Dynamic Motion of Organic Spacer Cations in Ruddlesden-Popper Lead Iodide Perovskites Probed by Solid-State NMR Spectroscopy

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Article

Dynamic Motion of Organic Spacer Cations in Ruddlesden−Popper

Lead Iodide Perovskites Probed by Solid-State NMR Spectroscopy

Clayton J. Dahlman, Rhys M. Kennard, Piotr Paluch, Naveen R. Venkatesan, Michael L. Chabinyc,*

and G. N. Manjunatha Reddy *

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sı Supporting Information

ABSTRACT: Layered hybrid organic−inorganic perovskites such as the lead halide

Ruddlesden−Popper (RP) series are solution-processable two-dimensional (2D)

materials with tunable optoelectronic properties. Dynamic interactions between the

ionic perovskite substructure and organic spacer cations impact optoelectronic properties

relevant for device applications. Here, the static and dynamic structures of linear

alkylammonium and aromatic spacers in lead iodide RP phases (n = 1) are characterized

at ambient temperatures using solid-state NMR (ssNMR) spectroscopy and compared

with previously reported crystal structures derived from X-ray diﬀraction. Rigid and

1

3

1

15

1

ﬂexible sites of spacers are distinguished by examining C{ H} and N{ H} cross-

polarization magic-angle spinning (CP-MAS) signal intensity build-up. Diﬀerent trends in site-speciﬁc rigidity are observed for short

and long alkylammonium spacers. Short spacers (e.g., butylammonium) are attached by strong aﬃnity interactions to lead iodide

octahedra, whereas longer spacers (e.g., dodecylammonium) are more rigid within the RP interlayer than near the octahedral surface.

Phenethylammonium and butylammonium spacers are similarly rigid, and we estimate that the local reorientation time scale of

13

phenyl rings is 10−100 μs by 2D C CP-variable contact (CP-VC) experiments. These ssNMR results indicate that the interplay

between spacer interactions with lead iodide octahedra (Coulombic and hydrogen-bonding) and van der Waals forces between

spacers is responsible for a variety of site-speciﬁc dynamics and local structural distortions at intermediate time scales (microsecond

to millisecond). This study demonstrates a general method to characterize nanoscale structures and site-speciﬁc dynamics that

contribute to structural and electronic disorder in functional optoelectronic RP phases.

INTRODUCTION

spatially separated by spacer cations, R, along the pseudoper-

■

8

−10

ovskite ⟨100⟩ direction.

For low n (i.e., n = 1, 2), the RP

Hybrid organic−inorganic perovskites (HOIPs), such as

methylammonium lead iodide (CH NH PbI , MAPbI ), are

solution-processable semiconductors with high crystallinity and

relatively low levels of electronically active defects.

series shows strong exciton binding and 2D behavior because

3

the Bohr radius is roughly the same dimension as a single PbI

6

21

1

−5

octahedron. The anisotropic structure of RP phases is

responsible for unique phenomena, including large exciton

The

addition of large insulating spacer cations (e.g., butylammo-

nium or phenethylammonium) separates the perovskite lattice

2

23,24

binding energies, optical anisotropy,

intrinsic magnetic

2

5

26

6

−14

dipole transitions, and Rashba and Dresselhaus couplings.

Spacer molecules template the semiconducting perovskite

into layered phases

with tunable quantum-conﬁned

electronic structures. The selection of spacers can change the

structure and optoelectronic properties of layered perovskites

by dielectric eﬀects and by modifying the inorganic

7

−9,27,28

substructure,

participate directly in electronic or optical interactions.

RP phases with larger n (i.e., n > 3), or mixed 2D/three-

but in some cases, they can also

29−31

7

−10

substructure.

Dynamic interactions between spacer mole-

32,33

dimensional (3D) composites,

have demonstrated en-

cules and lead halide octahedra, due to molecular vibrations or

thermal reorientation, have been postulated to impact

photoluminescence quantum yield (PLQY) and carrier

hanced ambient stability with respect to three-dimensional

11

counterparts such as CH NH PbI , tunable band gaps, and

3

1

5−20

good performance in solar cells and light-emitting di-

lifetimes.

However, the motion of organic spacers in

6

,34−36

odes.

layered perovskites is diﬃcult to characterize across a wide

time scale and with site speciﬁcity.

The Ruddlesden−Popper (RP) homologous series is a two-

dimensional (2D) layered HOIP analogue with a chemical

structure of R₂A_n₋₁B_nX₃_n₊₁, where X is a halide, B is a divalent

transition metal such as Pb, and A is a monovalent cation that

Received: October 8, 2020

Revised: December 30, 2020

Published: January 12, 2021

+

3

+

ﬁts within BX octahedral interstitials (e.g., CH NH or Cs ).

6

3

The n value deﬁnes the number of BX octahedra that are

6

©

2021 American Chemical Society

https://dx.doi.org/10.1021/acs.chemmater.0c03958

6

42

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5−78

Article

6

The dynamics of photocarrier generation and recombination

in low-dimensional perovskites can be correlated with the

structural arrangement and dynamic motion of spacer cations

across a wide range of time scales. The rigidity of diﬀerent RP

spacers is found to correlate with PLQY and carrier lifetimes in

moieties.

Dipole−dipole interactions between these

nuclei are sensitive to interatomic distances, relative

orientations of nuclear spins and molecular motions. Recently,

1D and 2D ssNMR spectroscopies have been used to study the

short-range structures of organic spacer cations in layered RP

phases and at the surfaces of nanoscale perovskite

1

9

RP phases, although the underlying mechanisms and the

associated time scales at which this inﬂuence occurs are

unclear. For instance, vibrational modes or phonons present in

spacer molecules adjacent to lead halide octahedra have been

observed to weakly couple with photogenerated exci-

67,79−83

particles.

In addition, ssNMR spectroscopy has been

used to understand the site-speciﬁc dynamics at diﬀerent time

scales, enabled by variable temperature experiments, multi-

nuclear NMR, and nuclear spin relaxation measurements and

1

5−17,20

19,40,41,71,72,74,77,78,84−86

tons.

Such vibrational motions occur at the time scale

analyses.

More recently, nuclear quad-

of photocarrier lifetimes (<100 ps) and are deﬁned by the

covalent and noncovalent interactions with the spacer. The

thermal motion of organic moieties can also aﬀect the time-

averaged crystal phase of hybrid perovskites, with dramatic

rupolar resonance (NQR) spectroscopy has been used in

conjunction with ssNMR spectroscopy to shed light on the

dynamics of perovskite halides.

70,85,87

Here, we present a relatively simple ssNMR approach to

elucidate interlayer structures and site-speciﬁc motional

dynamics in lead iodide RP phases (n = 1). Time-averaged

arrangements of alkylammonium and aromatic spacer cations

in the RP phases are ﬁrst characterized by 1D and 2D ssNMR

37,38

eﬀects on optoelectronic properties.

The time-averaged

structure, as probed by X-ray diﬀraction (XRD), suggests

disorder in the orientation of spacers that can perturb

electronic states. The incoherent thermal motion has been

observed at short time scales (picosecond to nanosecond) by

1

13

1

techniques. In particular, 2D H− C and H− H correlation

NMR spectra are used to distinguish the diﬀerent packing

arrangements of common spacer cations in RP phases. We next

characterize the dynamic motions of spacer cations by

39

inelastic neutron scattering. At longer time scales (micro-

second to millisecond), NMR spectroscopy reveals that

reorientation of spacers in RP phases occurs much more

19,40,41

1

13

1

15

slowly than carrier lifetimes.

In many cases, it is unclear

analyzing H → C and H → N cross-polarization (CP)

1

3

1

if the changes in the optoelectronic properties of RP phases are

due to changes in the time-averaged structure or the dynamics

intensity build-up curves, as well as the C− H dipole−dipole

13 1

interactions measured from 2D C{ H} CP-variable contact

(CP-VC) experiments. Notably, solid-state NMR measure-

ments and analyses show that spacer cations undergo relatively

slow motions at intermediate time scales (microsecond to

millisecond) at near-ambient temperatures. Rigid and ﬂexible

regions of spacer cations are identiﬁed for both linear

alkylammonium and aromatic spacer cations and compared

with the iodide salts of the same spacer cations. These results

indicate that the dynamic motion in RP spacers is variable

across the spacer molecule and correlates with the spacer size,

composition, and local environment. Solid-state NMR results

are compared with structures and isotropic displacement

parameters obtained from previously published single-crystal

X-ray diﬀraction reﬁnements of RP phases as a complementary

structural probe of light atoms (C, N, H) in the organic

spacers. Competition between noncovalent interactions, such

as Coulombic attractions between spacer cations and lead

octahedra and interspacer van der Waals forces, may lead to

observed variations in site-speciﬁc rigidity between diﬀerent

spacer cations. These results demonstrate the role of the lead

iodide octahedra in templating the crystalline spacer structure

and the sensitive balance of interactions that account for the

near-ambient dynamic structure of these 2D layered materials.

7

,10,18

of the spacers and octahedra.

