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A: Kinetics, Dynamics, Photochemistry, and Excited States

Ion-Pair Complexes of Pyrylium and Tetraarylborate as New Host-Guest Dyes: Photoinduced Electron Transfer Promoting Radical Polymerization Willy Glen Santos, Darya S. Budkina, Silvia H. Santagneli, Alexander N. Tarnovsky, Julio Zukerman-Schpector, and Sidney José Lima Ribeiro J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b03581 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 8, 2019

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The Journal of Physical Chemistry

Ion-pair Complexes of Pyrylium and Tetraarylborate as New Host-guest Dyes: Photoinduced Electron Transfer Promoting Radical Polymerization Willy G. Santos,a,c* Darya S. Budkina,b Silvia H. Santagneli,a Alexander N. Tarnovsky,b Julio Zukerman-Schpector,c Sidney J. L. Ribeiroa* a. Institute of Chemistry, São Paulo State University - UNESP, CP 355, Araraquara, SP 14801-970, Brazil b. Center for Photochemical Sciences, Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, United States c. Department of Chemistry, Federal University of São Carlos, UFSCar, CP 676, São Carlos, SP 13565-905, Brazil Supporting Information Placeholder ABSTRACT: Ultrafast transient absorption spectroscopy, NOESY-NMR and EPR spectroscopy shed light how π-π stacking interactions combined with electrostatic interactions can be used to form a stable ion-pair complexes between pyrylium and tetraarylborate ions in which the interaction of the π-delocalized clouds promotes the observation of new radiative processes and also electron transfer process excitation using visible light. The results exhibit a striking combination of properties: chemical stability, photophysical and photo-chemical events that makes these ion-pair complexes as a step towards the realization of chromophore/luminescent materials and also the usage as new mono-photoinitiator system in radical polymerization reactions.

I. Introduction Supramolecular chemistry describes complexes that are composed of two or more molecules or ions that are present in an unique structural relationships by forces other than those of covalent bonds. 1–9 These forces may include π−π interactions,1–5,10 ion−π interactions, and charge transfer interactions.3–5,11–14 Studies with anionic rings and cationic rings have shown that both ions may interact, generating a polarized π−π interaction with strong charge transfer character of the ground state.1–4,11,15 We asked whether both positively and negatively charged rings could form a polarized π-π stacking interaction, promoting an effective electron transfer process in the excited state. Explicit considerations are also pointed to highly polarized and conjugated systems,16–21 where a stable complex such as host-guest systems with donor–acceptor interactions may be formed using an electron donating and withdrawing functional groups.16,20–26 This complexation process could occur in a manner of face-centered or parallel displaced stacking, owing to efficient orbital overlap between the highest occupied molecular orbital (HOMO) of the donor and lowest unoccupied molecular orbital (LUMO) of the acceptor, which polarizes the electron density. 2,3,11,14,27,28 Indeed, π-π or ion-π systems formed between good electron donors/acceptors are well-known,3,11–13,28 and several systems may be characterized either by charge-transfer transitions in UV-visible absorption spectrum and/or by new emissive states in fluorescence/phosphorescence spectra.28,29 Ground-state charge-transfer (CT) complexes and structureproperty dependencies of heteroatom conjugated cores have been known for a long time.12,13 In the case of heterocyclic onium ions, they involve mainly pyridinium cations and inorganic counterions.28,30,31 Pyrylium salts are excellent electron-attracting molecules because of their positively charged oxygen atom in the heterocycle, which induces a non-uniform electron density

distribution with a high positive charge at positions 2, 4 and 6 of the aromatic ring. Pyrylium cationic molecules are electron acceptors by virtue of their electron deficiency and hence should be good oxidizing agents in their singlet and triplet excited states.32–35 Two members of this group, namely 2,4,6-trimethylpyrylium (1a ion) and 2,4,6triphenylpyrylium (1b ion) have found use as a sensitizer in photoinduced electron transfer reactions. 33,34,36 Table 1 shows the chemical structures of the 1a and 1b ions. As 1a ion (or 1b ion) is positively charged, only charge exchange takes place in the first moment after excitation.28 Therefore, the formation of the radical ions will be enhanced by a favorable reduction potential of the ground state, E° < -0.3 V vs the normal hydrogen electrode (NHE).37–39 Otherwise, the reversible halfwave oxidation potentials (E1/2ox) and reduction potentials (E1/2red) can be related to the energy of the highest occupied (EHOMO) and lowest unoccupied molecular orbitals (ELUMO), respectively. Tetraarylborates are anions for which the negative charge is associated with electrons in bonding orbitals. Consequently, oxidation of a borate will convert its two-electron carbon-boron bonds to ones having fewer electrons resulting in a concomitant reduction in bond energy, yielding aryl radical and triarylborane as end-products.27–29,40When borates anion is paired with cationic visible-light-absorbing compounds, tetraarylborates are good initiator systems for polymerization reactions and also have been used to probe for electron transfer reactions.27,40,41 In this publication, we study four novel donor/ acceptor ion-pair complexes (see Table1) based on tetraarylborate and pyrylium molecules. Building on a preliminary communication of proof-ofprinciple by means of transient absorption spectroscopy, NOESYNMR and electron paramagnetic resonance spectroscopy, the objective of this study is to explore ion pair−π interactions between pyrylium cations and tetraarylborate anions more comprehensively in the context of dynamics of transient species, characterization of radical species and use of photogenerated radical species as an effective initiator system for radical polymerization. Table 1. Chemical structures of the ions studied. Pyrylium (symbol) Tetraarylborate (symbol) R2

