Spectroscopic Signatures of Resonance Inhibition Reveal Differences in Donor-Bridge and Bridge-Acceptor Couplings

Di ﬀ erences in Donor −Bridge and Bridge −Acceptor Couplings

Article

Spectroscopic Signatures of Resonance Inhibition Reveal

David A. Shultz,* Martin L. Kirk,* Jinyuan Zhang, Daniel E. Stasiw, Guangbin Wang, Jing Yang,

Diana Habel-Rodriguez, Benjamin W. Stein, and Roger D. Sommer

Cite This: J. Am. Chem. Soc. 2020, 142, 4916 −4924

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sı Supporting Information

ABSTRACT: The torsional dependence of the ground state

magnetic exchange coupling (J) and the corresponding electronic

coupling matrix element (H_D_A) for eight transition metal

complexes possessing donor−acceptor (D-A) biradical ligands is

presented. These biradical ligands are composed of an S = 1/2

metal semiquinone (SQ) donor and an S = 1/2 nitronylnitroxide

(

NN) acceptor, which are coupled to each other via para-

phenylene, methyl-substituted para-phenylenes, or a

bicyclo[2.2.2]octane ring. The observed trends in electronic

absorption and resonance Raman spectral features are in accord

with a reduction in electronic and magnetic coupling between D

and A units within the framework of our valence bond

conﬁguration interaction model. Moreover, our spectroscopic

results highlight diﬀerent orbital mechanisms that modulate coupling in these complexes, which is not manifest in the

ferromagnetic J_S_Q_‑_B_‑_N_Nvalues. The work provides new detailed insight into the eﬀects of torsional rotations which contribute to

inhomogeneities in experimentally determined exchange couplings, electron transfer rates, and electron transport conductance

measurements.

INTRODUCTION

in the context of a valence bond conﬁguration interaction

VBCI) model, eq 1 and Figure 1. For the systems described

and the spectroscopic

observables are the mean SQ → B-NN charge transfer (CT)

energy (U) and the CT excited state singlet−triplet gap (K₀).

■

(

Persistent donor−bridge−acceptor (D-B-A) biradicals oﬀer a

unique and instructive platform to elucidate mechanisms of

bridge-mediated electronic coupling, which is central to our

understanding of orbital pathways that contribute to molecular

superexchange-mediated electron transfer and electron trans-

port in single-molecule electronic devices. The importance of

electronic coupling has been recently highlighted as a

transferable property of the bridge moiety that directly relates

superexchange (i.e., magnetic exchange coupling) to photo-

here, J_D_A≡ J

and H_D_A≡ H

SQ‑B‑NN

2_K

J_D_A

U − K

(1)

An intense SQ(π) → B-NN(π*) intraligand charge transfer

ILCT) transition is a deﬁning spectroscopic feature that is

(

induced electron transfer rates and electron transport

conductance) mediated by molecular bridges. Prior work

characteristic of an established contiguous π-communication

pathway in these D-B-A biradicals. Conﬁgurational mixing of

the ILCT excited state into the electronic ground state

(

from our laboratories has made extensive use of X-ray

crystallography, magnetic susceptibility measurements, optical

and magnetic spectroscopies, and electronic structure

computations to elucidate electronic and geometric structure

contributions to magnetic exchange coupling and electronic

coupling in D-B-A systems where the donor (D) is a

semiquinone radical (D = SQ; S = 1/2), the acceptor (A) is

the nitronylnitroxide radical (A = NN; S = 1/2), and the

bridge (B) can be any synthetically viable molecular frag-

contributes to the magnitude of J

(i.e., the ground

which is a ﬁt

SQ‑B‑NN

,5,7

singlet−triplet gap, Δ

= 2J_S_Q_‑_B_‑_N_N),

S‑T

parameter in the analysis of D−A biradical variable-temper-

ature magnetic susceptibility data. Since J

is propor-

SQ‑B‑NN

Received: January 10, 2020

Published: February 18, 2020

−8

ment.

The presence of a single, dominant π-superexchange

pathway connecting SQ and NN allows one to evaluate the

relationship between magnetic exchange coupling, J_S_Q_‑_B_‑_N_Nand

electronic coupling (H_S_Q_‑_B_‑_N_N) using experimental observables

2020 American Chemical Society

J. Am. Chem. Soc. 2020, 142, 4916−4924

916

Article

Figure 1. Left: Frontier NN(SOMO), SQ(SOMO), and NN(LUMO) fragment orbitals utilized in the VBCI model (right). Note that both the

SQ(SOMO) and the NN(LUMO) have π-bridge character, provided that the corresponding torsion angles are less than 90°. Right: VBCI model

illustrating SQ → B-NN CT conﬁgurations ( EC and EC; a 1b 2b ) that mix with corresponding ground conﬁgurations ( GC and GC; a 1b 2b )

to create the experimentally evaluated singlet −triplet gap (2 J _S _Q _‑_B _‑_N _N ).