An accurate characterization of both time-averaged and

dynamic interlayer structures of spacer molecules at diﬀerent

lengths and time scales has proven challenging, particularly at

near and above ambient temperatures. Although the positions

of carbon, nitrogen, and heavier atoms in spacer cations can be

resolved by single-crystal X-ray diﬀraction or total scattering

techniques, thermal displacement and structural disorder

17,42−45

obscure their precise structure.

In addition, the location

of hydrogen atoms, which inﬂuence noncovalent intermolec-

ular forces, are not resolved by typical X-ray diﬀraction

19,39

techniques. Neutron diﬀraction,

while sensitive to light

atoms and capable of resolving librational motion in spacers, is

also limited by crystalline disorder and rapid thermal motion.

Electron microscopy enables high spatial resolution but can

46−49

cause beam-damage in hybrid perovskites

and is a time-

averaged technique. Thus, electron microscopy cannot

elucidate structural dynamics without specialized ultrafast

5

0

techniques. Vibrational spectroscopies (e.g., Raman, infra-

red) of the spacer cations can resolve organic bonds and

1

5,17,19,45,51

conformations.

However, vibrational and phonon

modes across the organic species can be quite weak and

polydisperse, especially compared to the heavier metal halide

1

5−17,52,53

octahedral modes.

spacers can become glassy;

At higher temperatures, the

hence, dynamic motions

EXPERIMENTAL SECTION

27,42−45,54

■

(

Materials. Lead iodide (PbI ) was purchased from TCI America

2

are even more diﬃcult to deconvolute and compare.

99.99% trace metal basis). Phenethylammonium iodide, butylammo-

To this end, solid-state NMR (ssNMR) spectroscopy is a

suitable technique to study site-speciﬁc structures and

dynamics in 2D hybrid perovskites. ssNMR spectroscopy

methods have been employed to understand the self-assembly

nium iodide, octylammonium iodide, and methylammonium iodide

were purchased from Greatcell Solar. Dodecylammonium iodide was

synthesized by the reaction of dodecylamine (Aldrich, 98%) with 57%

hydriodic acid (Sigma-Aldrich) following the procedure outlined

88

55−57

previously. Organic solvents used were dichloromethane (anhy-

drous, >99.8%, Sigma-Aldrich) and γ-butyrolactone (>99%, Aldrich).

All chemicals were used as received.

of organic molecules

and to elucidate the arrangement

and motional dynamics of intercalated ionic surfactants in

other hybrid organic−inorganic layered phases, such as

5

8−64

Synthesis Methods. Butylammonium lead iodide {C4} PbI , was

2 4

organometallic clays.

In hybrid perovskite phases, one-

grown via slow crystallization from HI following a previously reported

1

2

dimensional (1D) and 2D ssNMR measurements of H/ H,

C, N/ N, Cs, Sn, and Pb nuclei have resolved

13

procedure. Octylammonium lead iodide {C8} PbI and dodecy-

2

4

1

3

14

15

133

119

207

lammonium lead iodide {C12} PbI were grown via antisolvent

2

4

diﬀerent local bonding environments of organic and inorganic

crystallization from γ-butyrolactone solution, following the referenced

6

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8

9

procedure. Phenethylammonium lead iodide {PhC2} PbI was

data was processed by applying the Fourier Transform of time-

domain signals along the direct (horizontal) dimension and in the

indirect (vertical) dipolar dimension.

2

4

grown using both procedures.

X-ray Diﬀraction Measurements and Analyses. X-ray

diﬀraction patterns were acquired using an Empyrean powder

diﬀractometer with a step size of 0.01313°, with a Cu Kα source,

operating with an accelerating voltage of 45 kV and a beam current of

0 mA, with samples rotated at 4 rpm to improve signal-to-noise

ratios.

Solid-State NMR Spectroscopy. RP phases of {C4} PbI ,

The sample temperature is regulated at 300 K using a Bruker BCU

II unit. In the case of fast MAS measurements at 18.8 T, we tested the

sample temperature in the 1.3 mm rotor as a function of MAS

2

07

4

frequency using PbNO and Pb chemical shifts were monitored as a

3

function of MAS frequency. The temperature of PbNO₃was

calibrated to 300 K using a protocol described in a previous

2

4

94

1

13

{

PhC2} PbI , {C8} PbI , {C12} PbI , and their precursor salts were

study. One-dimensional H and 2D H− H and H− C correlation

experiments of RP phases were carried out under identical

experimental conditions. For the measurements acquired at 9.4 T

and at 8 kHz MAS, we regulated the probe temperature at 300 K. Any

variation in the temperature of CP-MAS experiments carried out with

8 kHz MAS is expected to be within ±5 °C, which would not cause

phase transitions of the RP phases under this study.

2

4

2

4

2

4

packed either into 1.3 or 4 mm (outer diameter) zirconia rotors ﬁtted

with VESPEL or Kel-F caps, respectively. All solid-state magic-angle

spinning (MAS) NMR spectra were acquired either on a 9.4 T Bruker

AVANCE-II or 18.8 T Bruker AVANCE NEO NMR spectrometer

equipped with a 4 mm H-X or a 1.3 mm H-X probehead, respectively.

One-dimensional ¹H MAS NMR experiments were acquired by

1

coaddition of 16 transients. Two-dimensional (2D) H− H double-

quantum (DQ)−single-quantum (SQ) NMR spectra of all RP phases

90

RESULTS AND DISCUSSION

were acquired using a back-to-back (BaBa) sequence. A rotor-

■

synchronized t increment of 20 μs was applied with a recoupling time

of 40 μs, which corresponds to two rotor periods (2τ ). The indirect

H DQ dimension was acquired using 128 t increments, each with 16

coadded transients, corresponding to an experimental time of 3 h

using a 4 s recycle delay. The H-detected 2D H− C heteronuclear

correlation (HETCOR) spectra of RP phases were acquired with 94 t

increments, each with 16 coadded transients, using 4 s recycle delay.

Arrangement of Spacer Cations in n = 1 RP Phases.

The interfacial structures and site-speciﬁc molecular motions

of spacers in n = 1 lead iodide RP phases (R PbI ) are

1

r

1

2

4

investigated for four prototypical cations: linear alkylammo-

nium cations of varying lengths (n-butylammonium {C4}, n-

octylammonium {C8}, and n-dodecylammonium {C12}) and

a commonly studied aromatic cation (phenethylammonium

1

13

1

CP contact time was 0.10 ms. All 1D H MAS, 2D H− H DQ−SQ,

1

13

{

PhC2}). We have chosen these spacers as model systems

and 2D H− C HETCOR experiments were carried out at 18.8 T

using relaxation delays in the range of 2.3−4 s with 50 kHz MAS. For

owing to their diﬀerent structures that led to the diﬀerent

optoelectronic properties of RP phases.

1

6,12−14,19

these measurements, H T relaxation delays were 3.9, 2.3, 2.7, and 3 s

1

in {C4}, {C8}, {C12}, and {PhC2} spacer cations in RP phases,

respectively, as determined from saturation recovery measurements

Room-temperature X-ray diﬀraction (XRD) powder patterns

of {C4} PbI , {C8} PbI , {C12} PbI , and {PhC2} PbI are

_a_n_d_a_n_a_l_y_s_e_s_._A_l_l₁3C{ H} and N{ H} cross-polarization (CP)-MAS

1

15

1

2

4

2

4

2

4

2

4

shown in Figure S1 . A comparison of these powder patterns

with published single-crystal XRD reﬁnements conﬁrms the

presence of room-temperature phases of RP structures. All of

the linear alkylammonium RP phases have an orthorhombic

spectra were acquired at 9.4 T with 8 kHz MAS and 2 ms CP contact

time (unless otherwise speciﬁed) using relaxation delays in the range

of 2−3 s. Cross-polarization involves the simultaneous excitation of

1

13

15

H and C/ N nuclei to enhance the signals of the latter. N and

42−44

3

1

Pbca structure (Figure S2a ).

triclinic P1 space group, but monoclinic C2/m symmetry if

only the inorganic species are considered (Figure S2a).

{PhC2} PbI exhibits a

C 90° pulse durations were 4.0 μs, and the H 90° pulse duration

was 2.5 μs. The H spin−lattice relaxation times (T ) were measured

to be in the range of 2−3 s for these CP-MAS measurements, as

determined for inversion-recovery measurements and analyses. One-

dimensional ¹³C{ H} CP-MAS experiments were acquired at diﬀerent

CP contact times, 0.1, 0.2, 0.5, 1, 3, and 5 ms, by coadding 2048

transients with a H relaxation delay of 4 s, corresponding to an

experimental time of 2.4 h. One-dimensional N{ H} CP-MAS

experiments were acquired at diﬀerent CP contact times, 0.5, 1, 1.5, 2,

, 5, and 10 ms. Heteronuclear decoupling was applied during

2 4

1

̅

1

17,28

Octahedral tilts vary slightly between the diﬀ erent spacers,

although they all share the same tilt system (Figure S3 and

Table S1 ). Linear alkylammonium spacers pack with the same

methylene density as amorphous polyethylene within the

1

5

1

88

interlayer; therefore, the separation between lead iodide

octahedra is sensitive to spacer size and composition. RP

phases with linear alkylammonium spacers undergo phase

3

1

13

acquisition using the SPINAL64 sequence. The H and

experimental shifts were calibrated with respect to neat tetramethylsi-

lane (TMS) using adamantane as an external reference (higher parts

C

27,42−45,54

transformations at diﬀerent temperature,

but the

structures are all well-characterized at 298 ± 10 K. The crystal

structures of each of these previously published phases are

illustrated in Figure S2a along with anisotropic displacement

1

3

1

per million (ppm) C resonance, 35.8 ppm, and the H resonance,

1

.82 ppm). The ¹⁵N chemical shifts were referenced to neat CH NO

3

2

42−44

using powdered glycine at −347 as an external reference (see Table 2

of ref 91). To convert to the chemical shift scale frequency used in

protein NMR, where the alternative IUPAC (see Appendix A of ref

ellipsoids for Pb, I, C, and N atoms.

