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Spin-Selective Photoinduced Electron Transfer within Naphthalenediimide Diradicals Nathan T. La Porte, Joseph A. Christensen, Matthew D. Krzyaniak, Brandon K. Rugg, and Michael R. Wasielewski J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b06303 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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The Journal of Physical Chemistry

Spin-Selective Photoinduced Electron Transfer within Naphthalenediimide Diradicals Nathan T. La Porte†, Joseph A. Christensen†, Matthew D. Krzyaniak, Brandon K. Rugg, and Michael R. Wasielewski* Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, 2145 Sheridan Rd, Evanston, IL 60208, USA

Abstract: There has been increasing interest in the excited states of stable diradicals as means of manipulating their spin states for potential applications in quantum information science (QIS). In this work, we examine a set of diradicals composed of two stable naphthalene-1,8:4,5bis(dicarboximide) radical anions (NDI•–) bound either directly at their imide nitrogen atoms or through a series of benzene spacers resulting in diradicals with either singlet or triplet ground states. We use time-resolved near-UV, visible, near-IR, and mid-IR spectroscopy to show that the population in the singlet ground state can undergo photoinduced electron transfer upon excitation of one of the NDI•– radicals to produce the NDI0-NDI2–, while the corresponding triplet population cannot. In particular, spectroscopy in the wavelength region 330–450 nm and in the energy range 1450–1750 cm–1 is critical to distinguishing the two populations. By varying the connectivity between the two radical anions, we vary both the sign and magnitude of the singlet-triplet energy splitting (2J) of the diradicals, thereby varying the proportion of singlet and triplet ground state populations that are detected optically. EPR spectroscopy provides corroborating evidence for the ground spin state of the diradicals. This result has implications for using photoexcitation to manipulate the spin states of diradicals for QIS applications.

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INTRODUCTION Ultrafast photodriven charge transfer in electron donor-acceptor systems can produce spinentangled singlet radical pairs (RPs) in which subsequent spin evolution under the influence of environmental magnetic fields, usually due to electron-nuclear hyperfine interactions, results in a variety of spin coherence phenomena.1-4 These RPs can act as spin qubit pairs in molecular systems for quantum information science (QIS) applications.5-9 Spin polarization of the RP spin states is frequently observed using time-resolved electron paramagnetic resonance (TREPR) spectroscopy, while pulse-EPR spectroscopy is used to observe coherences between the RP spin states. In general, the small energy differences between the RP spin states precludes direct optical observation of their spin properties. However, the ability to employ powerful optical spectroscopy techniques to probe these phenomena is important for their potential use in QIS applications. One of the best characterized systems that uses optical techniques to interrogate spin states in this context is the nitrogen vacancy (NV-) center in diamond.10 These defect centers have the advantage that visible photons can be used to prepare them in a highly pure initial spin quantum state that has a ~5 ms coherence time at room temperature. This state can be manipulated using microwave photons, and the result of these operations can be read out using fluorescence emission from the defect center, which means that optically detected magnetic resonance (ODMR) techniques11 can be used to address single defect sites. Thus NV- centers in diamond fulfill several of the important criteria required for QIS systems.12 However, the main drawback of spin qubits based on defect centers is the difficulty of fabricating them at specific locations and selectively addressing different centers. In contrast, chemical synthesis can be used to build novel QIS systems from the bottomup, and their resulting properties can be tailored to specific applications. Here, we present results

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on a series of diradicals in which transient optical absorption techniques are used to distinguish between singlet and triplet diradicals. We have investigated a series of diradicals in which both radicals are naphthalene-1,4:5,8bis(dicarboximide) radical anions (NDI•–) covalently linked directly or using a series of benzene spacers (Scheme 1). The excited doublet state of NDI•– (*NDI•– ) has attracted attention recently1315

as one of a class of powerful reducing agents that can be produced by photoexciting NDI•– at

wavelengths as long as 780 nm.13,16,17 *NDI•– has an excited-state oxidation potential of –2.1 V vs SCE, making it capable of reducing a wide variety of chemical species, including catalysts commonly used for energy-demanding solar fuels reactions such as CO2 reduction.13 In addition, organic donor-acceptor dyads incorporating NDI•– have also been explored, chiefly by Majima and co-workers,15,18,19 where *NDI•– can transfer an electron to an adjacent chromophore. For example, they show that the reaction NDI•–-sp-NDI•–  NDI0-sp-NDI2– (sp = spacer) can occur. Given that NDI•–-sp-NDI•– is a diradical, the spin selectivity of this type of electron transfer O

R N O

O O

O

O

O

R N

N

R N

N

O

O

O

O

O

NDI-mPh-NDI

NDI O

O

O

O

R N

N

N

N R

O

O

O

O

O

O

O

R N

N

N

N R

O

O

O

O

NDI-pPh-NDI O

O

R N

N

O

O

N

O N O

O

NDI-Xy-NDI O

R=

N R O

NDI-NDI

Scheme 1. Structures of NDI and dimers studied in this work.