Figure 2. Bond-line drawings, thermal ellipsoid plots (cumenyl groups of the ancillary ligand, hydro-tris(3,5-dimethylpyrazolyl)borate

Cum,Me 21,22

(Tp

)

and hydrogen atoms have been omitted for clarity), donor−bridge and acceptor−bridge torsion angles (rounded oﬀ to the nearest

degree), donor−acceptor distances, and J

Complexes 1-Me Ph and 1-PhMe were prepared for this study.

exchange parameters (rounded oﬀ to the nearest wavenumber) of complexes studied in this work.

SQ‑B‑NN

tional to H_D_A²for a given SQ−bridge−NN complex, the

determined bond lengths, bond torsion angles, and the

distance between the donor and acceptor fragments. We

have shown that J_S_Q_‑_B_‑_N_Ndecays exponentially as a function of

distance (r_S_Q_‑_N_N) for oligo-thiophene and oligo-phenylene

bridges, and the experimentally determined decay constants

(β) are in agreement with those determined from electron

transfer kinetics, in accord with a bridge mediated super-

electronic coupling can be determined conveniently from the

ratio J_S_Q_‑_B_‑_N_N/J_S_Q_‑_N_N(=H_S_Q_‑_B_‑_N_N²/H_S_Q_‑_N_N²).

4,5,7

Here, J_S_Q_‑_N_Nis

the magnitude of the nonbridged “parent” biradical complex

for which both J_S_Q_‑_N_Nand H_S_Q_‑_N_Nhave been determined from

,5,7

detailed magnetic and spectroscopic studies.

Moreover, due

to the persistent nature of these biradicals, H_S_Q_‑_B_‑_N_Nand

J_S_Q_‑_B_‑_N_Ncan be directly correlated with crystallographically

exchange pathway.

917

J. Am. Chem. Soc. 2020, 142, 4916−4924

Article

Figure 3. Variable-temperature magnetic susceptibility plots displayed as χ_p_a_r_aT vs T plots for the two new complexes, 1-PhMe (blue open circles)

and 1-Me Ph (red open triangles). Fits include a weak, antiferromagnetic intermolecular Weiss correction of −0.51 K (1-PhMe ) and −0.28 K (1-

Me Ph); see the SI.

The degree of π−π-orbital overlap in conjugated D-B-A

systems plays a deﬁning role in modulating the resultant

electronic coupling. Wasielewski and coworkers have shown

structures with large donor torsion and small acceptor torsion

relative to the bridge, and vice versa. Thus, to complete this

series of complexes, we prepared 1-Me Ph and 1-PhMe (see

using oligo(phenylene-ethylene)-bridged donor−acceptor mol-

the Supporting Information (SI) for synthetic and crystallo-

2,3,15,20

ecules that the magnitude of H is related to the torsion

graphic details) using our standard procedures.

These

angles between the D, B, and A fragments, and the electronic

coupling possesses a cos(φ) dependence for each fragment-

fragment bond torsion within a D-B-A photoinduced electron

two new complexes possess the desired conformations, Figure

2, for selectively and independently reducing D-B and B-A

electronic couplings. For comparison, also shown in Figure 2

are the bond-line drawings and thermal ellipsoid plots of 1-Ph,

transfer (PET) system. Similarly, work by Mayor et al. has

shown a bond torsion dependence on the magnitude of the

conductance (g) in molecular systems. Additionally, Harriman

1-MePh, 1-PhMe, 1-p-MePhMe, 1-Me PhMe , and 1-BCO

2 2

that have been previously reported. Torsion angles for the

(substituted) phenylene-bridged biradical complexes were

determined using the mean planes of the SQ rings and bridge

rings (ϕSQ) and the O−N−C−N−O plane of NN and mean

plane of the bridge rings (ϕNN). These data are provided in

Figure 2.