Nevertheless, analysis

of XRD patterns of hybrid RP phases is challenging because

weak scattering and thermal disorder can limit the ability to

reﬁne positions of C, N, and H atoms. Fitting restraints based

on chemical models of bond lengths and estimated

9

2) reference is liquid NH at −50 °C, it is necessary to add 379.5

3

93

ppm to the experimental chemical shifts given in this study.

_A_l_l₂_D1 C-detected CP-VC experiments were carried out on a

3

13

1

3

Bruker AVANCE III 600 spectrometer (Larmor frequencies of

C

orientations are often used to reﬁne structures. Single-crystal

XRD reﬁnements indicate that longer spacers ({C8} and

1

and H were 150.90 and 600.13 MHz, respectively) equipped in 1.3

1

13

15

mm H− C− N probe head. The samples were spun at 50 kHz. RF

{C12}) interleave to accommodate this interlayer density

1

3

1

42,43

nutation frequencies were 125 and 75 kHz for C and H,

receptively, corresponding to 5/2 and 3/2 of MAS frequency. Low

(

Figure S2),

but alternative arrangements such as tilted or

45,95

disordered structures have also been proposed.

This

1

power H-decoupling was applied during the acquisition. The

relaxation delays were set to 3 and 11.8 s for {PhC2} PbI and

PhC2}I, respectively. The indirect t dimension was acquired using

0 increments, each with a rotor synchronous t incremental delay of

0 μs. Then, 1184 and 256 coadded transients were acquired

corresponding to the total experimental times of 53 and 43 h,

respectively, for {PhC2} PbI and {PhC2}I. Two-dimensional CP-VC

uncertainty is due to the diﬃculty in using X-ray diﬀraction

techniques to resolve local structures with disorder in materials

with mixtures of heavy and light atoms.

Solid-state NMR (ssNMR) spectroscopy provides a

complementary probe to elucidate the structure of RP spacers

with well-resolved atomic speciﬁcity. For example, intrinsically

2

6

{

1

5

1

2

6

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high sensitivity (100% natural abundance) and adequate

chemical shift resolution associated with H NMR enable

short CP contact time of 0.1 ms (Figure S4, Supporting

Information). Signal assignments obtained from 2D HETCOR

spectra (Tables S2 −S5) enable a comparison of the local

structures of methyl and methylene sites in each RP spacer.

The diversity of chemical shifts and 2D correlation peaks

observed in 2D HETCOR NMR spectra of the linear

alkylammonium RP spacers suggests the presence of diﬀerent

local bonding environments and interlayer arrangements

1

diﬀerent proton sites in RP phases to be distinguished. A

1

comparison of 1D H MAS NMR spectra of the diﬀerent RP

phases is shown in Figure 1 . For {C4} PbI , {C8} PbI , and

2

4

2

4

among −CH groups in diﬀerent spacer cations.

2

1

Next, we employed a 2D H− H double-quantum−single-

quantum (DQ −SQ) correlation NMR experiment (Figure 2 )

1

Figure 1. Solid-state 1D H NMR spectra acquired at 18.8 T ( H,

00.13 MHz) and at 50 kHz MAS of (a) {C4} PbI , (b) {C8} PbI ,

8

2

4

2

4

1

(

c) {C12} PbI , and (d) {PhC2} PbI RP (n = 1) phases. H signal

2 4 2 4

1

intensities correspond to diﬀerent H sites in linear alkylammonium

and phenylethylammonium cations, as depicted by colored circles.

{

C12} PbI (Figure 1a−c), ¹H signals corresponding to

2

4

terminal methyl, central methylene, and ammonium groups

in linear alkyl spacer cations are distinguishable. In the case of

1

{

PhC2} PbI , the H NMR spectrum is broadened compared

2

4

to the linear spacers (Figure 1d), but diﬀerent atomic sites can

1

still be distinguished. In all cases, the H chemical shift

+

1

^a^s^s^o^cⁱ^a^t^e^d^wⁱ^t^h^t^h^e^N^H3 groups in the n = 1 RP phases (7.2

Figure 2. Solid-state 2D H{ H} DQ−SQ NMR correlation spectra

1

ppm) is displaced to a higher frequency, as compared to the H

acquired at 18.8 T and at 50 kHz MAS of (a) {C4} PbI and (b)

PbI . The corresponding skyline projections are shown along

2 4

the top H SQ horizontal and H DQ vertical axes, respectively.

Correlated signal intensity originates from dipolar-coupled H− H

pairs and are depicted in colored circles.

2

4

+

chemical shift of NH groups in 3D CH NH PbI (6.3

{C12}

3

8

7

1

ppm). This result could be attributed to stronger aﬃnity

1

interactions (e.g., H-bonding) between RP spacer cations and

1

lead iodide octahedra. Although 1D H spectra can distinguish

1

+

H signals corresponding to NH₃groups, alkyl, and aromatic

groups in diﬀerent RP phases, information on inter- and

to characterize the intermolecular arrangement of spacers

within the RP interlayer. The DQ−SQ experiment allows

identiﬁcation of molecularly proximate and dipole−dipole

coupled H−H pairs at sub-nm (<5 Å) distances, which can

intramolecular packing cannot be obtained by the analysis of

1

D H NMR spectra alone.

1

13

Greater structural detail can be achieved with 2D H− C

correlation NMR spectroscopy by spreading ¹H and

13

C

1

either manifest as (i) on-diagonal 2D correlation signals H

signals into two frequency dimensions, facilitating the

DQ between the chemically equivalent atoms (shown in

1

13

assignments of H and C chemical shifts to particular atomic

sites in RP spacers. In addition, these techniques beneﬁt from

cross-polarization (CP) to transfer spin magnetization from

diagonal dashed lines), or (ii) oﬀ-diagonal signals between the

1

chemically distinct (or nonequivalent) H sites within the same

molecule or between the adjacent molecules (depicted in

1

13

15

1

abundant H nuclei to less-abundant nuclei (e.g., C and N),

horizontal dotted lines). H DQ chemical shifts (vertical axis)

13

15

1

leading to enhancements of dilute C and N signals.

Analyses of 2D H− C CP heteronuclear correlation

HETCOR) spectra thus enable the simultaneous assignments

of distinct H and C signals of C−H moieties. To assign the

H and C signals corresponding to directly bonded H− C

moieties in {C4}, {C8}, {C12}, and {PhC2} RP phases,

appear at the sum of the H SQ chemical shifts (horizontal

1

13

axis).

1

(

Analyses of 2D H− H DQ−SQ NMR spectra of diﬀerent

RP phases conﬁrm an interleaved arrangement of long alkyl

spacer cations within the RP interlayer. The 2D H DQ−SQ

NMR spectrum of {C4} PbI (Figure 2a) shows both

1

13

1

13

1

13

1

2

4

1

3

1

C{ H} CP HETCOR experiments were acquired using a

autocorrelation H DQ intensities along the diagonal and oﬀ-

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1

diagonal signal intensities between pairs of inequivalent H

RP phases can be distinguished by observing how cross-

polarized NMR signal intensities build-up as a function of CP

contact time. As mentioned in the above section, the CP-MAS

1

nuclei. Oﬀ-diagonal H DQ signals appear at 1.8 + 3.7 = 5.5

ppm for CH ↔ CH pairs, at 3.7 + 7.2 = 10.9 ppm for CH ↔

2

1

NH , and at 1.1 + 7.2 = 8.3 ppm for CH ↔ NH pairs

experiment involves the excitation of abundant H nuclei and

3

(

indicated by the red arrow). The closest CH ↔ NH

distance between the adjacent molecules in the reﬁned crystal

structure of {C4} PbI is ≈5 Å. Published crystal structures

subsequent magnetization transfer via dipole−dipole couplings

3

1

13

1

15

to rare nuclei (here, H → C or H → N), which enhance

1

3

15

the intensities of C and N signals. In particular, the central

2

4

from XRD analysis indicate that the end-to-end distance

CH groups in all RP phases have the same number of protons

2

between NH and CH groups in a single {C4} molecule is ≈5

covalently attached to them, and, to ﬁrst order, may be

3

4

1

13

Å, such that the CH ↔ NH oﬀ-diagonal peak (depicted in

expected to show identical H → C CP signal intensity build-

up. Nevertheless, segmental motions and site-speciﬁc molec-

3

red) is expected to show the signal from both inter and

intramolecular H−H proximities. It is noteworthy that the DQ

signal between the weakly dipolar coupled (i.e., distant) spin

pairs is attenuated when these spins are strongly dipolar

coupled with nearby spins through an eﬀect known as dipolar

1

3

1

ular reorientations that inﬂuence C− H dipolar couplings

also aﬀect the kinetics of CP transfer, as shown schematically

in Figure 3 a and described in more detail in the Supporting

9

6

truncation. For example, weak dipolar interactions in CH ↔

3

NH lead to very low-intensity DQ signal (red arrow) as

3

compared to the strong DQ signal intensity associated with

H−H pairs in CH groups. Thus, the longer spacer cations are

2

too large to produce a signiﬁcant oﬀ-diagonal peak

corresponding to an intramolecular H−H proximity between

the end groups. Assuming that the spacers are mostly extended

(

trans orientation), the distance from CH to NH is ≈15 Å

3

for {C12} and ≈10 Å for {C8}. The absence of characteristic

1

3

97

C signals of gauche conformations (Figures S7 and S8, vide

infra) indicates that spacers are indeed mostly trans-oriented.