process has not been considered in previous work, and can provide a means to control and manipulate the electron transfer processes in such systems. 3



Photochemical reactions involving RPs conserve electron spin angular momentum.20-22 In a diradical, such as NDI•–-sp-NDI•–, the energy level splitting between the singlet and triplet RP, 2J, i.e. the Heisenberg exchange interaction, ultimately determines its reactivity. Depending on the sign of 2J, the diradical can have either a singlet or triplet ground state, or if 2J ~ kT, both the singlet and triplet spin states can be populated based on Boltzmann statistics. We have shown previously that ultrafast photogeneration of *NDI•– does not perturb its spin configuration relative to an adjacent radical.23 Thus electron transfer from *NDI•– to NDI•– should only proceed from the singlet state of the diradical, which has implications for the observed rate and yield of electron transfer. In order to test the hypothesis that spin-selectivity plays a role in the photophysics of organic diradicals, a set of NDI•–-sp-NDI•– diradicals were synthesized (Scheme 1). The connectivity between the two NDI units was systematically varied across the series to probe the effect of spin exchange coupling on their photophysics. The spacers were selected to vary the substitution around the bridging phenyl ring (para in NDI•–-pPh-NDI•– and NDI•–-Xy-NDI•–, meta in NDI•–-mPhNDI•–), the torsional angle between the two NDI subunits (40.2° in NDI•–-pPh-NDI•– versus 23.3° in NDI•–-Xy-NDI•–), and the distance between the two NDI subunits (12.8 Å in NDI•–-pPh-NDI•– versus 8.4 Å in NDI•–-NDI•–). DFT-optimized geometries of each diradical are given in the Supporting Information. In this work, we demonstrate that the singlet and triplet states of the NDI•–-sp-NDI•– diradicals, where sp = pPh, Xy, and mPh, are both populated at room temperature, while only the triplet state is populated when there is no spacer. By carefully fitting the time-resolved spectroscopic data we are able to estimate the ratio of singlet to triplet population in each diradical and thereby obtain an estimate of 2J for each diradical. Additionally, we demonstrate that there are differences in the 4


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excited-state behavior of NDI•– when it is excited at its lowest energy absorption band versus a higher energy absorption band, and that these differences must be taken into account when analyzing spectroscopic data involving these chromophores. EXPERIMENTAL METHODS Materials. Solvents used for synthesis and spectroscopy were obtained from Fisher Scientific and used as received. All NDI derivatives were reduced using CoCp2 in slight excess in the glovebox and sealed under nitrogen. Detailed synthetic procedures and compound characterization are shown in the Supporting Information (SI). Steady-state optical spectroscopy. UV/Vis/NIR absorbance spectroscopy was performed on a Shimadzu UV-1601 spectrometer at 298 K. FTIR spectroscopy was performed on a Shimadzu IRAffinity-1 spectrometer at 298 K. Time-resolved

optical

spectroscopy.

Femtosecond

transient

UV/Vis/NIR

absorption

experiments were performed employing a regeneratively amplified Ti:sapphire laser system operating at 828 nm and a 1 kHz repetition rate as previously described.24,25 The output of the amplifier was frequency-doubled to 414 nm using a BBO crystal and that light was used to pump a laboratory-built collinear optical parametric (OPA) amplifier for visible-light excitation.26 Approximately 1-3 mW of the fundamental was focused onto a sapphire disk to generate the visible white-light probe spanning 430-850 nm, onto a 5 mm quartz cuvette containing a 1:1 mixture of H2O:D2O to generate a UV/visible white light probe spanning 385-750 nm, or onto a proprietary medium (Ultrafast Systems, LLC) to generate the NIR white-light probe spanning 850-1620 nm. The total instrument response function was 300 fs. Experiments were performed at a randomized pump polarization to suppress contributions from orientational dynamics. Spectral and kinetic data were collected with a CMOS or InGaAs array detector for visible and NIR detection, respectively, 5