Eﬀects of Steric Inhibition of Resonance Conjugation

on Magnetic Exchange Coupling. Since both the SQ and

NN radicals are S = 1/2 spin-bearing units, the relative

magnitudes of the bridge-mediated SQ-NN electronic coupling

et al. and Wenger and Hanss have shown that PET rates

vary as a function of the torsional dependence of H , while

Newton described an extended McConnell relationship that

accounts for both σ- and π-contributions to electronic

coupling. Using SQ−bridge−NN donor−acceptor biradical

complexes, we have recently shown that the magnitude of the

electronic and magnetic exchange couplings as a function of

bridge torsion angles span a well-described, smooth 3D

surface. These results are important since they show that

(

^HSQ‑B‑NN) in our series of complexes can be determined

the π-contribution to electronic coupling is ∼13 times greater

16,17

conveniently at high resolution by measuring the relative

magnitudes of their respective bridge-mediated SQ-NN

than the σ-coupling pathway, in accord with theory.

In this

paper, we detail how electronic absorption and resonance

Raman spectroscopies can be used in conjunction with

magnetic susceptibility measurements to develop a more

comprehensive understanding of steric inhibition of reso-

,7,23−26

^m^a^gⁿ^e^tⁱ^c^e^x^c^h^aⁿ^g^eⁱⁿ^t^e^r^a^c^tⁱ^oⁿ^s^,^JSQ‑B‑NN

This can be

accomplished using temperature- or magnetic ﬁeld-dependent

,27

spectroscopies (e.g., electronic absorption spectroscopy

EPR, ) or magnetic susceptibility measurements with the

singlet−triplet exchange splitting being equal to 2J

8,19

nance

biradicals.

in torsionally rotated donor−bridge−acceptor

SQ‑B‑NN

according to the Heisenberg exchange Hamiltonian, Eqn. 2:

RESULT AND DISCUSSION

̂ ̂

= −2J_S_Q_‐_B_‐_N_NS ·S

SQ‐B‐NN

■

(2)

Molecular Structures and Conformations. Our pre-

vious report on the torsional dependence of electronic and

exchange coupling lacked critical data points corresponding to

where the individual S are the spin operators for the S = 1/2

radical spins. Magnetic susceptibility data, plotted as the χ_p_a_r_a

918

J. Am. Chem. Soc. 2020, 142, 4916−4924

Journal of the American Chemical Society

Table 1. Magnetic Exchange (J ) and Electronic Coupling (H

ratio with 1-Ph for comparison.

Article

) Constants for Methyl-Substituted, Phenylene-

SQ=B‑NN

SQ‑B‑NN

Bridged SQ-NN Biradical Complexes

J_S_Q_N_N(cm⁻¹)

H_S_Q_N_N(cm⁻¹

)

ref

5, 20

compd

-Ph

-MePh

category

(deg)

SQ‑bridge

bridge‑NN

100

3632

3269

514

-Me Ph

this work

-PhMe

-p-MePhMe

-PhMe₂

3060

2179

1816

514

this work

-Me PhMe

-BCO

363

5 b

Electronic couplings determined by ratio with 1-Ph for which H_S_Q_‑_B_‑_N_Nwas determined by VBCI matrix diagonalization. Although the

superexchange pathways for 1-Me Ph, 1-Me PhMe , and 1-BCO may be/are dominated by σ-bonds, their electronic couplings are computed by

Figure 4. Experimental exchange coupling values vs the cosine-squared of torsion angles for SQ-Ph and Ph-NN. Square-blocked grid correlates

experimental J

values with experimental SQ-Ph and Ph-NN torsion angles: J

= 0.34 + 3.77cos (ϕ

) − 13.4cos (ϕ_P_h_‑_N_N) +

SQ‑B‑NN

SQ‑Ph

179cos (ϕ

) cos (ϕ ) + 16.3cos (ϕ_P_h_‑_N_N). See the SI for error analysis.

Ph‑NN

SQ‑Ph

product vs temperature for the two new complexes (1-Me Ph

NN bond torsions (14° and 64°, respectively). As a result,

H(1-MePh) ∼ 1.5H(1-p-MePhMe). We now compare the

data for complexes 1-PhMe and 1-Me PhMe . These two

complexes have nearly equal B-NN bond torsions (70° and

69°, respectively) but quite diﬀerent SQ-B bond torsions (48°

and 1-PhMe ), are displayed in Figure 3 (see SI for additional

magnetic data). Best ﬁts (derived from eq 2, see the SI) to the

data yield J

cm .