Therefore, oﬀ-diagonal peaks between the end groups can only

exist between the neighboring molecules, i.e., when the spacer

1

cations are interleaved. The 2D H DQ−SQ NMR spectrum of

{

C12} PbI (Figure 2b) shows intramolecular oﬀ-diagonal

2 4

features between NH (7.1 ppm) and nearby CH groups

3

2

(

dark gray, 3.6 + 7.1 = 10.7 ppm, and light gray, 1.9 + 7.1 = 9.0

ppm), but no intermolecular correlations with terminal CH or

3

CH groups (light blue, dark blue) in the broken red circle.

2

However, the DQ feature at ∼4.7 ppm (indexed as dark gray to

light blue, CH ↔ CH ) for long {C12} spacers may be due to

2

3

both intra- and intermolecular correlations between the

interleaved spacers. The published crystal structure shown in

1

Figure 3. (a) Schematic of polarization transfer from the adjacent H

13

sites to C sites in (C4) PbI , which is inﬂuenced by the motion of

functional groups such as CH groups in RP phases. (b) C{ H} CP

2

4

Figure 2c yields a nearest intermolecular CH ↔ CH (dark

13

1

2

3

gray to light blue) separation of 4.7 Å, which may be close

enough to cause weak intermolecular DQ signal intensity.

However, the peaks in the DQ spectrum in this region are

broad enough that intramolecular correlations are also

responsible for this signal. In addition, the dipolar truncation

may further constrain the accurate identiﬁcation of weak DQ

signal intensities originating from weakly coupled intermo-

signal intensities associated with diﬀerent methylene and methyl

1

3

1

groups in the C{ H} CP-MAS spectra of {C4} PbI show build-up

2

4

at diﬀerent rates as a function of CP contact time.

Information (Section S1 ). At short contact times (<0.1 ms),

1

3

only C signals associated with directly bonded C−H moieties

1

lecular H− H pairs. Therefore, it could be hypothesized that

this DQ feature implies, at least in part, intermolecular

correlations between the trans-oriented {C12} spacers, thus

(i.e., CH, CH , and CH ) are observed due to the short C−H

2

3

distances and large dipolar interactions (∼20 kHz) in the

absence of dynamic motion. Longer CP contact times (>0.1

1

3

1

ms) are required to observe C signals of quaternary carbons

because of the long C−H distances and a large reduction in the

strength of heteronuclear dipolar coupling constants. Faster

partial interleaving. The 2D H DQ−SQ NMR spectrum of

{

C8} PbI also shows no evidence of the inter- and/or

2 4

intramolecular CH ↔ NH 2D correlation signal (Figure S5).

3

1

magnetization transfer occurs for dipole−dipole pairs ( H →

Similar to {C12} PbI , these results are consistent with XRD

2

4

1

3

1

15

C or H → N) that are more rigid (i.e., slower molecular

reﬁnements; none of the separations between the end groups

in the published structure of {C8} PbI is less than 5 Å, and

the carbon sites show signiﬁcant thermal displacement (Figure

S2 ). While single-crystal XRD is an uncertain probe of the

reorientation and segmental motions) during the CP contact.

Therefore, the rate at which signal intensity build-up occurs

can be used to study molecular motions in solids at the time

scale of the NMR experiment (microsecond to millisecond).

2

4

positions of light atoms in RP structures with local disorder,

1

13

1

2

D H DQ−SQ NMR data are consistent with the interleaved

We analyzed C{ H} CP-MAS-NMR signal intensities as a

function of CP duration, as shown in Figure 3b for a series of

arrangement of spacer molecules proposed in published

structures (Figure S2).

1

3

1

C{ H} CP-MAS 1D ssNMR spectra of {C4} PbI acquired

2 4

Site-Speciﬁc Dynamics of Linear Alkylammonium

Spacers. The rigid and ﬂexible regions of spacer cations in

with diﬀerent CP contact times. We reasoned that the site-

speciﬁc variability in CP build-up rates originates from the

6

46

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diﬀerent local dynamic motions of spacers in RP phases, which

are sensitive to van der Waals interactions between the

adjacent molecules, Coulombic attractions between NH₃⁺and

the net negatively charged lead halide octahedra, and other

noncovalent interactions (e.g., H-bonding).

series of carbon atoms. This result could be attributed to the

faster rotational dynamics of CH (Figure 3a) than the

3

98

secondary carbon sites. These trends indicate that methylene

groups nearer to lead iodide octahedra in {C4} PbI are more

2

4

rigid than the methylene groups further away on the time scale

of the CP-MAS NMR experiment (microsecond to milli-

second).

Figure 4b compares the H → ¹³C signal intensity build-up

1

3

1

in the C{ H} CP-MAS spectra of {C4} PbI (Figure 4a)

2

4

The diﬀerent dynamic motions of spacer cations in RP

phases characterized by the analysis of CP intensity build-up

are corroborated by site-speciﬁc thermal disorder obtained

from XRD analysis. At room temperature, rotational and

librational motions typically occur on fast time scales

(

picosecond to microsecond), which can be probed by

sophisticated diﬀraction and NMR spectroscopy techni-

ques.

50,99−101

Nevertheless, the conventional XRD and 2D

homo- and heteronuclear ssNMR experiments presented in

this study lead to inherent time-averaged structures that can be

used as indicators of the site-speciﬁc thermal disorder (i.e.,

rigidity) of the spacer cations. The isotropic displacement

parameter (U ) from single-crystal XRD measurements can

iso

help to visualize the rigidity of spacer cations (i.e., an average

of U_i_s_ovalues of diﬀerent carbon sites in the organic layer) and

can be compared to the CP rate contents of diﬀerent carbon

1

13

sites. Figure 5 compares H → C CP build-up time constants

Figure 4. (a) A projection along ⟨110⟩ of the orthorhombic P unit

bca

1

3

1

cell of {C4} PbI at 293 K. (b) Solid-state 1D C{ H} CP-MAS

2

4

NMR build-up curves (circles) for the four diﬀerent carbon sites in

{

C4} of single crystals of {C4} PbI spectra acquired at 9.4 T, 8 kHz

2

4

MAS, and at 300 K. Monoexponential ﬁts of the data are also shown

(

lines). (c) Fitted CP rates, k , extracted from CP build-up curves

CP

for {C4} PbI (blue) and the precursor salt {C4}I (gray), for each

2

4

carbon site in {C4}. CP polarization times, T_C_P= 1/k , are indicated

CP

at right.

Figure 5. (a) Site-speciﬁc ¹³C{ H} CP build-up time constants, T

1

CP

(

{

=1/k ), for diﬀerent carbon atoms in alkylammonium spacers in

CP

C4} PbI , {C8} PbI , and {C12} PbI . (b) Calculated isotropic

acquired with diﬀerent CP contact times. All carbon sites reach

maximum intensity within 5 ms of contact time, and signal

intensities do not subsequently decrease over this range. This

behavior can be ﬁt to a stretched exponential model (eq S3 ) to

compare measured build-up kinetics. More details about this

model are discussed in the Supporting Information (Section

2

4

2

4

2

4

displacement parameters derived from room-temperature single-

crystal XRD measurements and analyses (see Figure S2 , Supporting

Information). Uncertainty in ﬁtted T_C_Pvalues is indicated by the

shaded regions for all three spacers (uncertainty is very small for

{

C4}).

S1 ). CP rate constants (k ) obtained from ﬁtting analyses are

CP

listed in Table S2. Relatively fast CP signal intensity build-up is

(shown as T_C_P= 1/k , Figure 5a) for diﬀerent carbon sites of

linear alkylammonium spacers with isotropic averages of

CP

+

3

associated with CH groups next to NH . CP build-up rates

2

(

k ) at carbon sites in {C4} PbI decrease with distance from

atomic displacement parameters (U , Figure 5b) obtained

CP

2

4

iso

+

NH₃(Figure 4c). The CP build-up rate scales with the

segmental dynamic motions and the number of dipole−dipole

coupled C− H moieties and their interatomic proximities

from reﬁnements of previously published single-crystal XRD

42−44

structures (Figure S2 ).