and a 8 ns pump-probe delay track (customized Helios, Ultrafast Systems, LLC). Transient spectra were averaged for at least 3 seconds. Gaps in the spectra shown are due to either scattering of the pump or idler beam, or regions not covered by the detectors. Samples prepared in DMF had an absorbance of 0.2-0.7 at the excitation wavelength and were irradiated in 2 mm quartz cuvettes with 0.4-0.8 μJ/pulse focused to ~0.2 mm diameter spot. Samples were stirred to avoid effects of local heating or sample degradation. The samples were prepared in the glovebox. Nanosecond transient UV/Vis/NIR absorption experiments were performed using the femtosecond excitation beam described above and a commercial spectrometer (Eos, Ultrafast Systems, LLC) utilizing a photonic crystal fiber ultra-broadband probe source. The pump polarization was randomized to suppress rotational dynamics. Samples were stirred to avoid effects of local heating or sample degradation. Femtosecond transient mid-IR absorption (fsIR) spectroscopy was performed using a commercial Ti:sapphire oscillator/amplifier (Solstice 3.5W, Spectra-Physics) to pump two optical parametric amplifiers (TOPAS-C, Light Conversion), one which provided a 100 fs, 605 nm excitation pulse and the other provided 100 fs pulses at 2150-1800 cm–1. The overall instrument response was 300 fs. The spectra were acquired with a liquid N2-cooled dual channel (2 x 64) MCT array detector that is coupled to a Horiba HR320 monochromator as part of a Helios-IR spectrometer (Ultrafast Systems, LLC). Samples with a maximum optical density of 1.5 at the excitation wavelength were prepared in DMF contained in a liquid demountable cell (Harrick Scientific) with 2.0 mm thick CaF2 windows and a 500 μm Teflon spacer. During data acquisition, the cell was mounted and rastered on a motorized stage to prevent sample degradation. Electron paramagnetic resonance spectroscopy. Continuous wave EPR measurements were performed with a Bruker Elexsys E580-X EPR spectrometer, equipped with a super-high Q 6


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resonator (ER4122-SHQE). The spectra were obtained at room temperature and at 77K using a liquid nitrogen dewar insert with non-saturating microwave power. The samples were prepared in 1.5mm ID x 1.8mm OD quartz EPR tubes (Vitrocom). The resulting spectra were processed in Matlab and fit using Easyspin.27 Computational Methods. Computations were performed using the Qchem package. Gas-phase geometries were optimized at the B3LYP/6-31G* level of theory, and singlet and triplet energies for NDI2– were calculated at the B3LYP/6-31+G* level. The frequency analysis of the minimum energy geometries confirmed that they represent the potential energy surface minima. RESULTS Synthesis. The syntheses of NDI-pPh-NDI, NDI-Xy-NDI, NDI-mPh-NDI and NDI-NDI are detailed in the Supporting Information and summarized in Scheme S1. Steady-state spectroscopy. The electronic absorption spectra of monomeric NDI, NDI•–, and NDI2– have been reported previously.16 All NDI•--sp-NDI•- diradicals exhibit electronic absorption and FTIR features that are essentially identical to those of the NDI•– monomer (Figure 1). In the electronic absorption spectrum of NDI•–, the main features are a small peak at 396 nm, a large peak at 472 nm, and smaller peaks at 603 nm, 696 nm, and 774 nm. In the NDI•--sp-NDI•diradicals, these peaks shift by no more than a few nanometers, and the ratios of their heights remain relatively constant. The one exception is the region between 475 nm and 610 nm, which in

7



(A)

(B)

Figure 1. Electronic absorption (A) and FTIR (B) spectra of the NDI•- monomer and the four NDI•--sp-NDI•diradicals in CH3CN, where the spacer (sp) is indicated in the legend.

the NDI•--sp-NDI•- diradicals exhibit slightly increased absorbance but no additional structure beyond that present in the monomer spectrum. The FTIR spectrum of the NDI monomer exhibits several absorptions in the C–C and C=O stretching regions whose energies are diagnostic of the NDI oxidation state (Figure 2). In neutral NDI, they lie at 1716 cm−1, 1679 cm−1, 1584 cm−1, and 1347 cm−1. Upon reduction with CoCp2 to yield NDI•–, the peaks shift to 1637 cm−1, 1601 cm−1, 1516 cm−1, and 1337 cm−1, respectively. Reduction with CoCp*2 yields NDI2–, with strong absorptions at 1604 cm−1, 1597 cm−1, and 1525 cm−1, as well as weaker absorptions at 1479 cm−1, 1428 cm−1, 1416 cm−1, 1388 cm−1, and 1380 cm−1.

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0.25

NDI NDI NDI2-

0.20

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


0.15 0.10 0.05 0.00

1700

1600

1500

1400

-1

1300

Wavenumber (cm ) Figure 2. FTIR spectrum of NDI monomer in different oxidation states in CD3CN: NDI (black), NDI•- (red), and NDI2- (blue).