R1

O

R1 B R2

R1

R2 R2

R1 = -Methyl R1 = -Phenyl

ACS Paragon Plus Environment

(1a) (1b)

R2 = -Hydrogen R2 = -Methyl

(2a) (2b)

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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performed to obtain data on the orbital contour, charges and spin distributions, and other properties.

II. Experimental Section Chemicals. Trimethylpyrylium chloride, triphenylpyrylium chloride, sodium tetra(phenyl)borate, sodium tetra-(ptoluyl)borate, 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO), and dichloromethane were used as purchased from Sigma-Aldrich. The monomer 2-hydroxyethyl methacrylate (HEMA) from SigmaAldrich containing 50 ppm of hydroquinone was vacuum distilled before use. UV−Vis Absorption and Emission Measurements. Absorption and fluorescence measurements were recorded on a Shimatzu UV 2550 spectrometer and a Hitachi F-4500 spectrofluorometer at 298 K, respectively. Femtosecond Transient absorption. The set-up used for the transient absorption measurements is based on a regeneratively amplified Ti:Sapphire laser system (Spitfire Pro, Spectra-Physics, 1kHz) that generates 35 fs (fhwm) 3.8 mJ pulses centered at 800 nm. The output is split 50:50 into two beams. The first beam is sent to a TOPAS-C optical parametric amplifier to produces 400 - 550 nm pulses, which were used for sample excitation. The second beam was attenuated, sent through a computer-controlled optical stage to adjust the time delay with respect to the excitation pulse, and then focused onto a 2 mm CaF2 window to produce a whitelight continuum (wlc) probe pulses spanning the 345-690 nm range. The wlc probe beam was focused to a 75 μm diameter spot at the sample and overlapped at a 6-angle with the excitation beam focused to a 150 μm diameter spot. A fraction of the wlc probe beam was split off before the sample to be utilized as a reference for the correction of the shot-to-shot pulse intensity fluctuations. The probe (after the sample) and reference beams were dispersed by a spectrograph and recorded using a dual CCD detector synchronized to the 1 kHz repetition rate. The difference between the decadic logarithms of a probe-to-reference intensity ratio measured at a specific position of the optical stage for excitation on and off represents the change in the sample absorbance (ΔA) at the corresponding delay time. The solutions were circulated through a flow cell with a 2-mm path length. The zero time delay is obtained from the non-resonant electronic response from neat solvents [1] measured at the same experimental conditions. The typical excitation energy was 3.1 µJ·pulse-1 and the linearity of ∆A signals with excitation energy confirmed that single-photon excitation is responsible for the measured data. The polarization of the excitation beam was set at the magic angle (54.7°) with respect to the probe beam to eliminate solute rotational signals. All experiments were performed at 294 K. Laser Flash Photolysis. Transient absorption spectra and time decay were measured on an Edinburgh instruments LP-920. A Quantel Brilliant Nd:YAG laser with third harmonic (355 nm) coupled to an Opotek Inc. Rainbow optical parametric oscillator was used to obtain excitation wavelength at 480 nm with 5 ns pulse. Electron Paramagnetic Resonance Measurements. X-band (9.5 GHz) EPR measurements were performed at 303 K using a TE102 rectangular cavity with 100 kHz magnetic field modulation and 0.5-1.0 G modulation amplitude (EMX plus, Bruker BioSpin, Rheinstetten, Germany). Light irradiation of the initiator system was performed using a LED laser at 410 ± 5 nm placed at 0.3 m from the cavity optical window. Measurements were performed in free oxygen solution.