= +25 ± 0.5 cm⁻and J₁_‑_M_e₂_P_h= +1.6 ± 0.1

‑PhMe

−

and 84°, respectively). As a result, H(1-PhMe ) ∼ 3.5H(1-

The McConnell superexchange model deﬁnes H_D_Aas the

product of the electronic coupling matrix elements that

describes the pairwise interactions (H , H ) within a D-B-

Me PhMe ). These results are conveniently illustrated graphi-

^c^a^l^l^y^,^aⁿ^d^a^p^l^o^t^o^f^m^a^gⁿ^e^tⁱ^c^e^x^c^h^aⁿ^g^e^,^JSQ‑B‑NN, versus the

product of the cosine-squared of the SQ-Ph and Ph-NN

torsion angles (ϕSQ‑B and ϕB‑NN, respectively, Figure 2 and

A triad divided by the energy gap between the localized donor

and bridge states (Δ ) as shown in eq 3.

Table 1) is provided in Figure 4. The two new complexes

occupy weaker coupling regions of the surface map.

H H

Δ_D_B

DB BA

H_D_A

(3)

Steric Inhibition of Resonance Conjugation Probed

by Electronic Absorption and Resonance Raman

Spectroscopies. The methyl groups, although sterically

When the dominant electronic coupling occurs through a

series of π-systems connected by σ-bonds, there is a torsional

rather small (d ∼ 2 Å), are eﬀective at rotating independently

dependence of H_D_Band H_B_Athat is described by eq 4, where i

or collectively the SQ donor relative to the bridge (1-MePh, 1-

p-MePhMe, 1-Me Ph, and 1-Me PhMe ) as well as the NN

and j index two adjacent π-units of the D-B-A system and H is

the intrinsic electronic coupling matrix element at cos ϕ = 0,

acceptor relative to the bridge (1-PhMe, 1-p-MePhMe, 1-

,11

where the constituent π-systems are coplanar.

PhMe and 1-Me PhMe ), Figure 2 and Table 1. When either

the SQ or NN moieties are “ﬂanked” by two methyl groups,

they are rotated so that their π-systems are nearly orthogonal

H = H cos ϕ

(4)

Since J_S_Q_‑_B_‑_N_Nvaries as H_S_Q_‑_B_‑_N_N²(eq 1), there will be a cos ϕ

dependence for J_S_Q_‑_B_‑_N_N. It is clear from the data in Table 1

that the H_S_Q_‑_B_‑_N_Ncoupling requires a strong interaction of the

bridge with both SQ/donor and NN/acceptor orbitals.

Consider the data in Table 1 for complexes 1-MePh and 1-

p-MePhMe. These two complexes have nearly equal SQ-B

bond torsions (48° and 47°, respectively) but quite diﬀerent B-

to those of the phenylene bridge (1-Me

Ph, 1-PhMe , and 1-

Me PhMe ) (Table 1). Due to steric diﬀerences between the

two radicals, two ortho-methyl groups are less eﬀective at

rotating the NN than they are at rotating the SQ. The smaller

internal bond angles of the 5-membered NN ring compared to

those of a 6-membered ring contributes to this eﬀect. The

attenuation of conjugation across the D-B-A triad that results

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J. Am. Chem. Soc. 2020, 142, 4916−4924

Article

Figure 5. Solution electronic absorption spectra (CH Cl at 298 K) for all complexes: (A) 1-Ph, 1-MePh, and 1-Me Ph; (B) 1-p-MePhMe, 1-

PhMe , and 1-PhMe; (C) 1-BCO and 1-Me PhMe .

from steric interactions between methyl groups and the SQ

and NN radicals dramatically attenuates the magnetic exchange

coupling in this series of biradicals, and this inhibition of

resonance is also manifest in the diﬀerent electronic absorption

spectra of these complexes (Figure 5). While all the spectra are

characterized by absorption bands originating from the

extended conjugation connecting SQ with NN across the SQ-

B-NN structure. At higher energies than the ILCT band, all of

the category A complexes display a distinct, structured CT

−

band at ∼27,500 cm with virtually unchanged intensity

across the series. We assign this band as the NN(SOMO) →

Ph-NN(LUMO) + Ph-NN(HOMO) → NN(SOMO) tran-

sition on the basis of the earlier spectral assignments made for

−

constituent chromophores (SQ, ∼12,000 cm ; 840 nm and

−

−1

30,31

Cum,Me

NN, ∼17,200 cm ; 580 nm, ∼27,000 cm ; 370 nm),

Cu(SQ-NN), Tp

Cu(SQ-m-Ph-NN), and Ph-

only those complexes with eﬀective donor−bridge−acceptor

extended π-conjugation exhibit a charge transfer transition of

NN, the results of TD-DFT computations, and the fact that

1-Ph, 1-MePh, and 1-Me Ph all lack steric interference that

−

varying intensity at ∼23,000 cm (∼435 nm).

would result in inhibition of resonance between the phenylene

bridge and the NN radical. Remarkably, the electronic

Comparison of these six spectra show that complexes with

methyl groups that can interfere sterically with the NN

acceptor possess ILCT bands with attenuated intensity that are

overlapped with broadened or shifted 27,000 cm⁻¹B-NN(π →

π*) bands. The electronic absorption spectra for these eight

compounds can be grouped into three categories: A, B, and C.