Transient Absorption Spectroscopy (fsTA and fsIR). Because transient absorption spectroscopy is used to show that there are multiple populations undergoing different photoinduced processes in the NDI•–-sp-NDI•– diradicals, a detailed analysis of the fsTA spectra of NDI•– is necessary. Since we have shown previously that excitation of the analogous perylenediimide (PDI) radical anions at different wavelengths can lead to different excited-state behaviors;28 we examined whether this is also the case with NDI•– by carrying out transient absorption experiments with 605 nm excitation and comparing the fsTA spectra to those obtained with 780 nm excitation at the lowest energy NDI•– absorption band. Species-associated spectra obtained from singular-value decomposition of the fsTA spectra obtained at both excitation wavelengths are shown in Figure 3. The fsTA spectra and fits to the kinetic traces obtained from the analysis given in Figures S1 and S2. As expected, the spectra obtained with λex = 780 nm fit to a single excited-state decay with a lifetime of τ = 142 ± 1 ps, identical within error to the lifetime obtained previously.16 This best-fit spectrum displays bleaches of all the NDI•– ground-state absorptions superimposed on a broad excited-state absorption that extends from 400–750 nm, and a sharper excited absorption at 354 nm. 9



(A)

5.0

*NDI•–  = 142 ± 1 ps

A x 103

 = 780 nm 2.5 0.0 -2.5 -5.0 350 400 450 500 550 600 650 700

Wavelength (nm)

(B) 2

**NDI•– 1 = 0.9 ± 0.3 ps *NDI•– 2 = 103 ± 0.3 ps

1

A x 103

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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 = 605 nm

0 2.5

-1

2.0 1.5 1.0

-2 -3

0.5 0.0 -0.5

340

360

380

400

420

440

350 400 450 500 550 600 650 700 750

Wavelength (nm) Figure 3. Species-associated spectra obtained from kinetic analysis of the fsTA spectra of NDI•– excited at 780 nm (A) and 605 nm (B) in CH3CN. Inset to (B): Expansion of the 350-450 nm spectra region.

When NDI•– is excited at 605 nm, however, a two-component kinetic model is required to fit the data, and spectral evolution is observed in the region below 450 nm associated with rapid internal conversion from a higher-lying doublet excited state (**NDI•–) with an absorption maximum at 360 nm and a significant shoulder at 375 nm to the lowest excited state (*NDI•–) in τ1 = 0.9 ± 0.3 ps, which is accompanied by the appearance of the 354 nm absorption. The decay of *NDI•– occurs

in τ2 = 103 ± 0.3 ps. Prior to this report, no fsTA data had been obtained for NDI•–

at probe wavelengths below 400 nm,16 so that these data provide new insights into NDI•– excited state dynamics.

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- 26 ps 0.6 ps 39 ps 110 ps 220 ps 580 ps

8 4

A x 103

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 -4 -8 1750

1700

1650

1600

1550 -1

1500

Wavenumber (cm ) Figure 4. FsIR spectra of NDI•– in CD3CN (λex = 605 nm).

Because the strongest spectral signatures of the three NDI oxidation states (neutral, radical anion, and dianion) are relatively broad and appear in a narrow region of the spectrum (340–420 nm), transient absorption experiments in the mid-IR region (fsIR) were carried out to distinguish the different species. CD3CN was used as the solvent due to the lack of solvent IR absorption bands in the region of interest. The fsIR spectrum obtained upon 605 nm excitation of NDI•– in CD3CN shows that the ground-state features at 1635 cm–1, 1600 cm–1, and 1514 cm–1 bleach instantaneously, while excited-state features at lower energies (1623 cm–1, 1535 cm–1, and 1498 cm–1) appear (Figure 4). An additional shoulder in the 1585–1550 cm–1 range is also observed. The energies of the bleaches do not exactly match the ground-state features observed in the FTIR spectra owing both to the lower resolution of the fsIR data and the overlap of the ground-state and excited-state features. The spectral features all decay with identical kinetics, matching the decay of the excited-state features in the fsTA spectra. Additional fsIR spectral data for NDI•– are given in Figure S3.

11



Having analyzed the effect of exciting an absorption of NDI•– other than the lowest energy band, we nevertheless chose to excite the diradicals at 605 nm. The main reason for this choice was practical, i.e. a much greater excitation pump energy could be achieved at 605 nm using our experimental apparatus, allowing for stronger transient signals with a higher signal-to-noise ratio more amenable to detailed analysis as long as the internal conversion rate of **NDI•– is taken into account in the kinetic model. All fsTA experiments on the diradicals were performed in CH3CN, while the fsIR spectra were acquired in CD3CN. First, we consider the three diradicals linked by a single aryl spacer, NDI•–-pPh-NDI•–, NDI•–Xy-NDI•–, NDI•–-mPh-NDI•–. The fsTA and fsIR spectra of each diradical display very similar features with slightly different kinetics. In order to highlight the salient features, the best-fit spectra from a global analysis of multiple wavelengths in the fsTA spectra (Figure 5) will be discussed. To aid in visualizing the differences between the spectra of the diradicals, only the wavelength range 330–450 nm, where the strongest differences are apparent, will be shown in the subsequent figures. However, all fitting was carried out across the full spectral range (330–1500 nm). The full TA spectra and kinetic traces with best-fit curves for NDI•–-pPh-NDI•–, NDI•–-Xy-NDI•–, and NDI•–-mPh-NDI•– are presented in Figures S4, S6, and S9, respectively, in the Supporting Information. In all three cases, the fsTA data are fit to a kinetic model that assumes a portion of the population does not undergo electron transfer (**NDI•–-sp-NDI•– → *NDI•–-sp-NDI•– → NDI•–sp-NDI•–) and a portion of the population does undergo electron transfer (*NDI•–-sp-NDI•– → NDI0-sp-NDI2– → NDI•–-sp-NDI•–); for mathematical details of the model see the Supporting Information. Because the lifetime of *NDI•–-sp-NDI•– in the population that undergoes electron transfer approaches the instrument response time, we were not able to fit the decay of (**NDI•–12