TR-FTIR Spectroscopy. A Jasco 6600 real-time Fourier transformed infrared spectrometer (TR-FTIR) with ATR accessory was used to follow the C=C double bond conversion versus time for polymerizations. The evolution of the near-infrared methacrylate C=C double bond peak was followed from 600 to 4000 cm−1. A LED@410 nm with 10 mW cm-2 at the sample position was used for the photopolymerization experiments. Photo-DSC. The bulk polymerization of HEMA photoinitiated by the ion-pair complex (1  10-3 mol L-1) was measured by photoDSC (TA DSC Q100 instrument) and a TA-PCA photo unit equipped with an UV− vis light source (200−520 nm). All photopolymerizations were performed under N2 flux (50 mL min−1) in an aluminum pan with 15 mg of monomer solution, and 5 W cm−2 irradiation was performed between 220 and 500 nm spectra range. A cutoff filter at 390 nm was used for measurements with visible irradiation. Polymerization of HEMA (bulk) in presence of the ion-pair complex (10-2 mol L-1) were performed by isothermal photocalorimetry. The conversion percentages, conversion %, were obtained integrating the area under the exothermic peak Ht. 𝑪𝒐𝒏𝒗𝒆𝒓𝒔𝒊𝒐𝒏 % =

𝑯𝒕 𝑯𝟎

× 𝟏𝟎𝟎

(1)

where ∆Ht is the reaction heat evolved at time t and ∆H0 is the theoretical heat evolved assuming total conversion. The experimental polymerization rate (RP) was calculated using 𝑹𝑷 =

𝑯𝒕/𝒅𝒕 𝑯𝟎

(2)

where dHt is the reaction heat evolved at dt and ∆H0 is the heat evolved assuming total conversion ( 60 kJ mol-1). ).27,40 X-Ray Crystallography. Crystals of the ion-pair complex spyrylium and tetraarylborate were grown in a saturated dichloromethane/THF solution at 268 K kept in the dark for two weeks. The data collection was performed using MoK radiation (l = 0.71073 Å) on a Bruker APEX II Duo diffractometer. All detailed information about the structural determination are summarized in the Table S1. NMR Spectroscopy. NMR spectra were recorded on Bruker Avance III HD (14.1T) spectrometer using non-deuterated residual signal as reference. Two-dimensional experiments gradientselected heteronuclear single quantum coherence (HSQC) and gradient-selected heteronuclear multiple bond correlation (HMBC), as well as unidimensional total correlation spectroscopy (1D TOCSY) and nuclear Overhauser effect spectroscopy (1D NOESY) were performed on 1a:2a and 1b:2a chemical systems. 1D NOESY was performed using selcssfnozs, which minimizes artifacts in NOESY spectra arising to the evolution of zeroquantum coherence of J-coupled spins during the mixing time. Lastly, 1D TOCSY was obtained using seldigpzs, in which a selective echo refocuses the selected spin, which is then transferred down the spin system by the DIPSI-II isotropic mixing sequence. Gel Permeation Chromatography (GPC) Analysis. Molecularweight (Mn and Mw) and dispersity of the polymer were determined by a GPC instrument (Shimadzu Prominence LC system) via a refractive index detector and DMF as eluent. The flow rate of the system was 1 mL min−1 at 35 °C. The molecular weights were calculated using polystyrene standards.

Density Function Theory (DFT) Calculations. DFT calculations, performed with the Gaussian 98 program using the three-parameter hybrid functional B3LYP with the 6-31G* basis set, were


Page 3 of 11

HOMO

(II, VI) (III, V)

LUMO

(6, 8) (7)

(2, 4) Solvent Signal

(IV)

h

1a:2a

8 5O 6 1 4 3 2 7 1a ion IV III V II VI B

H2O

1

H NMR

HOMO

h

1

H NOESY

LUMO 1b:2a

1

H NOESY

Irradiation

Figure 1. Crystal fragment of the 1b:2a ion pair complex and orbital contour plot with molecular electrostatic potential (MESP) revealing the intermolecular association mediated by π-π interactions. Colour codes: boron, brown; oxygen, red; and carbon, grey. The π-π interaction between two rings are emphasized as dashed lines. For reasons of clarity, hydrogen atoms are omitted.

0.0

0.1

-

(a)

1a with BF4 1a:2a complex 1a:2b complex 1b with BF4 1b:2a complex 1b:2b complex

Experimental Simulation-1 Simulation-2 Simulation-3 Simulated spectra (1 + 2 + 3)

(b)

Absorbance

0.4

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 nm

330 nm

0.0 300

400

500

600

700

800

Wavelength (nm)

300

400

500

600

Wavelength (nm)

Figure 3. (a) Uv-vis absorption spectra of four different ion pairs and the pyrylium precursor salts in solid phase (powder). (b) Experimental uv-vis spectra of the 1a:2a complex and simulated spectra obtained by deconvolution of the absorption shoulders, using dichloromethane as the solvent medium.