We note that the reﬁnement of

1

3

1

atomic displacement parameters can be very uncertain for light

atoms (i.e., C, N) in RP phases with local disorder; therefore,

ssNMR is particularly useful as a complementary probe of a

dynamic structure. A larger U_i_s_ofrom XRD indicates greater

atomic spatial disorder as the average of many X-ray scattering

events (∼femtosecond), either due to static disorder or

dynamic ﬂuctuations. CP build-up rates probe dynamic motion

(

see the Supporting Information). This simple relationship

between k_C_Pand dynamics of methyl and methylene groups in

spacer cations enables the rigid and ﬂexible regions of spacer

cations in the RP phase to be identiﬁed and compared. For

example, the terminal methyl group, despite an additional

1

directly bonded H, has the slowest CP build-up across the

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on the scale of microsecond to millisecond, which may

manifest as spatial disorder during X-ray scattering. Intra-

molecular trends in U_i_s_oare consistent with CP build-up rates;

and Tables S2 −S4 ). Unlike {C4}, carbon sites in the longer

alkylammonium spacers are not necessarily more rigid near

NH₃⁺groups. Instead, build-up rates for {C8} PbI and

2

4

+

for {C4} PbI , the CH groups near NH sites exhibit faster

{C12} PbI (Figure 5a) vary nonmonotonically with a distance

2

4

2

3

2 4

+

3

CP intensity build-up and smaller atomic displacement

compared to carbon sites further away. The agreement

between these techniques indicates that U_i_s_ovalues obtained

by XRD analysis are determined by dynamic ﬂuctuations that

aﬀect CP build-up, rather than static disorder, and conﬁrms

that site-speciﬁc CP build-up rates correlate with spatial

disorder in RP spacers.

from the −NH group and are generally slower (i.e., faster

motional dynamics) than in {C4}. For {C8}, motion increases

nonmonotonically with distance from −NH , and none of the

carbon sites are as rigid as the carbon site adjacent to NH

+

3

+

3

groups (i.e., C1) in {C4}. An even longer alkylammonium

spacer, {C12}, is in fact most rigid within the interlayer, with

faster thermal motion (i.e., larger T ) in the carbons closest to

CP

The restricted motional dynamics of carbon sites near

ammonium groups in {C4} RP phases can be attributed to

Coulombic interactions (i.e., surface anchoring) between

lead iodide surfaces. Atomic displacement parameters obtained

from XRD structures support these trends; {C4} and {C8}

both have lower U_i_s_o_t_r_o_p_i_c(i.e., less spatial disorder) near lead

iodide octahedra than at the terminal alkyl ends (Figure 5b),

though this trend is shallower in {C8}. However, {C12} has

larger U_i_s_o_t_r_o_p_i_cnear both terminal ends of the spacer

(ammonium and methyl groups) than at the interior carbon

sites. This result suggests that van der Waals interactions

between {C12} alkyl chains have a greater inﬂuence on the

thermal motion of methylene groups than Coulombic (or

other noncovalent) attractions between NH₃⁺and lead iodide

octahedra. Carbon sites in the iodide salts of {C8} and {C12}

+

spacer NH₃groups and the negative sites of lead iodide

octahedra. A comparison of CP build-up rates of {C4} in the

RP phase and in a separate phase without lead iodide

octahedra, the iodide salt ({C4}I), corroborates this surface

anchoring eﬀect. Each of the methylene carbon sites in {C4}I

has similar k_C_Pvalues, which are lower than the k_C _P values of

analogous methylene sites in the {C4} RP phase (Figure S6,

S9a, and Table S2 , Supporting Information). This result

indicates that {C4} undergoes more rapid and homogeneous

dynamic motion in the precursor iodide, i.e., in the absence of

any surface anchoring eﬀect. The similar values of k_C_Pfor the

terminal methyl groups in {C4} PbI and {C4}I reveal that

are generally more rigid (i.e., smaller T ) than analogous

CP

carbons in RP phases (Figures S7 −S10 and Tables S2 −S4 ),

+

3

which also demonstrates that local attractions between NH

2

4

lead iodide surface anchoring does not translate to greater

rigidity deeper within the {C4} PbI spacer interlayer region.

and lead halide octahedra have less inﬂuence than interspacer

van der Waals forces for longer alkylammonium cations. The

2

4

1

13

Site-speciﬁc dynamics have been measured in other hybrid

organic−inorganic layered composites, such as organometallic

clays, which share much in common with the lead halide RP

phases. For instance, layered cadmium thiophosphate can host

consistent results of both H → C CP-MAS build-up and

XRD atomic displacement parameters indicate that van der

Waals interactions signiﬁcantly contribute to the rigidity in

longer spacers such as {C12}, whereas interactions between

+

intercalated cetyltrimethylammonium (CTA ) that binds

NH and lead iodide sites anchor short alkylammonium spacer

3

1

3

1

anionic sites on the Cd-deﬁcient inorganic surface. C{ H}

cations (e.g., {C4}) near lead iodide octahedra.

+

CP-MAS build-up studies of CTA in these layered phases

The modes of motion in RP spacers are constrained by the

dense and crystalline packing of alkyl chains within the

indicates that carbon sites nearest to the ammonium group

have faster build-up, and thus slower motional dynamics, than

1

3

interlayer. C chemical shifts are sensitive to disorder and

conformational defects, in particular, to any gauche dihedral

62

carbons further away. Similar phenomena, described as a

noncovalent attachment (i.e., anchoring) of surfactant ionic

groups to charged surface sites by means of strong Coulombic

97

arrangements by the so-called γ-gauche eﬀect. For example,

when two methylene groups in an alkyl chain are in a γ

position relative to one another and in a trans/trans (tt)

+

forces, have been observed for CTA in other anionic

5

8

13

galleries, as well as for anionic surfactants in cationic galleries

conformation, the isotropic C chemical shifts are displaced to

6

0

(

e.g., dodecyl sulfate in Mg−Al layered double hydroxides).

higher frequencies compared with otherwise identical alkyl

chains in trans/gauche (tg) or gauche/gauche (gg) con-

formations, which are displaced to lower frequencies. Figures

Similarly, alkyl sidechains that are covalently bonded to

conjugated polymer backbones (e.g., poly(3-hexylthiophene)

1

3

1

(

P3HT)) show decreasing rigidity with distance from the

S7 and S8 show the 1D C{ H} CP-MAS NMR spectra for

{C8} and {C12} spacers, respectively. In the case of {C8} and

{C12}, the RP phase shows only one peak for the terminal

methyl carbon (16 ppm), whereas a small additional peak,

attributed to gauche conformations, is observed at lower

chemical shift (13, 14 ppm) for the iodide salts. The lack of

any γ-gauche eﬀect in the RP phases indicates that the

alkylammonium spacers are relatively well-ordered and

expected to be oriented in all-trans conﬁgurations with

minor perturbations around ﬁxed crystalline sites. Previously

reported inelastic neutron scattering measurements of

{C8} PbI also indicate that the room-temperature RP phase

102

backbone by CP build-up kinetics. The anchoring eﬀect of

ionic surfactants in layered galleries is more pronounced when

60

the surfactant packing density is lower or the temperature is

5

8

higher, likely because van der Waals interactions between

alkyl chains can also immobilize the surfactants. The similarity

between the site-speciﬁc dynamics of {C4} spacers in

{

C4} PbI and ionic surfactants in hybrid layered composites

2 4

suggests that anchoring also occurs for {C4} in RP phases.

The length of alkylammonium cations impacts site-speciﬁc

motional dynamics by tuning the balance of interactions

between RP spacers and lead iodide octahedra. Figure 5a

2

4

compares CP build-up rates (T_C_P= 1/k ) for diﬀerent carbon

is only capable of C rotations at the end groups (CH and

CP

3

sites for linear alkylammonium spacers of diﬀerent lengths

NH ) of the {C8} spacer, and inner carbon sites can only

3

13

1

39

(

{C4}, {C8}, and {C12}). A comparison of C{ H} CP-MAS

move slightly around ﬁxed positions. Thus, the lead iodide

spectra is shown in Figures S6 −S8 . All of the alkylammonium

spacers, either in RP phases or as iodide salts, have build-up

kinetics that ﬁt a stretched exponential function (eqs S3, S9,

octahedral surface eﬀectively templates the spacer cations into

adopting a more ordered crystalline phase than they would

otherwise form in the iodide salt because of the close-packing

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of spacers within the interlayer. By comparison, {C4} may not

be long enough for van der Waals interactions among

methylene groups to overwhelm entropic disorder and thermal

ﬂuctuations in the interlayer.

atoms and lower density of H atoms in the interlayer (Figure

S2b ), which may also aﬀect CP build-up kinetics. The faster

build-up is observed for the alkyl sites with two bonded H

−

1

atoms (i.e., C1 and C2, k_C_P= 8.5−8.6 ms ) than the C−H

−

1

Site-Speciﬁc Dynamics of {PhC2} PbI . Aromatic

sites (Cp2/3 and Cp4, k_C_P= 5−6 ms ) or the functionalized

−1

2

4

spacers are often used to synthesize RP phases with tunable

carbon sites in the phenyl ring (Cp1, k_C_P= 0.8 ms ). Similar

trends in CP build-up are observed in the iodide salt, {PhC2}I,

although build-up is faster in the RP phase, indicating that

interactions with lead iodide octahedra or adjacent spacers

decrease the motion of {PhC2} cations (Figure S11c and

29,30

dielectric and structural properties within the interlayer.