Category A (Table 1, Figure 5A) is exempliﬁed by 1-Ph, 1-

absorption spectrum of 1-Me Ph is strikingly similar to that

of 1-m-Ph, which possesses a cross-conjugated meta-phenylene

bridge (m-Ph). Thus, the loss of Ph-NN conjugation as a

function of torsional rotation produces a similar eﬀect to cross-

conjugation with respect to the loss of SQ → B-NN ILCT

intensity.

MePh, and, 1-Me Ph, and inspection of their geometric

Category B compounds (Table 1, Figure 5B) include 1-

structures indicates that they will display the highest degree of

Ph-NN π-conjugation in the series since there is no methyl

group steric interference with the NN moiety. Complex 1-Ph

has an unsubstituted para-phenylene bridge and exhibits an

intense intraligand charge transfer (ILCT) band near 23,000

PhMe, 1-p-MePhMe, and 1-PhMe , which display electronic

absorption spectra that are all quite similar. The category B

compounds possess a marked decrease in Ph-NN π-

conjugation when compared with the category A compounds

due to the increased torsional rotation about the Ph-NN bond.

The reduction in Ph-NN π-conjugation leads to charge transfer

absorption features being shifted to higher energy with respect

to the category A compounds. TDDFT computations on 1-

PhMe₂indicate electronic transitions responsible for the

−

cm . This band has been probed extensively by a combination

of electronic absorption, resonance Raman (rR), and hot band

spectroscopies. Additional insight into the nature of this

transition has been obtained within the context of DFT and

TD-DFT computations leading to the rigorous assignment of

this band as possessing appreciable intraligand SQ(π) → B-

−

absorption band centered at ∼26,000 cm possess contribu-

tions from SQ → B-NN and B-NN → SQ one-electron

promotions, both of which serve to promote magnetic

exchange coupling between the SQ and NN spins within a

VBCI model. The rather dramatic diﬀerences in spectral

features for 1-MePh and 1-PhMe indicate that bridge−

acceptor conjugation plays a greater role in ILCT band energy

−5,7,15

NN(π*) charge transfer (ILCT) character.

Thus, the

presence of an ILCT band in this energy region is a hallmark of

extended donor-bridge-acceptor π-conjugation in SQ-B-NN

biradicals. A similar ILCT band is observed for 1-MePh, but it

is slightly blue-shifted and possesses reduced intensity when

compared to 1-Ph. The ILCT band in 1-MePh retains the

vibronic structure observed in 1-Ph, which is associated with

an excited state distortion along in-plane SQ, NN, and

and intensity than does donor−bridge conjugation. Thus, the

electronic absorption spectra can distinguish between these

two bond torsion eﬀects at near parity of the exchange

coupling (e.g., 1-MePh vs 1-PhMe). Here, population of the

Ph-NN π* LUMO via a one-electron promotion creates the

ILCT state (Figure 1). The vibronic structure associated with

the NN(SOMO) → Ph-NN(LUMO) + Ph-NN(HOMO) →

NN(SOMO) transition is observed on the high energy side of

the broad CT feature for 1-PhMe and 1-p-MePhMe at

phenylene vibrational modes. The ILCT band assignment for

1-Ph and 1-MePh is also supported by their respective

resonance Raman spectra and excitation proﬁles, vide infra. We

observe a precipitous decrease in the ILCT band intensity for

-Me Ph since the phenylene bridge is rotated orthogonal to

the SQ plane (ϕ_S_Q_‑_b_r_i_d_g_e= 95°), and this leads to a loss of

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J. Am. Chem. Soc. 2020, 142, 4916−4924

predominantly the bonding combination of NN LUMO and bridge (phenylene) LUMO.

Article

Figure 6. (A) Resonance Raman proﬁle (red solid circles) for 1-MePh showing enhancement of the Ph(8a)-SQ quinoidal stretch in the ILCT

portion of the electronic absorption spectrum (black line). Raman enhancement of the quinoidal stretch is relative to the Na SO internal standard.