III. Results and Discussion A single crystal with 1:1 (cation/anion) stoichiometry has been obtained for the 1b:2a complex, and its structure determined by single crystal X-ray diffraction. The crystal structure reveals one ππ interaction between one of the aromatic rings of donor (tetraarylborate anion) and the acceptor (pyrylium core) unit. The centroid…centroid distance between these rings was found to be 3.6893(13) Å, which is a distance short enough to allow charge transfer transitions.28 See Figure 1 for crystal fragment structure. The space group and other crystallographic details are shown in Table S1-S6 (Supporting Information). In a non-complexed form, the 1b cation has three aromatic rings with essentially coplanar to pyrylium core and the bond lengths are considerably shorter than the carbon-carbon single bond, indicating delocalization of the π-systems over the aromatic rings.42 This coplanar structure is partially broken for the ion-pair complexes, wherein the dihedral angle found for the chemical bond between phenyl group and pyrylium core on the cation structure is 18.40 (the angle value predicted by the crystallographic results). In agreement with the crystallographic results, the 1H-NOESY NMR spectrum of 1a:2a and 1b:2a complex also reveals the π-π interaction between both ions as expected in host–guest complexes (see Figure 2). In fact, apart from obvious peaks due to the atoms that are close together in the same molecular fragment, the spectrum also shows all of the predicted cross-peaks relative to interactions between the cation moiety and the counterion (see Figure 2). In particular for the 1a cation, both H(2) and H(4) atoms show interionic contacts with H (II and IV) of 2a anion. The protons of CH3 groups (1 and 4 positions) also indicate interactions with the phenyl group of 2a anion. Further support for a proximity between

8

7

6

5

1

4

3

2

1

0

2a ion Figure 2.1H NMR Spectrum (600 MHz, CD2Cl2) and NOESY1D spectra irradiated at 2.47 and 7.4 ppm of the 1a:2a complex. Solvent signal is assigned to the protonated dichloromethane solvent.   ( H) / ppm

the pyrylium protons and the aromatic ring of 2a anion comes from the high value of the chemical shift in 2a anion (7.40 ppm; H(I)) into the complex form compared to that of the non-complexed 2a anion (6.30 ppm; H(I)). See Supplementary information for NMR characterization. The existence of interionic contacts in itself indicates that 1a cation (or 1b cation) and 2a anion in dichloromethane are mostly present as ion pairs. In both complexes 2a anion prefers the position that maximizes the π-interaction with the 1a cation. The crystal structure of the 1b:2a complex also reveals that a phenyl group from tetraarylborate structure and the pyrylium core is arranged in a parallel displaced configuration, as depicted in the representation of the crystal fragment (Figure 1). In this situation, the π-π interactions combined to opposite charges (electrostatic interaction) may promote a distortion of the electron clouds. In consequence of this effect, the charge transfer process is possible and new charge transfer absorption band may be observed in the visible-spectra range. Figure 3 shows the uv-vis absorption spectra of the 1a:2a complex in the solid (powder)and the liquid phases, where dichloromethane is used as solvent media. For the precursor salt of tetraarylborates (e.g. sodium tetraphenylborate and sodium tetra(p-tolyl)borate) the absorption bands in the ultraviolet spectra range are all due to π→π* transition. No evidence of an aggregation process was observed in our uvvis absorption, fluorescence and light scattering experiments. In agreement with the absence of aggregation, the Beer-Lambert law was found for an absorbing 1a:2a complex at 330 nm and 400 nm, both presenting molar extinction coefficient around 10-4 L mol-1 cm-1 (see Figure S1, SI). For the precursor salt of the 1b ion, the preexistence of two absorption bands in the visible absorption spectra is due to the intramolecular charge transfer, involving migration of electron density from phenyl group to the pyrylium core. Indeed, the absorption spectra of pyrylium cations can be characterized as to be due to the presence of oriented dipole moment vector along C2 axis passing through a phenyl group and the oxygen atom. In the dichloromethane solvent, for the 1b:2a or 1b:2b ion-pair complexes, the preexistence of intramolecular charge transfer from the phenyl to the pyrylium core may make difficult the observation of the intermolecular charge transfer band from the tetraarylborate anion (donor) to the pyrylium cation (acceptor). However, a red shift of the lowest absorption band is observed in the uv-vis absorption spectra (see Supplementary information for uv-vis absorption spectra), revealing that the presence of the tetraarylborate anion near to pyrylium core also has significant influence in the intramolecular charge transfer process.


