueat a faster rate than the unmodified parent compound during thePET reaction. Appending phenylethynyl groups onto the acenecore likely provides more resonance stability of the radical cationof the sensitizer. The long-lived radical cation may diffuse andform stable products by reaction with phthalaldehyde or other arylcompounds creating an extended conjugated network.

The nonvolatile, black residue from the PHA film with 10 pphrPAG and DMBA was analyzed by 1H-NMR to investigate thechemical structure, as shown in Figure S16. The residue wasbaked at 175 �C on a hotplate for 4 h to evaporate phthalalde-hyde while remaining below the melting point of DMBA. Theresidue had broad peaks from 6 to 8 ppm indicative of possiblearomatic derivatives. Furthermore, the mass of the postexposureresidue was greater than the original mass of PAG and DMBA inthe film. This suggests that phthalaldehyde participates in reac-tions with the DMBA cation radical.

The residue after photodepolymerization of a PPHA film loadedwith 10 pphr PAG with DMBA was analyzed by ESI to determine

Figure 12. Sunlight-induced depolymerization of 100 μm thick PPHA filmswith various loadings of FABA-PAG with DMBA over glass substrate overan 18 min sunlight exposure at noon. [Color figure can be viewed atwileyonlinelibrary.com] Scheme 3. Competing side reactions of sensitizer cation radical.




Figure 13. MALDI–TOF–MS analyses of residue after full sunlight-induced depolymerization of PPHA film with 10 pphr PAG and DMBA. [Color figurecan be viewed at wileyonlinelibrary.com]

Figure 14. Time-lapse photos of PPHA films with 1 pphr PAG and anthracene, CPTX, DMBA, BPET, and BTMP exposed to natural sunlight at noon overa 20 min period. [Color figure can be viewed at wileyonlinelibrary.com]



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the participation of PAG in the side reactions after PET. Theresults show that the iodonium cation was no longer present(i.e., the absence of 337 m/z peak); however, the borate anion waspresent (i.e., 679.1 m/z) suggesting that it does not play a signifi-cant role in the side reactions. MALDI–TOF–MS was used todetermine the molecular mass of the poly(aromatic) products toevaluate the extent of the conjugated framework, Figure 13.

The MS spectrum shows a number of aromatic derivatives as aresult of the side reactions with the DMBA radical cation. Aseries of main peaks (denoted in the spectrum) with their corre-sponding fragmentations were observed. The product formed inthe side reaction extends to nearly 1.5 kDa, which is 3.6 times themolar mass of DMBA. The separation between these peaks corre-sponds to about the molar mass of a phthalaldehyde monomer,about 134 Da. It is evident that lower loadings of the PAG andmodified PAHs are necessary to mitigate the undesirable sidereactions with phthalaldehyde that form nonvolatile, solidpoly(aromatics). Faster evaporating copolymers of aldehydes,such as with butanal, could also potentially mitigate sidereactions.

The sunlight-induced “transience” of the PPHA films with opti-mal loading of photocatalysts with PSs was exposed midday sun-light, as shown in Figure 14. Formulations of 1 pphr PAG withanthracene, CPTX, DMBA, BPET, and BTMP (1.2:1 PS to PAGratio) were solution cast into film. BTMP loaded films remark-ably began to depolymerize in less than 1 min and underwentcomplete depolymerization in 5.5 min. The PET efficiency (Ecc)of BTMP with iodonium salts are one to two orders of magnitudelower than the other sensitizers according to Table III. However,it is likely the higher near-UV absorbance excites BTMP to anupper excited state where electron transfer is more efficient. Thisis shown in Table II and Figure S14 where the bimolecularquenching rate increased by an order of magnitude when evalu-ated at the second singlet excited state. PPHA only, PPHA withanthracene, and PPHA with 1 pphr PAG films did not undergodepolymerization in sunlight after 1 h. The PPHA with PAG filmdid not significantly depolymerize because of the limited amountof UV radiation absorbed by PPHA in sunlight. Thus, a sensitizeris needed to activate the PAG in sunlight. All of the sensitizedPPHA films with PAG eventually lose their mechanical integrityduring depolymerization and absorb into the texwipe.

CONCLUSIONS

PAHs with disubstituted phenylethynyl groups were found to beefficient sensitizers of onium salts to induce acid-catalyzeddepolymerization of PPHA and PPC films. Thermodynamicallyfavorable sensitizers for PET reaction, as evaluated by the Rehm–Weller equation, show a strong correlation with the fluorescencequenching rates in solution and Ecc within films. Pentacenederivatives showed long fluorescence lifetimes and high quench-ing rates in the upper excited singlet state levels. The solubleanthracene, tetracene, and pentacene derivatives were effective inthe photodegradation of polymers with short sunlight exposuretimes. This study has provided a pathway of developing a class ofphotochemical triggers whose exposure wavelength can be tunedfrom the UV region into the entire visible light region for the

purpose of initiating the depolymerization of polymers for a widevariety of transient applications.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of theDARPA ICARUS program, C. G. Willson for technical discussions,the Science and Technology of Advanced Materials and Interfacesnetwork at Georgia Institute of Technology, and Matthew Cooperfor experimental assistance with fluorometer studies.