(

B) Frontier Kohn−Sham orbital fragments that ﬁgure prominently in the ILCT transition (vertical gray arrows) and in the VBCI mechanism for

magnetic exchange coupling. Dominant SQ-Ph and Ph-NN fragment orbital contributions are shown for simplicity. Note that the LUMO is

Figure 7. (A) From top to bottom: Solid state (asterisk: Na SO internal standard) resonance Raman spectra for 1-Ph, 1-MePh, 1-PhMe, and 1-p-

−

MePhMe; λ_e_x= 407 nm. Note the reduced intensity of the ∼1600 cm Ph(8a)-SQ quinoidal stretch (highlighted gray) as a function of the

diﬀerent SQ-Ph and Ph-NN ring−ring torsion angles indicating the eﬀect of inhibiting extended conjugation as a function of these bond torsions.

These data have been normalized to the 800 cm⁻peak, which is tentatively assigned as an in-plane SQ stretch, for comparative purposes. (B)

Intensity of the Ph(8a)-SQ quinoidal stretch for these compounds as a function of the magnetic exchange coupling, J_S_Q_‑_B_‑_N_N. The point at the origin

is the quinoidal rR enhancement expected when the SQ and NN moieties are completely decoupled, and the solid line is simply a guide to the eye.

−

∼

27,500 cm . Intensity in this region is diminished in 1-

fully consistent with complete electronic and magnetic π-

decoupling of the spin-bearing units and dramatically reduced

exchange coupling constants. Thus, the electronic absorption

spectra, which represent a weighted distribution of all

accessible torsional conformations in solution, correlate

remarkably well with the eﬀects of individual D-B and B-A

bond torsions determined from the solid-state structures

determined by X-ray crystallography.

Our magnetic susceptibility and electronic absorption data

clearly show that the conformation of the phenylene bridge

relative to both the SQ and NN radical moieties is critically

important in deﬁning the magnitude of the donor−bridge−

acceptor π-coupling and the nature of the ILCT spectral

features. Resonance Raman spectroscopy is a powerful tool for

PhMe due to increased torsional rotation about the Ph-NN

bond leading to a loss of conjugation.

The category C (Table 1, Figure 5C) compounds are

comprised of 1-Me PhMe and 1-BCO, for which both SQ

and NN radical π-systems are decoupled due to either the

complete absence of a π-system bridge (1-BCO) or large SQ-

Ph and Ph-NN bond torsions that eﬀectively eliminate donor−

acceptor π-conjugation in 1-Me PhMe . Thus, neither 1-

Me PhMe nor 1-BCO (Figure 5C) possess strong charge

transfer absorbance features in the ILCT region of the

−

spectrum or in the higher energy (i.e., 27000 cm )

NN(SOMO) → Ph-NN(LUMO) + Ph-NN(HOMO) →

NN(SOMO) region of the spectrum. This observation is

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Article

Figure 8. (A) SQ−bridge−NN conjugation maximized for no/small bond torsions between donor-bridge and between acceptor-bridge. The

quinoidal resonance form (right) illustrates the vibronically active SQ-phenylene(8a) bridge mode. (B) Steric inhibition of resonance for SQ donor

rotated out of conjugation with bridge. (C) Steric inhibition of resonance for NN acceptor rotated out of conjugation with bridge.

probing the nature of excited state distortions relative to the

equilibrium ground-state geometry, and can therefore be used

to probe the role of the bridge and the degree of donor−

acceptor coupling through the observation of resonantly

A plot of the relative Raman intensity for the ∼1600 cm⁻

SQ-Ph(8a) quinoidal stretch as a function of J

SQ‑B‑NN

presented in Figure 7B. The “break” in the data points occurs

−

at ∼75 cm , and this corresponds to the methyl group

substitution pattern changes between the category A

> 70 cm ) and the category B complexes

^JSQ‑B‑NN < 70 cm ), eﬀectively highlighting the sensitivity of

the resonance enhancement as it relates to SQ-B vs B-NN

bond torsions.

,32

enhanced bridge modes.

With respect to 1-Ph, rR

−

complexes (J

(

spectroscopy has shown that vibrations in the 1200−1590

SQ‑B‑NN

−

cm region derive from SQ and NN in-plane stretching

vibrations, and the most resonantly enhanced Raman band is

−

−1

observed at ∼1600 cm . This ∼1600 cm band has been

rigorously assigned as the in-plane Ph(8a)-SQ stretching

vibration, where the phenylene bridge component of this

stretching frequency is similar to the ν₈_aquinoidal stretch of

the doubly degenerate benzene e₂_gvibration.