The Journal of Physical Chemistry 18 (a)

A (mOD)

A (mOD)

0 ps 0.1 ps 0.2 ps 0.3 ps 0.5 ps

14 (b)

0 ps 0.1 ps 0.2 ps 0.3 ps 0.5 ps

0

0

A (mOD)

0.5 ps 0.8 ps 1 ps 2 ps

6

3 ps 6 ps 10 ps

0.5 ps 0.8 ps 1.0 ps 2.0 ps

A (mOD)

6

0 0 400

500

600

Wavelength (nm)

400 450

700

600

Wavelength (nm)

700

Figure 4. Transient absorption spectra recorded at different times delay in dichloromethane solutions. Laser pump at 440 nm was used for the (a) 1a:2a and (b) 1a:2b complexes. Figure S2-S5 shows excitation and fluorescence emission spectra of the 1a:2a complex in dichloromethane solutions. The difference between the excitation and absorption spectra suggests that the excited states of the complex relax via a mechanism that bypasses this emissive state (em = 510 nm). The Stokes shift between the fluorescence excitation and fluorescence also indicates a significant energy changes (6000 cm−1 or 17 kcal/mol). To get more detail about these ion-pair complexes, Density Functional Theory (DFT) calculations were carried out in order to evaluate the singlet and triplet properties such as HOMO and/or LUMO orbital contour, dipole moment, spin density and electrostatic potentials (see Table S7). In the singlet state of the 1a:2a complex, the orbital contour of tetraarylborates shows that one of the four aryl rings contribute an additional two electrons to populate the boron p-orbital. Hence the boron atom has a partial negative charge and the conjugated π-electrons are shifted slightly toward the different ions due to the electrostatic interactions. Figure 1 (right side) shows the contour maps with electrostatic potential. The mapping of molecular electrostatic potential (MESP) has been carried out on pyrylium/tetraarylborate complex to bring out the common set of electrostatic characteristics of complexes. The MESP isosurface plots have been generated and the critical point characteristics estimated in terms of difference of coloration (red for negative values and blue for positive values). The MESP also indicates the presence of π-π stacking interactions combined with charge shifts between the pyrylium core and the aryl group of the tetraarylborate moieties, in which the p-conjugation-spans the two-dimensional sheets of each ion by use of a strong π-π staking and oriented electrostatic interactions in the z-axis. This type of ion-pair complex permits the distortion and increase the proximity of the p-delocalized clouds between two closed rings. Due to the p-delocalized clouds between two closed rings, one ring from the anion and the other ring from the cation, the charge transfer and electron transfer processes are possible. However, the respective values of Ered (~ -0.4 eV) and Eox (~ 0.8 eV) )43,44 potential values of pyrylium cation and tetraarylborate anion indicates that the pyrylium in the ground state may not be used to oxidize tetraarylborate anions, as predicted by the positive values for the Gibbs free energy (G > 0). 𝐷+

𝐴

𝐷

𝐴―

( ) ― 𝐸( )

∆𝐺𝑒𝑡 = 𝐸

𝐷+

𝐴

𝐷

𝐴―

( ) ― 𝐸( ) ― ∆𝐸

∆𝐺𝑒𝑡 = 𝐸

500 ps 1.0 ns 1.5 ns 2.3 ns

10 ps 20 ps 50 ps 200 ps

A (mOD)

500 ps 1000 ps 1500 ps 2300 ps

10 ps 20 ps 50 ps 200 ps

This new component, ΔE00, indicate that the pyrylium cation in the triplet-state may be used to oxidize tetraarylborate anions, as predicted by the well-known Rehm-Weller equation (4). In point of fact, the Gibbs energy change for the electron transfer reaction in the triplet state is shown as an exergonic process (ΔG = - 0.94 eV).

3.0 ps 6.0 ps 10.0 s

0 2

0 2 A (mOD)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 11

(3)

00

(4)