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**NDI--NDI(*NDI--NDI-) 1 (*NDI--NDI-) NDI2--NDI

A (normalized)

1.00


1.00

3

0.75

3

0.75

0.50

0.25

0.25

0.25

•–

-0.25

NDI -mPh-NDI 350

4

375

•–

400

425

450

Wavelength (nm)

•–

-0.25 NDI 330 450

NDI -pPh-NDI

-0.25

350

4

NDI2-

2

0.00

0.00

•–

375

400

425

Wavelength (nm)

•–

-Xy-NDI 360

•–

400

Wavelength (nm)

440

10

NDI2-

2

NDI2-

5 0

3

0.75

0.50

0.00


1.00

0.50

A x 103

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

-2

-27.1ps 1.14ps 10.5ps 107ps 2.93ns

-4 -6 1750

1700 1650

-27.1ps 1.14ps 10.5ps 107ps 254ps 6.08ns

-2 -4 •–

NDI -mPh-NDI 1600 1550

•–

1500

-6 1750

Wavenumber (cm-1)

1700 1650

0 -5 •–

•–

NDI -pPh-NDI 1600 1550

Wavenumber (cm-1)

1500

-10 1750

-27.1ps 2.05ps 10.4ps 130ps 7.32ns

NDI -Xy-NDI

1700 1650

1600 1550

•–

•–

1500

Wavenumber (cm-1)

Figure 5. Top row: Species-associated spectra obtained from the multi-wavelength analysis of the fsTA spectra of the indicated NDI diradicals in CH3CN. Spectra from each diradical are normalized to the maximum of the green spectrum to facilitate comparisons of the relative intensities of each species. Bottom row: fsIR spectra of the NDI diradicals in CD3CN (λex = 605 nm).

sp-NDI•– → *NDI•–-sp-NDI•– in that population. The amplitudes of the species-associated spectra are indicative of the population of that species. The

**NDI•–-sp-NDI•–

and 3(*NDI•–-sp-NDI•–) spectra in all three diradicals resemble the

species-associated spectra for

**NDI•–

and *NDI•– of the monomer, suggesting that their

assignments as the vibrationally hot and cool excited states, respectively, in the non-electrontransfer population is correct. The best-fit lifetimes for those species match the lifetimes of these two species in the monomer as well. As expected, the 1(*NDI•–-sp-NDI•–) spectra in all three compounds closely resemble the 3(*NDI•–-sp-NDI•–) spectra; however, the best-fit lifetime of 1(*NDI•–-sp-NDI•–)

is much shorter than the lifetime of 3(*NDI•–-sp-NDI•–), where 1(*NDI•–-sp-

NDI•–) decays with time constants of 2.3 ps, 7.8 ps, and 14.0 ps for NDI•–-pPh-NDI•–, NDI•–-Xy13



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NDI•–, and NDI•–-mPh-NDI•–, respectively. As species 1(*NDI•–-sp-NDI•–) decays, NDI0-spNDI–2 forms. This species displays the same ground-state bleaches as all the other species, and has four prominent induced absorptions at 357 nm, 378 nm, 395 nm, and 418 nm. These correspond to each of the two largest peaks in the ground-state absorption spectra of NDI0 (measured at 361 nm and 381 nm) and NDI2– (measured at 400 nm and 423 nm).16 This charge-shifted state NDI0sp-NDI2- decays much more slowly than it forms, with a lifetime of 610 ps in NDI•–-mPh-NDI•– and approximately 6 ns in NDI•–-pPh-NDI•– and NDI•–-Xy-NDI•–, where the lifetimes for NDI0pPh-NDI2- and NDI0-Xy-NDI2- were determined from fits to the nsTA data shown in Figures S7 and S10, respectively. Interestingly, when sp = mPh, the spectra of 3(*NDI•–-sp-NDI•–) and 1(*NDI•–-sp-NDI•–)

have approximately equal magnitudes, and when sp = pPh and Xy, the

magnitude of 3(*NDI•–-sp-NDI•–) is approximately three times that of 1(*NDI•–-sp-NDI•–). This difference in the ratio of non-electron-transferring and electron-transferring populations, will be discussed further below. A summary of the kinetics is presented in Table 1. Table 1. Time constants for photophysical processes.