CONCLUSIONS

■

Donor−bridge and bridge−donor bond torsions in molecular

donor−bridge−acceptor triads dramatically aﬀect both elec-

tronic and magnetic D−A couplings by modulating the relative

contributions of bridge-mediated σ- and π-orbital pathways.

These eﬀects are important for increasing our understanding of

bond-torsion induced inhomogeneities in experimentally

determined exchange couplings, electron transfer rates, and

electron transport conductance measurements. In order to

explore the eﬀects of donor−bridge and bridge−acceptor

torsional rotations on electronic and magnetic coupling, we

have spectroscopically and magnetically interrogated a series of

donor−phenylene−acceptor biradical compounds that are

modiﬁed only by the judicious placement of diﬀerent numbers

of methyl substituents on the p-phenylene bridge fragment.

This allows for the bond torsion eﬀects on magnetic and

electronic coupling to be probed at high resolution and at

parity of donor, acceptor, donor−acceptor distance, and the π-

orbital nature of the bridge fragment.

A rR excitation proﬁle has been constructed for the ∼1600

−

cm SQ-Ph(8a) quinoidal stretch of 1-MePh (Figure 6A) that

clearly displays resonance enhancement of this mode as the

laser is tuned into the ILCT band. This proﬁle is similar to that

constructed for 1-Ph and indicates that the observed resonance

enhancement derives from a dominant excited state distortion

along the SQ-Ph(8a) quinoidal stretching coordinate in the

ILCT excited state. The reason for resonant enhancement of

this vibration becomes apparent by observing the nature of the

donor (SQ SOMO) and acceptor (Ph(8a) + NN LUMO)

Kohn−Sham orbitals, Figure 6B, that are associated with the

SQ(π)SOMO → NN-Ph(π*) one electron promotion

character that dominants in the ILCT excited state. In addition

to 1-Ph and 1-MePh, the Ph(8a)-SQ quinoidal stretch is also

observed in the rR spectra of 1-PhMe, and 1-p-MePhMe

(

Figure 7) but with reduced relative intensity compared to the

−

Electronic absorption spectra of complexes with suﬃcient

steric interactions between the donor and bridge and/or the

acceptor and bridge display dramatically modiﬁed ILCT bands

relative to 1-Ph. In our SQ-p-phenylene-NN biradicals, torsion

of the bridge-NN bond can be distinguished from torsion of

the SQ-bridge bond by the energies and band-shapes of the

vibrations in the 1200−1590 cm region. The number of

−

vibrations in the 1200−1590 cm region of the rR spectra for

-Ph, 1-MePh, 1-PhMe, and 1-p-MePhMe and their

frequency positions are all distinctly diﬀerent and reﬂect how

the SQ and NN in-plane vibrations are aﬀected by

progressively decoupling them from the phenylene bridge as

a function of the SQ-Ph and Ph-NN bond torsions. The order

−1

ILCT band and the ∼27000 cm B-NN(π−π*) band.

Torsions about the SQ-B bond only result in a less intense

and blue-shifted ILCT band, with the intense and structured B-

NN(π−π*) band remaining constant. In contrast, torsions

about the B-NN bond result in merged ILCT and B-

NN(π−π*) bands. When the phenylene bridge is orthogonal

to the SQ donor and NN acceptor (i.e., 1-Me PhMe ), these

(1-Ph > 1-MePh > 1-PhMe > 1-p-MePhMe) of reduced rR

intensity for the SQ-Ph(8a) quinoidal stretch tracks with the

decrease in exchange coupling within this series and correlates

with the magnitude of SQ-Ph and Ph-NN bond torsions,

reﬂecting the progressive resonance inhibition across the series.

The results underscore the importance of a highly delocalized

LUMO that can serve as the acceptor orbital in electronic

excitations that originate from the SQ SOMO, since the

SQ(SOMO) → B-NN(LUMO) one-electron promotion

ﬁgures prominently in the VBCI description of magnetic

exchange in these donor-bridge-acceptor biradicals (Figure 1).