where E(D+/D) and E(A/A−) correspond to the donor and acceptor ground state potentials, respectively, ΔE00 is the transition energy between the vibrationally relaxed ground and excited states of the fluorophore (ΔE00 ~ 2.14 eV for the ion-pair complex between tetraphenylborate and triphenylpyrylium). In the triplet state of 1b:2a complex, the spin density distributions show that the two unpaired electrons are mainly localized in the pyrylium structure. However, for the 1a:2a complex, the unpaired electron is also predicted with low contribution in the phenyl group of the tetraphenylborate, which may indicate an effective borate participation in the triplet-state nature. The phosphorescence emission of the 1b:2a complex (or 1b:2b complex) reveals similar spectra and lifetime behavior of the precursor triphenylpyrylium salt (1b cation with tetrafluorborate as counter ion), indicating that the triplet-state character of the 1b:2a complex is mainly due to the pyrylium structure side. For the non-complexed 1a ion, the fluorescence or phosphorescence emission is not observed in the visible spectra region. However, in the complex form with 2a anion, fluorescence and phosphorescence emissions are observed for the 1a:2a and 1a:2b complexes, which indicates the formation of new emissive states from excited singlet and triplet states. In an effort to directly detect the early stages of the excited state formation of these three compounds, their transient absorption spectra and associated decay kinetics were monitored through femtosecond pump–probe experiments in dichloromethane solutions. Figure 4 shows the transient absorption spectra of 1a:2a (Fig. 4a) and 1a:2b (Fig. 4b) complexes upon excitation at 440 nm and 520 nm, respectively. This excitation wavelength overlaps exclusively with the CT band. The ground state bleach (negative signal) is observed at 450 nm around 100 and 200 fs after excitation, with concomitant induced absorption bands (positive signals) around 400 nm and between 500 and 750 nm. These absorption bands have a decay time constant of around 1000 fs, and the ground state bleach signal partially recovers on the same time scale. In fact, excitation into the CT-band populates the initial Franck−Condon charge-transfer (CT) state. The stimulated emission band around 450 nm ‒ 510 nm is attributed to the singlet excited state formed via back charge transfer of the initial CT* state. For 1b:2a and 1b:2b complexes, only one induced absorption band at 550 nm is observed at 100 fs after excitation in the CTband. This absorption band has a decay time constant around 1000 fs with concomitant increase of the stimulated emission around 480 ‒ 500 nm, as also observed in the steady-state fluorescence spectra at 298 K. See Supplementary information for transient absorption spectra 1b:2a and 1b:2b complexes (Figure S6). Nanosecond laser flash photolysis measurements give insight into the transient absorption at 400 nm. We observed biexponential decay kinetics (see Figure S7), which were attributed to the triplet excited state deactivation. This assignment is confirmed by quenching of this transient absorption band in the presence of molecular oxygen.

After excitation in the CT-band (exc = 410 nm), the triple state is populated with a new negative contribution for the equation (3).


Page 5 of 11 CH3

(a) 1a:2a complex

H

Experimental Simulation

H3C

O

*

CH3

* *

*

344

346

*

348

Experimental Simulation

N

*

H

H

H

H

H

H

H

H

pyrylium radical H H

O

H

O

H H

H

H H

Experimental Simulated

*

*

Field (mT)

* * *

H

(d)

TMPO-Phenyl adduct radical

During 5 min of Irradiation at 410 nm After 50 min of Irradiation at 410 nm

pyrylium radical

*

(c)

(b) 1b:2a complex

H

* 350

344

346

348

344

350

Field (mT)

* *

* 346

*

*

348

* 350

344

*

* *

346

Field (mT)

*

*

348

350

Field (mT)

Figure 5. Recorded EPR spectra in solutions of (a) 1a:2a complex and (b) 1b:2a complex in the presence of TMPO and absence of oxygen molecule. (c) is the experimental and simulated spectra of the (*) Phenyl-TMPO• radical adduct. (d) experimental and simulated spectra with hyperfine structure of the 1b-radical. Phenyl radical kd2

B

Ion Pair * kISC Complex

hCT

3

R

Ion Pair * ket Complex

R

hF

1

Ion Pair Complex

O

R

R

hP

Pyrylium + Tetraarylborate

kd1

B

+ Monomer

Radical Pair R

O

R

Low efficiency

+ Monomer

1

B

High efficiency

Polymer

Pyryl radical

0

Scheme 1. Photochemical and polymerization mechanisms which use pyrylium/tetraarylborate as a photoinitiator system, where ket is the electron transfer rate constant and kd1 and kd2 are the dissociation constants of the radical pair and tetraarylborate anion. 5

[1a:2a] [1a:2b] [1b:2a] [1b:2b]

(b)

[1a:2a] [1a:2b] [1b:2a] [1b:2b]

RP X 10

-3

-1

(a)

(min )

120

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0 0

10

20

30

40

Time (min)

50

60

70

0

10

20

30

40

50

60

70

Time (min)

Figure 6. (a) Percentage conversion and (b) polymerization rate of HEMA photoinitiated by the ion-pair complex (1 × 10−3 mol L−1) using visible light (λ > 390 nm) and pure monomer. Figure S7a shows nanosecond transient absorption spectra recorded for the 1a:2a complex after excitation at 355 nm. We observed a short-lived component (τ1 = 10.7 μs; 43 %) and a longlived component (τ2 = 41.9 μs; 57 %). For the 1b:2a complex, the two components were observed with τ1 = 0.6 μs (33 %) and τ2 = 11.6 μs (67 %). Figure S8 shows the EPR signal of the 1a:2a and 1a:2b complexes at 77 K during continuous excitation at 410 nm. Both signals are observed as an anisotropic mixture of tetraarylborate radical and pyryl radical signals around g = 2.000. The tetraarylborate contribution on the EPR spectra is confirmed when the 2a anion is replaced by the 2b anion. In this situation, the hyperfine structure is confirmed as product of interactions between both radicals In the presence of spin-trapping molecule such as 3,3,5,5tetramethyl-1-pyrroline-N-oxide (TMPO), the pyryl radical is easily characterized after 50 min of irradiation at 410 nm. Figure 5 show the spectra of the pyryl radical with multiline structure (the signal centered at 347.5 mT). The simulation of the pyryl radical is observed as redline in Figure 5d and the coupling constant values are shown in Table S8 and S9. In the absence of TMPO molecules, both radical signals are not observed at 298 K. This testify the dynamics of radicals to recover the initial complex in the ground-state, bypassing the states involved in the back-electron transfer process.