Interactions between the adjacent phenyl groups within the

organic layer have been hypothesized to increase the rigidity of

RP phases and decrease electronic disorder relative to linear

17,19

1

alkylammonium spacers.

Aromatic spacers also contain

Table S5 ). Broad signals in the 1D and 2D H MAS NMR

bulky groups that are not present in the linear alkylammonium

spectra of {PhC2} PbI , compared to {PhC2}I (Figure S12),

2

4

1

13

species; thus, a diversity of site-speciﬁc H → C CP build-up

kinetics and diﬀerent dynamic motions are expected for

further indicate that templating by the lead iodide octahedra

increases the rigidity of {PhC2}. Atomic displacement

parameters, U_i_s_o_t_r_o_p_i_c, of phenyl carbon sites in {PhC2} PbI

{

PhC2} in RP phases.

2

4

17

Figure 6a illustrates the arrangement of {PhC2} spacers in

PhC2} PbI , and Figure 6 b labels the diﬀerent carbon sites.

are similar across all carbons (Table S6 ), so the diﬀerences in

CP build-up rates of protonated and nonprotonated carbon

sites in {PhC2} are likely due to diﬀerences in the number of

protons rather than dynamic motion. At low CP contact times

{

2

4

(

<0.1 ms), the CP intensity build-up and rate constants are

governed by the dipolar interactions between the directly

bonded C−H moieties and the local dynamics associated with

them. In the case of quaternary carbon atoms, without any

directly bonded H, the CP build-up rather depends on the

distant C−H dipolar interactions, which cannot be directly

related to the local dynamics. Thus, the diversity of carbon

sites in {PhC2} complicates a direct comparison of local

dynamics within the molecule and with other spacers.

Nonetheless, the local dynamics of alkyl carbon sites near

lead iodide octahedra can be compared between {PhC2} and

1

13

linear alkylammonium spacer cations. H → C CP build-up

rates associated with the methylene alkyl carbon sites near

NH (C1 and C2) in {PhC2} are chemically similar to the

3

same sites in {C4}, {C8}, and {C12} spacers in RP phases.

Methylene groups (C1) in both {PhC2} and {C4} spacers

have much faster build-up (Figures 4c and 6c) than equivalent

carbon sites in {C8} or {C12} spacers in RP n = 1 phases

(

8

Figure 5a). The CH group nearest to the lead iodide surface

2

C1) in {PhC2} PbI has a relatively slower build-up (k

=

2

4

CP

−1

−

1

.6 ms ) than the same site in {C4} PbI (k = 23.4 ms ),

2

4

CP

which indicates that {C4} is more rigid at the lead octahedra

interface. Prior temperature-dependent T₁relaxation rate

studies of n = 1 lead bromide RP phases (complemented by

neutron scattering measurements) asserted that {PhC2}

exhibited more rigid structures and packing interactions than

̅

unit cell of

{

C4}, but these diﬀerences can be attributed to the local

19

motion at the ﬂexible methyl end of the linear spacer.

Overall, the analysis of CP build data supports, at least in part,

the conclusion that {PhC2} spacer cations exhibit a more rigid

structure than {C4} spacer cations when averaged across the

entire molecule.

The phenyl ring rotation in {PhC2} around the Cp1−Cp4

axis is an important reorientation mode that can aﬀect both the

dynamic spacer motion and dielectric environment of RP

2

4

phases. {PhC2} has a C rotational symmetry of the phenyl

2

ring that allows for rapid reorientations even if the alkyl

carbons (C1 and C2) are rigid at CP-MAS NMR time scales

(microsecond to millisecond). The ortho, para, and meta sites

(Cp2/3 and Cp4) of the phenyl ring in {PhC2} PbI have

Although carbon sites in {PhC2} exhibit a range of ¹³C{ H}

CP-MAS-NMR CP build-up rates (Figure 6c), these values are

comparable to the linear alkylammonium spacers (Figure 5a).

1

2

4

{

(

PhC2} contains carbon atoms in secondary CH alkyl sites

C1 and C2), phenyl C−H sites (Cp2/3 and Cp4), and a

similar CP build-up rates (Figure 6c), comparable to the

analogous CP build-up rates associated with alkyl carbons (C1

2

functionalized quaternary carbon site (Cp1). Compared to the

and C2). Thus, CP build-up indicates that C rotation may be

2

linear spacers, {PhC2} has a slightly higher local density of C

limited in {PhC2} spacers. This result is intriguing when

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1

03

compared to earlier NMR measurements of temperature-

rigid C−H bond (D

≈23 kHz).

In comparison, the

CH,max

40,41

dependent T relaxation rates in {PhC2} PbI .

C₂rotation

lower D coupling of ≈13 kHz for the iodide salt, {PhC2}I,

2

4

CH

has been observed to account for peak broadening of the meta

and ortho carbons (Cp2 and Cp3) in {PhC2} at elevated

temperatures (heated to 368 K), which is not observed for the

functionalized (Cp1) or para carbons (Cp4). An activation

energy of 25 kJ mol⁻was estimated for the phenyl ring

rotation in {PhC2} PbI , which is signiﬁcantly lower than in

indicates that the {PhC2} can undergo rotational motion in

other phases. This result indicates that phenyl groups are rigid

about the C₂rotation axis in {PhC2} PbI at room

2

4

temperatureconsistent with previous observations by

1

NMReven though {PhC2} may rotate at elevated temper-

19,40,41

atures.

According to published structures from single-

2

4

many crystalline and amorphous solids of similar phenyl

crystal XRD, {PhC2} spacers do not interdigitate or exhibit

π −π stacking in the n = 1 RP phase at room temperature

(Figure S2). However, intermolecular CH−π interactions may

stabilize the structure. Based on the published structure from

XRD, the separation between the nearest H atom and the

phenyl plane in between {PhC2} spacers is slightly larger

(≈2.9 Å) and more distorted (C−H-phenyl plane angle

≈140°) than in common aromatic systems with strong CH−π

interactions (e.g., H-phenyl distance ≈2.7 Å, C−H-phenyl

4

0,41

species.

Thus, we cannot rule out the impact of phenyl

1

13

rotation about the C axis on the H− C dipolar interactions

2

1

13

and H → C CP build-up rates at ambient temperatures. To

this end, we performed 2D cross-polarization variable contact

(

CP-VC) experiments to directly probe the C rotational mode

2

of {PhC2}. Such experiments have been previously employed

to understand the side-chain dynamics of phenyl rings in

1

03

13

1

polypeptides. Dipolar-mediated 2D C{ H} CP-VC experi-

13

104

ments (Figure 7 ) correlate C chemical shifts in the horizontal

angle ≈150° in strongly interacting CH−π systems). C−H

bond lengths determined by XRD can be poorly resolved or

shortened (often up to 0.1 Å) due to weaker scattering from H

atoms; therefore, the CH−π interaction length in {PhC2} PbI

2

4

is consistent with strongly interacting systems. The crystalline

packing of spacers in the RP interlayer may also augment the

interaction. In addition, CP-VC measurements demonstrate

that, at ambient conditions, phenyl rings in {PhC2} spacers are

rotationally rigid within intermediate time scales (microsecond

1

to millisecond). One-dimensional H and 2D H− H DQ−SQ

1

correlation spectra further corroborate this result; strong H

DQ signal intensities arising from dipolar interactions between

(

rigid) alkyl and phenyl carbon sites are observed in the RP

phase (Figure S12 ).

Spacer Dynamics through Build-up of H → ¹⁵N CP-

MAS Intensity. While site-speciﬁc dynamics of carbon sites in

RP spacer cations are highly sensitive to molecular structures

and interlayer arrangements, dynamic motions closer to the

lead iodide octahedra are likely more impactful on the

inorganic substructure than ﬂuctuations deeper within the

interlayer. Since the ammonium group in RP spacers carries

the cationic charge and binds with negative sites on the lead

iodide surface, and may also participate in H-bonding with

1

3

1

Figure 7. Comparison of the solid-state 2D C{ H} CP-VC NMR

1

spectra acquired at 14.1 T ( H, 600.13 MHz) for (a) the

{

PhC2} PbI (n = 1) RP phase and (b) the {PhC2}I precursor

2 4

salt. The separation of signals Δ = (D /√2) that leads to the

CH

estimation of ¹³C− H dipole−dipole couplings of (a) 20.8 kHz and

1

(b) 12.6 kHz.