charge transfer features are dramatically attenuated. This is

supported by the charge transfer spectrum of 1-BCO, which

does not possess a bridge capable of π-mediated electronic

coupling with SQ and NN. Thus, electronic absorption

spectroscopy provides a unique and powerful probe of D-B-

A electronic structure as a function of diﬀerential bond

922

J. Am. Chem. Soc. 2020, 142, 4916−4924

Journal of the American Chemical Society

orcid.org/0000-0001-8241-9160

Article

torsions, allowing for one to distinguish the eﬀects of spectral

changes as a consequence of the SQ-B torsion from those of

the B-NN torsion (compare Figure 3A to B). These individual

Martin L. Kirk − Department of Chemistry and Chemical

Biology and Center for High Technology Materials, The

University of New Mexico, Albuquerque, New Mexico 87131-

0001, United States; orcid.org/0000-0002-1479-3318;

Email: mkirk@unm.edu

electronic eﬀects are not revealed by the magnitude of J_S_Q_‑_B_‑_N

since this exchange parameter reﬂects the eﬀects of both SQ

and NN torsions relative to the bridge that combine to

attenuate J_S_Q_‑_B_‑_N_N, (Figure 2, Table 1).

Authors

Jinyuan Zhang − Department of Chemistry, North Carolina

State University, Raleigh, North Carolina 27695-8204, United

States

Daniel E. Stasiw − Department of Chemistry, North Carolina

State University, Raleigh, North Carolina 27695-8204, United

States; orcid.org/0000-0003-0526-7372

Guangbin Wang − Department of Chemistry, North Carolina

State University, Raleigh, North Carolina 27695-8204, United

States

Jing Yang − Department of Chemistry, The University of New

Mexico, Albuquerque, New Mexico 87131-0001, United States;

When unencumbered by steric interactions, SQ → NN

resonance delocalization as depicted in Figure 8A describes the

eﬀects of ILCT (SQ → B-NN) excited state character being

admixed into the electronic ground state. The magnitude of

this resonance delocalization eﬀect is also revealed in the

magnitude of the relative resonance enhancement for the

ν(Ph(8a)-SQ) quinoidal stretching mode, which is vibronically

active in the ILCT transitions (Figure 7). Thus, the two

resonance structures in Figure 8A represent limiting valence-

bond descriptions of the ground state (GS) and the ILCT

excited state (EC) within the two-state VBCI formalism of

Figure 1. Conﬁguration interaction provides a mechanism for

admixing quinoidal structure into the ground state wave

function and enhancing the electronic coupling. The degree of

quinoidal character in the ILCT excited state (EC) is

conveniently probed by rR spectroscopy, which provides a

convincing picture of the change in electronic coupling by

monitoring the decrease in resonance enhancement of the

Ph(8a)-SQ quinoidal stretch as a function of reduced SQ-B-

NN planarity (Figure 7). However, when methyl groups are

positioned on the bridge to force rotation of either the SQ

donor (Figure 8B) or the NN acceptor (Figure 8C) with

respect to the bridge, contributions from this key quinoidal

resonance structure to the electronic ground state are

Diana Habel-Rodriguez − Department of Chemistry, The

University of New Mexico, Albuquerque, New Mexico 87131-

001, United States

Benjamin W. Stein − Department of Chemistry, The University

of New Mexico, Albuquerque, New Mexico 87131-0001, United

States

Roger D. Sommer − Department of Chemistry, North Carolina

State University, Raleigh, North Carolina 27695-8204, United

States; orcid.org/0000-0003-1422-5967

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.0c00326

attenuated and can even be eliminated (e.g., 1-Me PhMe ).

Notes

This eﬀectively leads to (1) dramatic decreases in the

ferromagnetic exchange interaction (J_S_Q_‑_B_‑_N_N) that couples

the SQ and NN radical spins in the ground state, (2)

signiﬁcant changes in the optical spectra, and (3) a reduction

in excited state distortions along the Ph(8a)-SQ quinoidal

stretch coordinate. In summary, the use of rR and electronic

absorption spectroscopies, coupled with magnetic suscepti-

bility measurements, reveal the eﬀects of steric inhibition of

resonance in torsionally rotated donor−bridge−acceptor

biradicals at high resolution to provide deep insight into

orbital and conﬁguration interaction contributions to magnetic

and electronic coupling.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

D.A.S. thanks the National Science Foundation (CHE-

464085 and CHE-1764181) for ﬁnancial support. M.L.K.

acknowledges the National Science Foundation (NSF CHE-

1900237) for ﬁnancial support.

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■

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ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/jacs.0c00326.

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AUTHOR INFORMATION

Corresponding Authors

David A. Shultz − Department of Chemistry, North Carolina

State University, Raleigh, North Carolina 27695-8204, United

States; orcid.org/0000-0001-8121-6812; Email: shultz@

ncsu.edu

■

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