Design of Polyuretanes and Related Hydrogen-Bonding Supramolecular Materials
with Charge Carrier Transporting Properties
Tsuneaki Sakurai,1 Shu Seki,*1 Suhrit Ghosh,2 Kazunori Sugiyasu3, Masayuki
Takeuchi,31Department of Molecular Engineering, Graduate School of Engineering, Kyoto
University.
Nishikyo-ku, Kyoto 615-8510, Japan
2
Polymer Science Unit, Indian Association for the Cultivation of Science, Kolkata700032, India
3
Molecular Design & Function Group, National Institute for Materials Science(NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
（第一回 ポリウレタン国際技術振興財団研究助成）
Introduction
Polyurethane is a widely used polymer backbone carrying multiple urethane bonds.
These urethane linkages are capable of forming both intramolecular and intermolecularhydrogen (H-) bonds in both solution and solid state, affording various kinds ofsupramolecular materials with organized structures.[1–3] Controlled H-bondings can berealized by periodically arranging urethane bonds as well as analogous amide and urea
functionalities in polymer main chains. One of the unique conformationally organizedstructures for those polymers is folded state as a single polymer chain. Such folding
polymers have been categorized into “foldamer”, where helical or layered nanostructures
are formed with the help of periodic H-bonding formations among adjacent/nonadjacentmonomer units.[4–6] Classical polyurethane itself has elastic or thermoplastic characters,
but no optical or conductive properties are equipped due to the absence of conjugatedmoieties. However, by means of the polyurethane scaffold providing periodically aligned
urethane functionalities, it is possible to incorporate π-conjugated units into either mainor side chains periodically.
In our study, naphthalenediimide (NDI), a famous n-type semiconducting πconjugated motif, was introduced as a pendant group of a classical polyurethane backbone.
The charge carrier transporting property was studied by our noncontact microwave-based
technique.[7,8] As a result, significantly larger electron mobility and longer lifetime ofcharge carriers were observed for the folded tubular assembly of the NDI-appended
polyurethane than those for unfolded polymer, likely due to a better delocalization of thecharge carriers in the integrated tubular assembly consisting of stacked NDI arrays inside
the multilayer wall.[9]Furthermore, inspired by this polyurethane scaffold having H-bonding units, wedeveloped porphyrin-based supramolecular polymers with three types of nanostructures.
Nanofiber, nanosheet, and nanoparticle morphologies were developed from the identicalmonomer by finely tuning the π-π stacking, H-bonding, and van der Waals interactions
among alkyl chains that can be controlled by the preparation conditions. The observed
photoconductivity and charge carrier mobility for these nanostructured films were foundto be different, reflecting the nanostructures providing different charge transporting
pathways.[10]Principle of noncontact assessment technique of charge carrier mobility in softmolecular aggregates.
Till date, highly sophisticated assessment techniques have been developed for the
analysis of charge carrier mobility such as time-of-flight (TOF),[11] field-effect-transistor(FET),[12] space-charge-limited-current (SCLC),[13] impedance spectroscopy (IS)analyses.[14] The analyses give the value of mobility as a consequence of long-range
translational motion of charge carriers along electric field vector E applied between a setof electrodes apart as a few hundreds mm. The statistically deduced value of mobility isoften impacted by the presence of charge carrier injection barriers at the electrodeinterfaces, structural instability and defects, impurities, etc, leading to screen the intrinsic
potentials of the charge conducting nature of the materials. Particularly in case ofmolecular assembly, even if the structure has been stabilized via relatively stronghydrogen bonding networks, the fabrication procedures of the devices for the mobility
assessments cause severe damages on the structures/interfaces, resulting in the far lower
values of mobility than the ones originally the materials are expected. This is the case of
the strong demands for the contactless evaluation of charge carrier mobility in soft
molecular materials, and we have developed a series of assessment techniques refers astime-resolved microwave conductivity (TRMC) measurements via the short-range
interaction of electromagnetic waves in GHz range (microwaves) and charge carriers
photo-injected into the molecular assembly without electrodes attached.[7,8,15–17]
Short-range (nm-scaled) oscillating motion is induced by the interaction between
microwaves and charge carriers, and the fundamental Maxwell’s formulae on theinteraction are given as followings in semiconducting materials with negligible
conductivity of .
 E    m  m E   mE 
t 
t