37

+

neighboring I atoms, dynamic motion of NH₃should be

particularly important. Analysis of CP build-up kinetics of N

1

5

axis with C−H dipole−dipole couplings, D_C_H, in the vertical

indirect axis. The frequency separation in the vertical

dimension of CP-VC spectra is related to the modulation of

nuclei can provide complementary insights into spacer motion

at the immediate interface with lead iodide octahedra.

Although N NMR is a useful probe of site-speciﬁc structures

1

13

15

H− C dipolar couplings, which scales with the molecular

and dynamics, inherently low sensitivity (gyromagnetic ratio, γ,

motions of diﬀ erent functional groups, as described in more

detail in the Supporting Information. Although the −CH

groups (Cp2, Cp3, Cp4) show diﬀerent frequency separations

that can be directly correlated to local molecular motions, it is

less straightforward to establish similar correlations in carbon

1

5

1

of N is an order of magnitude lower than H) and low

1

5

natural isotopic abundance ( N, 0.4%) often limit the use of

N ssNMR techniques. One-dimensional N{ H} CP-MAS

1

5

15

1

ssNMR spectra of both RP phases and precursor salts exhibit

1

5

sites with multiple protons attached to them, e.g., CH and

well-resolved N signals between −335 and −330 ppm, which

15

2

CH groups. Therefore, measurements of dipolar splittings in a

diﬀer slightly from the analogous N chemical shifts of

methylammonium cations in CH NH PbI (MAPbI ) (−347

3

2

D CP-VC spectrum provide a more accurate picture of

3

87

rotational dynamics speciﬁc to phenyl groups in {PhC2}

spacers, i.e., beyond what is attainable with CP build-up

kinetics alone.

ppm). These displacements are consistent with the diﬀerence

in H NMR chemical shifts between NH groups in RP

phases (≈7.2 ppm) and in MAPbI (≈6.3 ppm), and are

attributed to diﬀerent noncovalent interactions between the

1

+

3

8

7

3

Figure 7 compares 2D CP-VC spectra of {PhC2} within the

RP phase and iodide salt. Dipolar frequency splittings (Figure

oriented spacer ammonium groups and the lead iodide surface.

1

5

1

7

, inset arrows) for the phenyl carbon sites (Cp2/3 and Cp4)

In addition, the comparison of N{ H} CP-MAS NMR

are much higher in the RP phase, {PhC2} PbI (D = √2Δ

spectra also reveals distinguishable NH peaks, with subtle

2

4

CH

3

=

≈

20.8 kHz), than in the precursor iodide salt, {PhC2}I (D_C_H

diﬀerences in chemical shift between each spacer (Figure S13),

which could be attributed to slight variations in the local

chemical environments at RP interfaces.

√2Δ = 12.6 kHz). For the RP phase, the D coupling of

CH

21 kHz is closer to the theoretical maximum D value for a

CH

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Article

A comparison of ¹⁵N{ H} CP build-up curves for diﬀerent

1

diﬀerent motional modes and local bonding interactions of

15

1

15

1

spacer cations is given in Figure 8 a. N − H dipole−dipole

NH and CH groups in RP spacers. N{ H} intensity build-

3 2

up of {C4} is similar to {PhC2}, and much slower than {C8}

or {C12}, suggesting that NH₃⁺groups in smaller cations are

more mobile near lead iodide octahedra (Figure 8b). By

1

3

1

comparison, C{ H} CP data indicate that {C4} and {PhC2}

spacers are much more rigid at alkyl groups near the lead

iodide surface (Figure 5), especially compared to the alkyl

groups of longer {C8} and {C12} spacers. The NH group

3

participates in both Coulombic interactions with negative sites

at the lead octahedral interface and local H-bonding

interactions with octahedral I atoms. The C rotation of the

3

NH₃group has been reported to dominate motion in

{

C8} PbI through inelastic neutron scattering measurements

2 4

39

at room temperature, and is expected to dictate CP build-up

kinetics for the other spacers as well. NH rotation should not

3

strongly aﬀect Coulombic interactions with the negatively

charged lead iodide octahedra but may change the strength of

directional H-bonding interactions with I atoms. The dynamics

of NH rotation can be strongly perturbed by H-bonding with

3

nearby I atoms, as shown in temperature-dependent studies of

methylammonium lead halide (MAPbI ) and formamidinium

3

37,38

lead halide (FAPbI₃).

The closest N−I distance for each

RP spacerestimated from reﬁned XRD structures in Table

Figure 8. Solid-state 1D ¹⁵N{ H} CP-MAS build-up kinetics for each

1

S1 ranges from ≈3.6 to ≈3.7 Å and is consistent with

spacer cation in the RP phase and in the associated iodide precursor

+

3

1

5

1

separations between NH and I in strongly H-bonding

salt. (a) Solid-state N{ H} CP-MAS ssNMR build-up curves

105

1

5

systems.

Thus, the dynamics of NH rotation should be

(

{

9

markers) for terminal ammonium N of single crystals of

3

^c^l^o^s^e^l^y^lⁱⁿ^k^e^d^t^o^t^h^e^l^o^c^a^l^a^t^o^mⁱ^c^eⁿ^vⁱ^r^oⁿ^m^eⁿ^t^a^r^o^uⁿ^d^t^h^e^N^H3

C4} PbI , {C8} PbI , {C12} PbI , and {PhC2} PbI acquired at

.4 T, 8 kHz MAS, and 300 K. Monoexponential ﬁts of the data are

2

4

2

4

2

4

2

4

group.

1

5

1

also shown for {C4} and {PhC2} spacers, along with biexponential

The trend in N{ H} build-up kinetics may also be

correlated with apical octahedral tilts, which are sensitive to

the arrangement and local interactions of spacer molecules

ﬁts for {C8} and {C12} spacers (dashed lines). (b) Fitted CP rates,

1

5

1

^kCP^,^e^x^t^r^a^c^t^e^d^f^r^o^m N{ H} CP build-up curves for each of the n = 1

lead iodide RP phases (colored) along with each corresponding

(

Figure S3 ). The octahedral tilt systems, and tilt angles about

ammonium iodide precursor salt (gray). CP polarization times, T_C_P

/k , are indicated at right.

=

each axis for the tested RP phases, are derived from published

structures obtained from single-crystal XRD and are listed in

Table S1 . The longer spacer cation, {C12}, has greater apical

octahedral tilts (10.5°) than the shorter alkyl spacers (5.5°).

1

CP

1

3

1

interactions are relatively weaker than C− H dipolar

1

15

interactions; thus, H → N CP requires much longer CP

This evidence suggests that dynamic rotations of NH and

3

1

15

contact times to attain intensity build-up. Therefore, H → N

intensity build-up is measured and compared as a function of

CP contact times of up to 10 ms. In the n = 1 RP phases,

time-averaged octahedral distortions are related and depend on

the size and composition of the spacer. H-bonding interactions

between NH

and I are directional and can induce octahedral

3

37

1

5

1

{

PhC2} and {C4} have relatively slower N{ H} build-up

tilting as the temperature is lowered; therefore, the rotational

dynamics of NH probed by N{ H} build-up kinetics is also

3

−

1

15

1

rates (k_C_P= 0.4 ms ) than the longer alkylammonium spacers

k_C_P= 1.0 ms for {C8} and 1.4 ms for {C12}), which can

−

1

−1

(

expected to shed light on the time-averaged structure of RP

phases. Intermolecular van der Waals forces in the RP

interlayer, which are stronger and more templating for longer

be attributed to diﬀerent proton densities and local molecular

motions. The presence of a large number of CH groups in the

2

{

C8} and {C12} spacer cations may enhance the build-up rate

alkyl spacers, may inﬂuence NH rotational dynamics by

3

by spin polarization from distant CH groups (i.e., three or

altering the tilt angle and local stereochemistry of methylene

groups near lead iodide octahedra. It is unclear how precisely

dynamic spacer motion inﬂuences, or is inﬂuenced by, the

time-averaged structure of lead iodide octahedra. Nonetheless,

these results demonstrate that spacer motion is highly localized

2

more bonds away from NH groups). The iodide salts of each

3

1

5

1

spacer exhibited similar trends but slightly lower N{ H}

build-up rates than the RP phases (Figure 8b). Build-up curves

for {C8} and {C12} also show a gradual decrease in signal

intensity after a maximum of around 3 ms. This decrease is

commonly attributed to the homonuclear relaxation, T of

in RP phases and that the motion of diﬀerent alkyl, NH , or

3

other functional groups should be analyzed independently to

study the structure and properties of layered hybrid perov-

skites.

1

ρ

1

polarized H nuclei (see the Supporting Information, Section

). To account for this phenomenon, N{ H} CP build-up

9

8

15

1

curves for {C8} and {C12} were ﬁt to a biexponential model

with variable k , b, and k parameters (eq S3 and Tables

Implications of Spacer Dynamics on the Local

Structures and Properties of RP Phases. The relevance

of spacer dynamics to the functional properties of RP phases

depends on the time scales of molecular reorientation. Spacer

motions can aﬀect the optoelectronic behavior of RP phases

through several means: by actively coupling with electronic or

phonon modes at time scales of photocarrier and exciton

CP

I,H

S2 −S5). The same model can be applied to CP build-up

curves for other samples, but T₁relaxation would be too slow

ρ

(

T₁_ρ₍_H₎≫ 10 ms) to ﬁt the measured data.