The oxidized tetraarylborate species is observed as a radical species at 77K and absence of TMPO molecules. However, in the presence of TMPO molecules and temperature of 298 K, the chemical stability of tetraarylborate radical is low and new paramagnetic species is confirmed as a new adduct Phenyl-TMPO radical (Figure 5), as consequence of phenyl radical release from the oxidized tetraphenylborate structure. The simulation of the phenyl radical is shown in red-line (see Fig. 5c), using symbols (*) to specify the max signal peaks. See Table S8 and S9 for the coupling constant values. The phenyl radical trapped by TMPO molecule shows similar spectral profiles and coupling constants previously reported by Buettner,45 aN = 15.8, aH = 24.4 in aqueous media (pH = 7.0), using DMPO molecule. To examine the radical polymerization behavior of vinyl monomer such as 2-Hydroxyethyl methacrylate (HEMA), the polymerization of the HEMA with 1a:2a complex was carried out at 298 K in bulk conditions. The use of Real-Time Attenuated Total Reflection (ATR)–FTIR spectroscopy and PHOTO-DSC technique both allow the monitoring the polymerization reactions. By isothermal photo-calorimetric analysis at 330 K under irradiation in the visible spectra range (λ > 390 nm). The polymerization rate (RP) was calculated using equation (2). As observed in Figure 6, using different complexes as photoinitiator system, the polymerization of HEMA is confirmed and the maximum polymerization rate is also observed after some time of reaction. The photochemical and polymerization mechanism are shown in scheme 1. After 30 min of photopolymerization with 1a:2a complex (1 × 10−3 mol L−1) as initiator system, the DSC result indicates that 69% of monomer is converted to the polyHEMA. The polymerization process was also performed with in situ ATR–FTIR spectroscopy. In the IR-spectrum one absorption peaks was of interest: the C=C stretch at 1635 cm-1, characteristic of the monomer (see Figure S9). The spectra evolution indicates that the band decrease of the monomer concentration becomes detectable and thus, changes in the polymer ratio can also be identified.


n


Polymer

b H2C

O

aH 3 C

c H C

e

2

a

OH

O

d

H

bH

C H2

b'

H 2O

L

Monomer

C

O

H3 C

c H C

O

Polymer active center

e

2

e

c

a

OH

L

b’

e

6

5

DMSO

d c

B

b 1

2

3

L

B

L

Triaryborane

d

4

L

C

d H2

L b

Page 6 of 11

a

C O

O

O

HO

ppm

Figure 7. 1H NMR (600 MHz, DMSO) spectra of polymer and monomer residues after 40 min of photopolymerization (  > 390 nm) using 1a:2a complex (1 × 10-3 mol L-1) as initiator of bulk polymerization.

C O

L L

HO

L

B

L L

B C O

L

O

C O

Experimental Simulation-1 Simulation-2 Simulation-3 Simulation-4 Sim 1+2+3+4

Low reactive radical adduct

O HO HO

Scheme 2. Suggestive chemical mechanism to form a stable adduct radical and its chemical rearrangement to form a non-reactive polymer.

18

19

20

21 22 Elution time (min)

23

24

25

Figure 8. Deconvolution of GPC elution curve using 1a:2a complex as photoinitiator system. Table 2. GPC data results using 1a:2a or 1b:2a complex (1 × 10-3 mol L-1) as photoinitiator system. Momomer HEMA was used in the bulk polymerization %