B 




  m  m B   mB 
t 
t


(1),
(2),

where B is magnetic field, m and m are magnetic permeability and permittivity,
respectively. In case of E(r) (and B(r)) given as a sinusoidal standing wave, thesimultaneous equations of (1) and (2) are converted in terms of E(t) as,E ( r )   m i   m  2 E ( r ) .

(3)

Thus, the complex dielectric constant of the medium with small electrical conductivity isgiven as, m  i


,

(4),

and the above Maxwell equation is rearranged and simplified using  as
E ( r )   m  2 E ( r )(5) .

This is suggestive of a direct correlation with the complex dielectric constant andconductivity of the materials. As seen in eqs. (4) and (5), the trace of E(t) gives directly
complex , and deducing quantitatively and separately the contribution of imaginary partin  immediately allow us to estimate  of the materials in the interaction.
The value of mobility (), the final target of the assessment, is in direct relation with
conductivity as,(6), en
where e and n are elementary charge and charge carrier density, respectively. Mobility
measurement along with the above scheme is advantageous to realize a non-contact, nondestructive screening process without the need for device fabrication. However unlike theconventional measurement techniques, electrical amplification techniques are notapplicable for this scheme via evolution of current (voltage) by external amplifier systems.
To achieve highly sensitivity in the measurement with wide dynamic range, a microwave
cavity system is the primary choice to promote effective interactions of probing
microwave with the injected charge carriers, facilitating dramatic increase in thesensitivity which is in trade-off relation with response time of the system.[18,19]
Assuming an electromagnetic wave cavity with a Q value, the loss ofelectromagnetic waves in the cavity (1/Q) cannot be measured directly by inserting
probes, and the power (Pr) of microwaves reflected from the cavity and its transientchanges (Pr) is the only observable. The Q value can be further divided into Qu, Qs, andQc corresponding to the cavity coupling with waveguide, the interaction with sample instatic states, and the interaction with conductive species injected transiently, respectively.
The Qu and Qs components remain constant during measurement of microwave power
loss, so these can be included in Q. The relationship between Q, Qu, and Q can be
expressed as:
1 1
1


(7).
Q QS QU
Prior to carrier injection, the cavity is in resonance at the frequency of f0 (= ω0/2). If theinsertion of samples into the cavity only causes perturbation of the cavity parameters, thestatic loss of microwave is small enough, the change is given as,
1
1
1
1
1



 d QS QU QC QU
Q 

(8).

Simultaneously, we define the static shift of microwave resonance angular frequency as
. The normalized reflection (PR) of the microwave from the cavity is now given as,2

     1    d   4
0    Q  

PR 
2
2
    1 
   4
 0   Q 

2

(9).

Herein excess charge carriers are injected non-contactly into the sample via mainly photocarrier generation processes upon excitation to laser pulses within response time of themeasurement system, leading to the change of Q value into Qc. The PR after the interactionof the charge carriers changes into PR’ as,2

    11 
  4
0   QC QU PR ' 
22
    1 
   4
 0   Q 

2

(10).

The net contribution from the dielectric loss by the charge carriers is,111  
(11), Q  QC QS
and the change in the normalized reflected microwave power PR = PR’ – PR reveals to
depend linearly on 1/Q as the following form derived by the rearrangements of eqs (9),(10), and (11),PR  PR '  PR 

1
1

QC QS

1
1
2 




2
2
    1   QS QC QU    4
 0   Q 

1
d  
1Q  
22
Q
   1  1 
   2
 0  2  Q 

and simplified as,1PR   
Q

(12).