The divergence between trends in ¹N{ H} and C{ H}

5 1

13

1

build-up kinetics with spacer size can be attributed to the

6

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1

5−18,52

generation and recombination (≪1 ns),

by generating

conﬁrm that long linear alkylammonium spacers (e.g., n-

dodecylammonium) interleave in close-packed trans-arrange-

ments within the RP interlayer. In addition, rigid and ﬂexible

sites of linear alkylammonium and bulky aromatic spacer

cations are identiﬁed and distinguished by analyzing and

comparing H → C and H → N CP signal intensity build-

up. In linear alkylammonium spacers, the thermal motion of

methylene groups depends on spacer length, whereby shorter

{C4} spacers are most rigid near the octahedral surface, but

longer spacers (e.g., {C12}) are more rigid within the

interlayer. ssNMR results suggest that rotational rigidity of

disorder in the local dielectric or electrostatic environment due

106

to slower dynamic motion (≫1 ns) or by inﬂuencing the

time-averaged tilting, bonding, and disorder of octahedra (long

time scales). Each of these processes can change nonradiative

recombination rates and aﬀect the performance of RP phases

for photovoltaic, light-emitting, or sensing applications.

Analysis of CP intensity build-up provides a useful

comparison between the dynamics associated with structurally

similar molecules across intermediate time scales. However, it

is less straightforward to extract quantitative correlation times

of molecular reorientation from this technique. CP build-up

rates are convoluted by motional dynamics, densities, and local

1

13

1

15

NH groups in RP spacers, on the other hand, increases with

3

spacer length and appears to correlate with octahedral tilting in

the time-averaged crystalline structures. We rationalize that a

balance between Coulombic and hydrogen-bonding interac-

tions with lead iodide octahedra, and van der Waals forces

between spacers within the RP interlayer, is responsible for

such trends in site-speciﬁc dynamics. Aromatic {PhC2} spacers

appear to have similar CP build-up kinetics to short

alkylammonium spacers near lead iodide octahedra (e.g.,

{C4}), suggesting that aromatic and alkylammonium spacers

are similarly rigid in RP phases. The time scale of phenyl

rotation in {PhC2} is estimated to be 10−100 μs by measuring

with 2D CP-VC NMR. For all spacers, the lead iodide

octahedral surface templates the structure and increases the

rigidity of spacer cations compared to iodide salts of the same

cations. The proposed ssNMR approach is relatively simple to

implement and can be applied to study interfacial structures

and dynamics of spacer cations in low-dimensional perovskite

halides. Future investigations may expand on these techniques

1

chemical environments of H sites in the vicinities of carbon or

9

8

nitrogen sites. Temperature-dependent measurements of CP

build-up or ssNMR relaxation rates can be used to estimate

reorientation activation energies, and thus absolute correlation

times, but this method requires stable RP phases that do not

undergo transformations across a wide temperature range (ΔT

∼

100 K). Many of the RP phases undergo thermal phase

transformations around room temperature; therefore, we can

only make relative comparisons of molecular motion between

samples. For example, previous ssNMR measurements have

estimated a room temperature molecular reorientation time, τ_c,

for {PhC2} spacers in n = 1 RPs of about 20 μs based on

temperature-dependent peak broadening by T relaxation.

Our room-temperature CP-VC measurements of {PhC2} PbI

40

2

4

(

Figure 7a) indicate a reorientation mode in {PhC2} at time

scales slightly faster than the rigid limit of the technique (∼100

40

μs), consistent with the earlier estimate. We ﬁnd that the

other spacers have similar time scales of reorientation in RP

phases because CP build-up time constants for carbon sites in

2

using variable temperature H NMR spectroscopy and T , T ,

1

1ρ

and T relaxation measurements to provide complementary

2

{

C4} PbI , {C8} PbI , and {C12} PbI span the same order of

insights into the site-speciﬁc dynamics of spacer cations in the

RP phase. This analysis informs the molecular design of spacer

cations that take into account length, shape, functional groups,

and the roles of diﬀerent noncovalent interactions to create

hybrid organic−inorganic layered perovskite materials with

improved environmental stability and optoelectronic proper-

ties.

2

4

2

4

2

4

magnitude as carbon sites in {PhC2} PbI (Figures 5 and 6).

2

4

Indeed, the longer alkylammonium spacers seem to be, on

average, similarly rigid to aromatic spacers such as {PhC2} in

RP phases. At these time scales, spacer reorientation occurs

much more slowly than carrier lifetimes or phonon vibrations.

Therefore, it becomes less relevant to attribute electron−lattice

coupling to the molecular dynamics of spacer molecules

probed by NMR spectroscopy. Rather, these reorientations can

change the static structure (e.g., octahedral tilting, Figure S3)

of RP phases by modulating H-bonding between NH groups

and octahedra, and contribute to local disorder near lead halide

octahedra, similar to the Urbach energy measured from the

absorption band edge of semiconductors.

higher-frequency vibrations and phonon modes may still exist

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.chemmater.0c03958.

■

*

sı

3

107

Theoretical aspects of NMR cross-polarization kinetics

and the CP-VC experiment; powder XRD patterns of

ground RP single crystals used in the study;

representations of reﬁned crystal structures for each

measured RP phase to indicate spacer arrangements and

We note that

15−17,20

within spacer molecules,

ssNMR measurements.

but are not probed by these

1

13

octahedral tilting; 2D H{ C} HETCOR NMR spectra

CONCLUSIONS

1

■

of each RP phase; 2D H{ H} DQ−SQ correlation

Low-dimensional hybrid perovskite RP phases combine the

electronic defect tolerance of 3D perovskites with improved

ambient stability and tunability through compositional control

of spacer cations. It is crucially important to characterize and

understand how the organic and inorganic substructures

interact and aﬀect the structure and functional properties at

near-ambient temperatures. This study employs multinuclear

NMR spectra of {C8} and {PhC2} RP phases; 1D

13

1

15

1

C{ H} and N{ H} CP-MAS NMR spectra for each

13

1

RP phase and precursor iodide salt; C{ H} and

15

1

N{ H} CP build-up curves of precursor iodide salts;

and tables of ﬁtted CP kinetic parameters for each RP

phase and iodide salt (PDF)

1

13

15

(

H, C, N) solid-state NMR to gain insight into the

AUTHOR INFORMATION

Corresponding Authors

structure−dynamics relationships of the organic moieties in RP

■

1

13

phases. Homo- ( H− H) and heteronuclear ( H− C) 2D

NMR correlation experiments complement previously pub-

lished single-crystal XRD reﬁnements of RP phases and

Michael L. Chabinyc − Materials Department, University of

California, Santa Barbara, Santa Barbara, California 93106,

6

52

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Chem. Mater. 2021, 33, 642−656

Chemistry of Materials

pubs.acs.org/cm

Article

United States; orcid.org/0000-0003-4641-3508;

Email: mchabinyc@engineering.ucsb.edu

G. N. Manjunatha Reddy − Univ. Lille, CNRS, Centrale Lille,

Univ. Artois, UMR 8181UCCSUnité de Catalyse et

Chimie du Solide, F-59000 Lille, France; orcid.org/0000-

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(

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(

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002-8283-2462 ; Email: gnm.reddy@univ-lille.fr

Karunadasa, H. I. A Layered Hybrid Perovskite Solar-Cell Absorber

Authors

with Enhanced Moisture Stability. Angew. Chem. 2014, 126, 11414−

Clayton J. Dahlman − Materials Department, University of

11417.

California, Santa Barbara, Santa Barbara, California 93106,

United States; orcid.org/0000-0002-4555-4846

Rhys M. Kennard − Materials Department, University of

California, Santa Barbara, Santa Barbara, California 93106,

United States

Piotr Paluch − Centre of Molecular and Macromolecular

Studies, Polish Academy of Sciences, 90-363 Lodz, Poland

Naveen R. Venkatesan − Materials Department, University of

California, Santa Barbara, Santa Barbara, California 93106,

United States; orcid.org/0000-0003-2086-5274

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Complete contact information is available at:

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15) Mauck, C. M.; France-Lanord, A.; Hernandez Oendra, A. C.;

Dahod, N. S.; Grossman, J. C.; Tisdale, W. A. Inorganic Cage Motion

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Material synthesis and structural characterization were

supported by the U.S. Department of Energy, Oﬃce of

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Defense Science and Engineering Graduate Fellowship for

ﬁnancial support. G.N.M.R. gratefully acknowledges the

ﬁnancial support from University of Lille and the IR-RMN-

THC FR-3050 CNRS France for conducting solid-state NMR

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https://dx.doi.org/10.1021/acs.chemmater.0c03958

Chem. Mater. 2021, 33, 642−656

Article Doi

Dynamic Motion of Organic Spacer Cations in Ruddlesden-Popper Lead Iodide Perovskites Probed by Solid-State NMR Spectroscopy

DOI: 10.1021/acs.chemmater.0c03958

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Article abstract of DOI:10.1021/acs.chemmater.0c03958

Full text of DOI:10.1021/acs.chemmater.0c03958

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