NDI•–

τ**NDI → *NDI (ps) 1.3 ± 0.3

τ*NDI → GS (ps) 105 ± 3

τCS (ps) -

NDI•–-mPh-NDI•–

τCR (ns) -

1.1 ± 0.3

110 ± 15

2.3 ± 0.3

0.61 ± 0.01

NDI•–-pPh-NDI•–

0.3 ± 0.3

120 ± 10

7.8 ± 0.3

4±1

NDI•–-Xy-NDI•–

0.6 ± 0.3

130 ± 15

14.0 ± 0.3

4±1

NDI•–-NDI•–

1.5 ± 0.3

110 ± 1

-

-

The fsIR spectra of the three diradicals confirm that there is a population that undergoes the charge-shift reaction (Figure 5). At early times, the spectra resemble those of the NDI•– monomer. 14


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Within the first ten picoseconds, however, small peaks at 1711 cm–1 and 1677 cm–1 appear, while additional spectral density in the 1580–1550 cm–1 and 1500–1475 cm–1 regions also appears. The two low-energy peaks are consistent with those discussed above in the FTIR spectrum of NDI0, while the other additional density is consistent with the FTIR spectrum of NDI2– (Figure 2). Additional fsIR and spectral data for NDI•–-pPh-NDI•–, NDI•–-Xy-NDI•–, and NDI•–-mPh-NDI•– are given in Figure S5, S8, and S11, respectively. (A)

(B)

Figure 6. (A) Best-fit spectra from multi-wavelength analysis of fsTA spectra of NDI•–-NDI•– in CH3CN with excess CoCp2 (λex = 605 nm). (B) fsIR spectra of NDI•–-NDI•– in CD3CN with excess CoCp2 (λex = 605 nm).

We first attempted to fit the fsTA data obtained from the NDI•–-NDI•– diradical to the same model used above, in which a fraction of the population undergoes electron transfer and a fraction 15



does not; however, no satisfactory fit was obtained using that model. Instead, the data are best fit using a simpler model in which none of the population undergoes electron transfer, just as in the NDI•– monomer (Figure 6A). In accordance with a model in which no electron transfer occurs, the fsIR spectra for NDI•–-NDI•– (Figure 6B) resemble those of the monomer, with no NDI0 or NDI2signals appearing at any time. Additional fsTA and fsIR data are presented in Figures S12 and S13. EPR Spectroscopy. The continuous-wave EPR spectrum of monomeric NDI•– at room temperature in CH3CN shows the expected 13 line hyperfine pattern,29 while featureless broad lines are observed for the diradicals (Figure S14). The broad lines most likely result from a doubling of the number of hyperfine splittings when 2J for the diradicals is large combined with a contribution from residual monoradical.30 Upon cooling NDI•–-NDI•– to 77K, its EPR spectrum displays features characteristic of a triplet state, where a simulation of the spectrum yields the zerofield splitting constants D = 133 MHz and E  0 (Figure 7). The large signal in the middle of the spectrum results from the monoradical impurity that often accompanies the diradical. Using the

2

Intensity (arb. units)

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 16 of 30

Data Fit

1

0

-1

-2 325

330

335

340

345

Magnetic Field (mT) Figure 7. EPR spectrum NDI•–-NDI•– in CH3CN at 77K and  = 9.456 GHz.

16


350

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point dipole approximation, where 𝐷 = ― 2785 mT ∙ Å3/𝑟3,31 the measured D value yields an average distance between the two spins of r = 8.3 Å, which compares favorably with the 8.4 Å distance obtained from DFT calculations (see Supporting Information). By definition, the sign of 2J is opposite for triplet and singlet diradicals;32 here we adopt the convention that a positive 2J represents a ferromagnetic (triplet) ground state as indicated for NDI•–-NDI•– in Figure 7. In contrast, NDI•–-mPh-NDI•– gives no EPR spectrum at 77K, which indicates that its ground state is a singlet, while both NDI•–-pPh-NDI•– and NDI•–-Xy-NDI•– show weak EPR spectra, which once gain favor a singlet ground state at 77K (Figure S15). These weak spectra are too noisy to simulate; moreover, given that the distance between the NDI•- units in both NDI•–-pPh-NDI•– and NDI•–-Xy-NDI•– is about 12.8 Å, the value of D should decrease to about 37 MHz (or 1.3 mT for g = 2), and thus any features due to the triplet are buried beneath the residual NDI•- monoradical signal in the center of the spectrum. DISCUSSION Electron transfer behavior in singlet versus triplet populations. Using the known redox potentials for the NDI0/•– and NDI•–/2– redox couples,16 the published NDI singlet and triplet excited state energies,33 the NDI•– doublet excited state energy,16 the NDI2– singlet excited state energy and the calculated NDI2– triplet excited state energy, we are able to calculate the overall energies (relative to the NDI•–-sp-NDI•– ground state) of each potential product of excited-state decay after excitation of the NDI•–sp-NDI•– diradicals (Figure 8). As is clear from Figure 8, while the singlet excited diradical is able to undergo electron transfer to produce a low-energy charge-shifted state, no such state exists within the triplet manifold. Instead, to undergo electron transfer, one or the other of the NDI•– moieties must end up in an excited state. The lowest-energy charge-shifted state within the triplet manifold is 17