(13).

Now, once back to the eq. (4), the ratio of real and imaginary parts of the dielectric
constant is often called as a dielectric loss factor of tan as, '' 

(14).
 '  'In contrast, the Q factor of the microwave cavity is defined also by the ratio of microwave
power storage in the cavity (W) and its consumption (L) as,tan  

1
L

Q 0W

(15).

W is estimated simply by the averaged electric field strength of sinusoidal standing waves
with angular frequency of  = 2/T in the cavity over the time as,' ' E0 2(16).
E 0 cos t dt T 
2For L, we assume Joule’s heating by the local oscillating motion of charge carriers via theW   '  E 22

2interaction with microwave. Aaveraged Joule’s heating is given by eq. (14) as,L  E 2   ' ' E 2   ' E 2 tan(17),

and averaged over the time as, ' tan  ' tan E 0 2

 E0 cos t dt T2
Substitution of eq. (15) by eqs. (16) and (18) leads,
L

2

2

 ' tan E 0 2 
1
L



tan 
Q  0W0 0 ' E0 2 2

(18).

2

(19),

and if the change of micorwave frequency can be neglegible even after the interaction
with the transiently injected charge carriers, we obtain,1   tan  
 PR
(20),
Q
 '
which is suggestive that the precise tracing of the change in the reflected microwave
power from the cavity give immediately an estimates of conductivity change in thematerials loaded in the cavity via transietn charge carrier injection.
Combination of transient absorption spectroscopy (TAS) and TRMC for totally noncontact assessment of charge carrier mobility
To deduce finaly the value of mobility of charge carriers, as seen in eq. (6), thenumber of charge carriers (n) is indispensable for fully experimental assesment. A
pioneering work was reported by Warman et al. at Delft University of Technology(Netherlands) using microwaves ranging at 10–50 GHz (microwaves) for conductivity
measurements combined with transient absorption spectroscopy.[20,21] This technique isreferred to as pulse-radiolysis time-resolved microwave conductivity (PR-TRMC), as itgenerates transient charge carriers in the materials upon exposure to a pulsed high-energy
electron beam. Because of the homogenous ionization of the primary component of themedium, the density of the generated charge carriers can be determined without
performing other independent measurements, allowing a direct evaluation of charge
carrier mobility from the accurate detection of dielectric loss. In contrast, the authors havefocused on the flash-photolysis TRMC (FP-TRMC) utilizing exciton dissociation into
charges as a charge-carrier injection scheme.[7,8,15–17] Transient absorption spectroscopy
has been combined with the TRMC apparatus, allowing us to simultaneous determinationof charge carrier density (n) injected into the materials non-contactly. The details of thetotal set of apparatus are described in elsewhere.[16,17] The present FP-TRMC systems
are successfully applied to reveal the electronic conductive nature of molecularaggregates stabilized by hydrogen bonding networks via urethane moieties, and we
discuss in the following sections. In eq. (20), the normalization of PR is typically carriedout by the ratio of actual power reflection from the cavity prior to carrier injection (Pr /W) and its change by the injection (Pr / W) as follows,PPR  rPr

(21),

and with the proportional factor A in eq. (20):1 0  0  r   Pi  2 A     1
2Q   Pr 




1

(22),

we obtain the following equation by the rearrangement of eq. (20) as, P   A  r 
 Pr 