1a:2a

MW  103 (g mol-1) 86.6 126.1 90.6 55.4 27.5

Đ

Total 1 2 3 4

Mn  103 (g mol-1) 65.7 124.5 89.5 53.5 25.7

1.3 1.0 1.0 1.0 1.1

100 33 32 23 12

1b:2a

Total 1 2 3 4

36.6 76.5 58.7 30.1 10.5

51.2 77.4 59.0 33.4 11.1

1.4 1.0 1.0 1.1 1.1

100 32 21 41 6

Initiator System

Peak#

As observed in Figure 7, the assignment of the resonance peaks in the 1H-NMR spectrum leads to the accurate evaluation of the monomer and the content of monomeric unit. The proton resonances of the methyl groups (–CH3) at 0.7–1 ppm, the –CH2– CH2– group at 3.4–4.2 ppm and –OH at 4.7–4.9 ppm in HEMA monomer and PolyHEMA are clearly resolved and distinguished. See supplementary information for other NMR results. The presence of hydrogen signal 4.7–4.9 ppm of the polymer also indicates that the alcohol group does not participate efficiently in parallel reactions. Furthermore, the end polymer obtained in the photopolymerization reaction was soluble in dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) solvents, suggesting the absence or low content of reticulated polymer via cross-linking reactions.

To investigate the effect of two different photoinitiators (1a:2a and 1b:2a) in the final molecular weight of polymer, after 40 min of irradiation in the visible region ( > 390 nm), PHEMA samples were obtained at 40 °C and solubilized in DMF solvent (15 g L-1) prior to GPC analysis. The GPC data results (Table 2) shows that the polymer solution had a major distribution followed by other three low contributions thus giving a low Mw/Mn (Đ) values. Other GPC component may also be observed around 23.7 minutes, which was attributed to oligomers traces with low percentage value (< 1 %). However, as observed in Figure 8, the deconvolution of GPC elution curve indicates that the simulation curve with only four components (Sim 1+2+3+4) almost represents the experimental curve. Đ values around 1.3 – 1.4 indicates a non-conventional value for free radical polymerization as previously observed in other studies with HEMA.27,40,41 Actually, Đ values less than 1.5 are usually observed in specific and controlled polymerization reactions such as reversible addition−fragmentation chain-transfer polymerization (RAFT), atom transfer radical polymerization (ATRP) and ring-opening metathesis polymerisation (ROMP) that allows accurate control over the molecular weight and length.46–55 Triarylborane molecules are good electron acceptor molecules that may react with electron donors or Lewis bases such as amine molecules, sulfur based structures, carbanion and electron-rich radical species56–60 to form stable adduct radical species. In this sense, we suggest that the low value of weight dispersity is due to the competitive and parallel reactions to suppress the active center in polymer radical center. This chemical mechanism is shown in Scheme 2. For non-based boron molecule, no firmly established example of the bimolecular homolytic substitution (SH2) was found at a simple saturated carbon center. Conversely, once an organic free radical is generated in the polymer structure, this active center may reacts with organoborates, where the empty p-orbital of organoboranes may works as Lewis acid species.56,57,61,62


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IV. Conclusions

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In summary, the mechanistic investigations highlighted that the competition between the radiative and reductive pathways of the pyrylium cation governs the complex behavior, leading to fluorescence and phosphorescence emission in the first case and free radical polymerization in the second case. π-π stacking interactions combined with electrostatic interactions may be used to form a stable ion-pair complexes between pyrylium and tetraarylborate ions in which the interaction of the π-delocalized clouds of both ions promotes: (a) π-orbital delocalization into z-axis, promoting a “quasi-chemical bond” character to the π-π stacking interaction; (b) observation of new radiative processes (i.e. fluorescence and phosphorescence) and also electron transfer process excitation using visible light. The radicals (i.e. phenyl and pyrylium radicals) were identified and attributed to the dissociation process of ion pair complex in the triple state, after electron transfer process from the tetraarylborate anion to the pyrylium core. Indeed, the results exhibit a striking combination of properties: chemical stability, photophysical and photo-chemical events that makes these ion-pair complexes as a step towards the realization of chromophore/luminescent materials and also the usage as new mono-photoinitiator system in radical polymerization reactions. By GPC data analysis, the polydispersity was found around 1.3 – 1.4, meaning that the photopolymerization reaction with pyrylium-tetraarylbroate ion pairs may be performed in a controlled pathway. The extension of this work to improve photophysical and photochemical properties of new ion-pair complexes between pyrylium and tetraarylborates that may all efficiently absorb light, emit light and make efficient photoinitiator are of great interest for future research.

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ASSOCIATED CONTENT Supporting Information Experimental details, additional spectra, characterization and crystallographic data (PDF). Data for 1b:2a complex (CIF) Corresponding Author

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*E-mail [email protected] (W.G.S.). *E-mail [email protected] (S.J.L.R.).

Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT

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The authors are grateful for the financial support and grants from CNPq (150080/2018; 303207/2017-5). ANT acknowledges CHE0923360 and CHE-1626420. W.G.S. thanks D. R. Cardoso (IQSCUSP) for the EPR and LFP facilities (FAPESP 2017/01189-0 and 2011/51555-7).

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