REFERENCES

1. Uzunlar, E.; Schwartz, J.; Phillips, O.; Kohl, P. A.J. Electron. Packag. 2016, 138, 20802.

2. Saha, R.; Fritz, N.; Bidstrup-Allen, S. A.; Kohl, P. A. Micro-electron. Eng. 2013, 104, 75.

3. Uzunlar, E.; Kohl, P. A. J. Electron. Packag. 2015, 137,41001.

4. Monajemi, P.; Joseph, P. J.; Kohl, P. A.; Ayazi, F. In 11thInternational Symposium on Advanced Packaging Mate-rials: Processes, Properties and Interface; IEEE: Atlanta,GA, USA, 2006; p 139.

5. Hua, Y.; Henderson, C. J. Chem. Inf. Model. 2013, 53,1689.

6. Jayachandran, J. P.; Reed, H. A.; Zhen, H.; Rhodes, L. F.;Henderson, C. L.; Bidstrup, S. A.; Kohl, P. A.J. Microelectromech. Syst. 2003, 12, 147.

7. Jiang, J.; Phillips, O.; Keller, L.; Kohl, P. A. ECS J. SolidState Sci. Technol. 2017, 6, N163.

8. Fu, K. K.; Wang, Z.; Dai, J.; Carter, M.; Hu, L. Chem.Mater. 2016, 28, 3527.

9. Hernandez, H. L.; Kang, S.-K.; Lee, O. P.; Hwang, S.-W.;Kaitz, J. A.; Inci, B.; Park, C. W.; Chung, S.; Sottos, N. R.;Moore, J. S.; Rogers, J. A.; White, S. R. Adv. Mater. 2014,26, 7637.

10. Hwang, S. W.; Lee, C. H.; Cheng, H.; Jeong, J. W.;Kang, S.-K.; Kim, J. H.; Shin, J.; Yang, J.; Liu, Z.;Ameer, G. A.; Huang, Y.; Rogers, J. A. Nano Lett. 2015, 15,2801.

11. Phillips, O.; Schwartz, J. M.; Engler, A.; Gourdin, G.;Kohl, P. A. In Proceedings of the 67th Electronic Compo-nents and Technology Conference (ECTC); IEEE: Orlando,FL, USA, 2017; p 772.

12. Gourdin, G.; Phillips, O.; Schwartz, J.; Engler, A.; Kohl, P.In Proceedings of the 67th Electronic Components andTechnology Conference (ECTC); IEEE: Oralndo, FL, USA,2017; p 190.

13. Spencer, T. J.; Kohl, P. A. Polym. Degrad. Stab. 2011,96, 686.

14. Phillips, O.; Schwartz, J. M.; Kohl, P. A. Polym. Degrad.Stab. 2016, 125, 129.

15. Ohlendorf, P.; Ruyack, A.; Leonardi, A.; Shi, C.;Cuppoletti, C.; Bruce, I.; Lal, A.; Ober, C. K. ACS Appl.Mater. Interfaces. 2017, 9, 25495.

16. DiLauro, A. M.; Phillips, S. T. Polym. Chem. 2015, 6, 3252.




17. Phillips, S. T.; Robbins, J. S.; Dilauro, A. M.; Olah, M. G.J. Appl. Polym. Sci. 2014, 131, 40992.

18. Diesendruck, C. E.; Peterson, G. I.; Kulik, H. J.; Kaitz, J. A.;Mar, B. D.; May, P. A.; White, S. R.; Martínez, T. J.;Boydston, A. J.; Moore, J. S. Nat. Chem. 2014, 6, 623.

19. Phillips, S. T.; Dilauro, A. M. ACS Macro Lett. 2014, 3, 298.

20. DiLauro, A. M.; Robbins, J. S.; Phillips, S. T. Macromole-cules. 2013, 46, 2963.

21. Seo, W.; Phillips, S. T. J. Am. Chem. Soc. Commun. 2010,16802, 9234.

22. Dilauro, A. M.; Zhang, H.; Baker, M. S.; Wong, F.; Sen, A.;Phillips, S. T. Macromolecules. 2013, 46, 7257.

23. Dilauro, A. M.; Lewis, G. G.; Phillips, S. T. Angew. Chem.2015, 127, 6298.

24. Kaitz, J. A.; Moore, S. Macromolecules. 2013, 46, 608.

25. Kaitz, J. A.; Diesendruck, C. E.; Moore, J. S. J. Am. Chem.Soc. 2013, 135, 12755.

26. Kaitz, J. A.; Diesendruck, C. E.; Moore, J. S. Macromole-cules. 2013, 46, 8121.

27. Kaitz, J. A.; Lee, O. P.; Moore, J. S. Mater. Res. Soc. Com-mun. 2017, 5, 191.

28. Park, C. W.; Kang, S.-K.; Hernandez, H. L.; Kaitz, J. A.;Wie, D. S.; Shin, J.; Lee, O. P.; Sottos, N. R.; Moore, J. S.;Rogers, J. A.; White, S. R. Adv. Mater. 2015, 27, 3783.

29. Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W.J. Am. Chem. Soc. 2002, 124, 14284.

30. Schwartz, J. M.; Phillips, O.; Engler, A.; Sutlief, A.; Lee, J.; Kohl,P. A. J. Polym. Sci. Part A: Polym. Chem. 2017, 55, 1166.

31. Schwartz, J. M.; Engler, A.; Phillips, O.; Lee, J.; Kohl, P. A.J. Polym. Sci. Part A: Polym. Chem. 2018, 56, 221.

32. Kaitz, J. A.; Moore, S. Macromolecules. 2014, 47, 5509.

33. Schwartz, J. M.. Ph.D. Thesis, Georgia Institute of Technol-ogy, 2017.

34. Fan, B.; Trant, J. F.; Yardley, R. E.; Pickering, A. J. Macro-molecules. 2016, 49, 7196.

35. Crivello, J. V. J. Photopolym. Sci. Technol. 2008, 21, 493.

36. Crivello, J. V.; Jang, M. J. Photochem. Photobiol. A Chem.2003, 159, 173.

37. Crivello, J. V.; Bulut, U. J. Polym. Sci. Part A: Polym. Chem.2005, 43, 5217.

38. Dadashi-Silab, S.; Doran, S.; Yagci, Y. Chem. Rev. 2016,116, 10212.

39. Shi, S.; Croutxé-Barghorn, C.; Allonas, X. Prog. Polym. Sci.2017, 65, 1.

40. Wallraff, G. M.; Allen, R. D.; Hinsberg, W. D.;Willson, C. G.; Simpson, L. L.; Webber, S. E.;Sturtevant, J. L. J. Imaging Sci. Technol. 1992, 36, 468.

41. Patra, D.; Malaeb, N. N.; Haddadin, M. J.; Kurth, M. J.J. Fluoresc. 2012, 22, 707.

42. Brömme, T.; Oprych, D.; Horst, J.; Pinto, P. S.; Strehmel, B.RSC Adv. 2015, 5, 69915.

43. Brömme, T.; Schmitz, C.; Moszner, N.; Burtscher, P.;Strehmel, N.; Strehmel, B. ChemistrySelect. 2016, 1, 524.

44. Kabatc, J.; Ortyl, J.; Kostrzewska, K. RSC Adv. 2017, 7,41619.

45. Schmitz, C.; Halbhuber, A.; Keil, D.; Strehmel, B. Prog. Org.Coat. 2016, 100, 32.

46. Shiraishi, A.; Kimura, H.; Oprych, D.; Schmitz, C.J. Photopolym. Sci. Technol. 2017, 30, 633.

47. Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2003,5, 4245.

48. Pramanik, C.; Miller, G. P. Molecules. 2012, 17, 4625.

49. Schmidt, R.; Göttling, S.; Leusser, D.; Stalke, D.; Krause, A.-M.;Würthner, F. J. Mater. Chem. 2006, 16, 3708.

50. Lakowicz, J. R. Principles of Fluorescence Spectroscopy. 3rded.; Springer: New York, NY, USA, 2006.

51. Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986, 86, 401.

52. Turley, W. D.; Offen, H. W. J. Phys. Chem. 1984, 88, 3605.

53. Campbell, S. A. The Science and Engineering of Microelec-tronic Fabrication. Oxford University Press: Oxford, UK,Vol. 476, 2001.

54. Dektar, J. L.; Hacker, N. P. J. Org. Chem. 1990, 55, 639.

55. Breton, G. W.; Vang, X. J. Chem. Educ. 1998, 75, 81.

56. Tsuda, M.; Hata, M.; Nishida, R. I. E.; Oikawa, S. J. Polym.Sci. Part A: Polym. Chem. 1997, 35, 77.

57. Feinberg, A. M.; Hernandez, H. L.; Plantz, C. L.;Mejia, E. B.; Sottos, N. R.; White, S. R.; Moore, J. S. ACSMacro Lett. 2018, 7, 47.

58. Lee, K. M.; Phillips, O.; Engler, A.; Kohl, P. A.; Rand, B.ACS Appl. Mater. Interfaces. 2018, 10, 28062.




Sunlight photodepolymerization of transient polymersINTRODUCTIONEXPERIMENTALMaterialsGeneral Synthesis Procedure for Acene DerivativesCharacterization

RESULTS AND DISCUSSIONOptical Properties of PSsGibbs Free Energy of PETPhotosensitivity of Sensitized Polymer Films

CONCLUSIONSACKNOWLEDGMENTSREFERENCES

sunlight photodepolymerization of transient...

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