(23),

where  and ’ are assumed to be constant as  = 0 and ’ = 0r because of negligibly
small amount of injected charge carriers into the system. Pi is the power of microwave
injected into the cavity, and typically fixed at 1-10 mW in our system. Transient charge
carriers are injected by photo-ionization by ns laser pulses with the duration of a few ns
which is short enough to the response time of the cavity system. Here upon
photoexcitation, the carrier density of n (m–3) is injected into the system, giving thechange of electrical conductivity as,  en (24),

where  (m2 V–1 s–1) is the sum of the mobility of negative and positive charge carriers.
Unfortunately quantitative contribution from negative (electron) and positive (hole)carriers cannot be discusses separately in case of photo-carrier injection without selective

quenching of carriers by additives. TAS measurement of photo-generated carriers(electrons/radical anions and holes/radical cations) gives simultaneously the kinetic traces
OD(t) as well as the quantum efficiency () of the transient species with their molar
extinction coefficients, and similarity of the kinetic traces of (t) and OD(t) canrecognize the major contribution from charge carrier species. Under a light excitation
with the photon density I0 (m–2), the number density of absorbed photons can be derived
from a linear absorption coefficient of the target molecules F (m–1) as FI0, andconsequently the value of n is given by,n   I 0 F(25).

From eqs. (23) (24) and (25), finally we obtain,  

A  Pr eI 0 F  Pr (26).

With the value of  determined by TAS, TRMC measurement allows us to fully
experimental assessment of charge carrier mobility in a variety of materials.[7,8] The
present system has been successfully applied to the urethane-based molecular aggregates,
leading to the intrinsic charge transport properties.
Synthesis of polyurethane carrying NDI pendants as side chains.
P1 was synthesized (Scheme 1) by a step- growth polymerization between
hexamethylene-diisocyanate (M1) and a NDI-containing diol (M2) in the presence ofsmall amount of (s)-(−)-2-methyl 1-butanol as a “mono-functional impurity” with a
specific ratio to ensure capping of all the chain-ends by the chiral alcohol. The poly
condensation was carried out in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO)
catalyst in THF to produce the desired polyurethane (P1) as a light yellow powder in 75%
yield. The end g roup analysis in 1H NMR estimated the molecular weight of P1 to be8000 Da that matched reasonably well with the number average degree of polymerization=13, estimated from the Carothers’s equation. GPC chromatogram of P1 in THF showeda unimodal distribution with weight-average molecular weight (Mw) = 7600 Da (PDI =1.72) that corroborated well with the value obtained from end group analysis.

Self-assembled structures of polyurethane carrying NDI pendants.
Self-assembly of P1 was tested in a nonpolar solvent, methylcyclohexane (MCH).
At 5.0 mM concentration, the polymer was soluble at high temperature and upon allowing
to settle at room temperature, the solution gradually became more viscous and eventually
resulted in a nearly transparent green emitting gel (Figure 1) after 72 h at 3.0 mM. To
have a closer look at the morphology of the gel, high resolution transmission electron
microscopy (HR-TEM)-images were recorded that showed nanotubular structures for theaged sample (Figure 1). The inner diameter and wall thickness of the tubes were
measured to be 85 ± 5 nm and 26 ± 2 nm, respectively, while the length extended over
5−9 μm. In contrast, the HR-TEM images of a freshly prepared solution showed hollow
spherical objects. UV-vis absorption spectra of P1 in a good solvent such as CHCl3

Scheme 1. Synthesis of polyurethane P1 carrying NDIs.

showed sharp absorption bands in the range of 300–400 nm with obvious vibronic
features indicating presence of monomeric NDI -chromophores. Nevertheless, in a
nonpolar medium like MCH, a significant lowering of intensity with concomitant reversalof the intensities of the peaks at 382 and 361 nm were noticed owing to π–π interaction.
The absorption bands in MCH did not exhibit red-shift as typically observed for offset πstacking of NDI chromophores, suggesting interchromophoric interaction without
longitudinal displacement between the adjacent NDI units. Concentration dependent UVvis experiments showed no spectral change in the range of 10–3–10–5 M with theFigure 1. Schematic illustration of proposed self-assembly model of P1.
Reproduced with permission from Ref. 9. Copyright 2015 American Chemical Society.

absorption intensity varying linearly with concentration indicating “intrachain” NDI–
NDI stacking. Therefore, folding of single polymer chains was supported. Corroboratingthese experimental observations, it was proposed (Figure 1) that P1 spontaneously
adopted a folded conformation in MCH by intrachain H-bonding among the urethanegroups, leading to stacking of the pendant NDI units.