3*NDI2–sp-


Excited

Charge Shifted

[*NDI•––NDI•–] 1.60 eV

NDI2––NDI0 0.47 eV

Ground

CS with excited NDI dianion

Page 18 of 30

CS with excited NDI neutral

Singlet

1

[NDI•––NDI•–] 0 eV

1

1*


1*

NDI0–NDI2– 3.62 eV

CS state does not exist

Triplet

3

[NDI•––NDI•–] 0 eV

3

[*NDI•––NDI•–] 1.60 eV

3*


3*

NDI0–NDI2– 2.50 eV

Figure 8. Energies of the relevant NDI-sp-NDI dimer states

NDI0, which is calculated to have an energy of 1.82 eV above the ground state (for details of the calculation of the energies of possible product states see the Supporting Information). It is possible that charge transfer could occur to the

3*NDI2–-sp-NDI0

state from the initial excited state

populated by the 605 nm laser pulse, which has an energy of around 2.05 eV, however that process is likely to be slow due to the small ∆G (-0.23 eV or less). The fsTA data show that the singlet and triplet states of the NDI•–-pPh-NDI•–, NDI•–-Xy-NDI•– and NDI•–-mPh-NDI•– diradicals are populated at room temperature. Upon photoexcitation of NDI•–, the population that is initially in the diradical singlet ground state undergoes charge transfer, while the population that is initially in the diradical triplet ground state does not undergo the charge transfer. An energy-level diagram depicting the photoinduced electron transfer behavior and showing the lifetime of each process in these dyads is shown in Figure 9. In contrast, for the NDI•–

18


Page 19 of 30

3

[*NDI•–-sp-NDI•–]

1

[NDI•–-sp-NDI•–] sp =

IC < 1.5 ps

IC < 1.5 ps

[3*NDI2–-sp-NDI0]

Energy

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


CS = 2.3 ps (mPh) 7.8 ps (pPh) 14.0 ps (Xy) not observed (none)

IC ~ 110 ps

605 nm CR = 0.6 ns (mPh) 4-6 ns (pPh) 4-6 ns (Xy)

[NDI0-sp-NDI2-]

2J > 0 3

[NDI•–-sp-NDI•–]

1 •– •– 2J < 0 [NDI -sp-NDI ]

Figure 9. Energy-level diagram showing photoinduced electron transfer pathways in NDI•–-sp-NDI•– dimers.

-NDI•– diradical, the fsTA data at 295K and the EPR data at 77K are both consistent with a triplet ground state, and a 2J value that is large enough to prevent significant population of the singlet diradical, even at 295K. To understand why this is the case, we must consider the factors that determine the population of the diradical singlet and triplet states. Effect of varying coupling. If the diradical singlet and triplet states are isoenergetic, we should expect to see a 1:3 ratio of singlet to triplet based on the multiplicity of each state. This situation occurs if 2J  0; however, if 2J is larger, deviations from this ratio are expected with increased population of the lower-energy state. The relative populations of the diradical singlet and triplet states were determined from their species-associated spectra given in Figures 5 and 6. To determine the relative populations of each state, we compare the magnitudes of the ratios of the two peaks characteristic of the *NDI•– state at 338 nm and 353 nm in each species-associated spectrum (Table 2). Averaging the ratios at the two different wavelengths, the singlet:triplet population ratios of NDI•–-pPh-NDI•– and NDI•–-Xy-NDI•– are close to the expected 1:3 ratio suggesting that the singlet and triplet states are near isoenergetic, so that within the error of the 19



measurement we cannot determine whether the singlet or triplet diradical is lower in energy. However, the weak EPR spectra obtained at 77K may indicate that the singlet diradical is lower in energy at that temperature. In contrast, the singlet:triplet population ratio of NDI•–-mPh-NDI•– is close to 1:1, suggesting that the singlet state is significantly lower in energy. Assuming Boltzmann statistics, the 1:1 ratio corresponds to 2J  -230 cm-1 at 295K (see Supporting Information). This is consistent with the absence of an EPR signal for NDI•–-mPh-NDI•– at 77K. Finally, in NDI•–NDI•–, no singlet population is observed at all, indicating that the coupling between the two unpaired electrons is large enough that the diradical singlet state is not thermally populated at 295K. To estimate the lower bound of 2J, if we assume that a 10% population of the singlet state is readily observable, this implies that 2J > 230 cm–1 (see Supporting Information). Table 2. Ratio of magnitudes of species-associated spectra at specified wavelengths of the lowest-energy excited state of the singlet and triplet populations in dyads NDI•–-pPh-NDI•–, NDI•–-Xy-NDI•–, and NDI•–-mPh-NDI•–. Dyad Wavelength (nm) Ratio NDI•–-pPh-NDI•–

NDI•–-Xy-NDI•–

NDI•–-mPh-NDI•–

338

3.70

353

3.97

338

2.79

353

3.73

338

1.01

353

1.16

It is notable that 2J is negative in NDI•–-mPh-NDI•–, the meta-linked diradical, implying a singlet state that is lower in energy than the triplet state. At first glance, this seems to be a violation of the Ovchinnikov parity model,34 and more recent calculations of Barone et al.,35-39 which 20


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successfully predict the sign of 2J for a variety of organic diradicals in which para- and metaphenyl bridges link the two radicals.40-42 In fact, following the parity model, a singlet ground state is expected for NDI•–-pPh-NDI•– and NDI•–-Xy-NDI•–. There is sufficient rotational flexibility about the single bonds joining the NDI•– units and the pPh and Xy spacers that the interaction between the  symmetric SOMOs of NDI•– units and the  system of the spacer dominate. However, for NDI•–-mPh-NDI•– steric hindrance between the two NDI•– units across the mPh spacer results in a stronger interaction between the SOMOs of NDI•– units and the  system of the spacer. This results in spin polarization of the electrons in the  bonds that result in an antiferromagnetic interaction between the two NDI•– radicals.43 We observed comparable behavior previously in comparing the rates of electron transfer from a naphthalene-1,8-dicarboximide radical anion to NDI linked via their imide nitrogen atoms at the para- and meta-positions of benzene.44,45 We found, somewhat unexpectedly, that the electron transfer rate for the meta isomer was about 8 times faster than for the para isomer. Since it is well known that 2J is proportional to the rate of electron transfer,46 this indicated that the exchange coupling was commensurately larger for the meta isomer. If the exchange interaction was propagated strictly by the  systems of these components, the rate for the para isomer should have been faster. Utilization of the  bonds, of which there is one less in the meta isomer relative to the para isomer, resulted in the observed rate increase. It is also possible that a small contribution from a direct interaction between the donor and acceptor could have also contributed. Thus, we suggest that a similar interaction is responsible for the observed singlet ground state of NDI•–-mPh-NDI•–.

CONCLUSION 21



The NDI•–-sp-NDI•– diradical is capable of existing in either a singlet or a triplet ground state with the singlet ground state alone being capable of undergoing electron transfer after excitation. Analysis of the fsTA spectra at λ < 400 nm and λ > 800 nm is crucial to determining whether a population of the NDI•– excited state undergoes electron transfer, as those regions contain strong excited-state features diagnostic of the *NDI•– state. Additionally, analysis of the fsTA spectra of the NDI•– monomer at λ < 450 nm revealed that excitation at 605 nm produces a vibrationally hot excited state whose relaxation time is approximately 1.3 ps. Incorporation of this cooling process into the kinetic model is crucial for accurately fitting the overlapping processes taking place in the singlet (electron transferring) and triplet (non-electron transferring) populations. The ratio of the populations of the singlet and triplet state depends on the connectivity between the two NDI•– units, which determines the sign and magnitude of 2J. By examining the species-associated spectra obtained from the fits of the fsTA data for each diradical, the singlet:triplet population ratio is estimated and used to provide a rough estimate of 2J, which are in line with experimentally determined and computationally predicted values for similar organic diradicals. Radical ion excited states are promising high-energy species that can engage in a variety of redox processes relevant to catalysis as well as spin-selective photophysics important to QIS applications. In both fields, it is essential to understand how spin selectively determines the course of electron transfer processes. ASSOCIATED CONTENT Supporting Information Additional details, including complete synthetic procedures, spectroscopic techniques, procedures for data analysis, and results of calculations can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. 22


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Author Information Corresponding Author [email protected] ORCID Michael R. Wasielewski: 0000-0003-2920-5440 Matthew D. Krzyaniak: 0000-0002-8761-7323 Nathan T. La Porte: 0000-0001-7467-4460 Author Contributions †N.T.L and J.A.C. contributed equally to this work. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENTS This research was supported by the Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0019356. This publication was made possible by NPRP grant #9-174-2-092 from the Qatar National Research Fund (a member of Qatar Foundation) (N.T.L.). NMR and MS measurements in this work were performed at the IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the State of Illinois and International Institute for Nanotechnology (IIN). We thank Dr. Saman Shafaie for collecting high-resolution mass spectrometric data.

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