Figure 2. (a) Conductivity transients and (b) kinetic traces at 490 nm of transient absorption spectra
for P1 films dropcast from MCH gel (red), MCH solution (blue), and CHCl3/MeOH solution (green)upon photoexcitation at 355 nm with photon density of 9.1 × 1015 photons cm–2 pulse–1.

Noncontact evaluation of charge transporting property of polyurethane carrying
NDI pendants.
The electron-transporting property of the well-defined supramolecular architecturesof P1 was studied by FP-TRMC. As noted earlier, FP-TRMC technique applies electricfield of microwave that probes the local-scale charge carrier motions and thus enables thedevice-less evaluation of charge carrier transport. Upon photoexcitation by 355 nm laser
pulses, a dried-gel film of P1 on a quartz plate, prepared by drop-casting of MCH gel,showed a prompt rise followed by a slow decay in the conductivity transient (Figure 2),
indicating that π-stacked NDIs in folded P1 indeed provide charge transport pathways. In
fact, a drop-cast film of P1 from CHCl3/MeOH solution, consisting of unfolded state ofP1, displayed much smaller conductivity. The observed maximum Σμ values (3.5 × 10−5
cm2 V−1 s−1 and ~0.7 × 10−5 cm2 V−1 s−1 for gel in MCH and solution in CHCl3/MCH,respectively) indicated five times larger conductivity for P1 in folded state compared to
unfolded form. A dropcast film from freshly prepared MCH solution, including vesicleshaped assembly, also showed a transient conductivity whose maximum value was quite
similar to MCH gel state. In TAS, all the three films exhibited a characteristic absorption
band at around 490 nm indicative of NDI radical anion species, where their kinetic
profiles were similar to those of transient conductivity. Thus, electron transporting
property was confirmed. After comparing the absorption coefficient of NDI•− at 490 nm
with the observed maximum TAS intensity, we evaluated the φmax values as 1.5 × 10−2
and 1.8 × 10−2 for the dropcast films from MCH gel and CHCl3/MeOH solution,
respectively. These φmax values are similar to that of reported organic crystals and thus
indicate similar carrier generation mechanism. Finally, the electron mobility (μe) was
calculated by combining the above max and (Σμ)max values. The calculated electron
mobility values were 2 × 10−3 cm2 V−1 s−1 and 4 × 10−4 cm2 V−1 s−1 for the dropcast films
from MCH gel and CHCl3/MeOH solution, respectively, indicating strong impact offolded structure on charge carrier mobility. Of further interest, although the films from
MCH gel and freshly prepared MCH solution showed the similar electron mobility, theformer allowed much longer half- lifetime of charge carriers (τ1/2) than that from the latter(τ1/2 of ca. 40 and 5 μs, respectively). Taking into account this observation along with thefact that TRMC/TAS measurements were carried out under air, it is conceivable that
confinement of NDI arrays within the folded polymer wall in nanotubularly assembled
P1 reduced the quenching probability by oxygen in one hand and provided long-range
electron transporting pathways on the other hand, leading to the observed remarkableprolongation of negative charge carrier’s lifetime.
Development of supramolecular polymers of porphyrin chromophores with amide
functionalities.
ZnP (Figure 3) is a porphyrin derivative that undergoes living supramolecularpolymerization[10,22,23] to afford nanofibers (NF) with H-aggregation of porphyrin
chromophores or nanoparticles (NP) with J-aggregation. The presence of hexyloxy
chains leads to the further integration of NPs into nanosheets (NS). These nanostructures
are formed with the combination of π-π stacking of porphyrin cores, H-bondings of amide
units, and van der Waals interaction of hexyloxy chains. NPs were prepared by the heatcool procedure of ZnP in MCH. Then, aging of that solution resulted in the formation ofNSs. On the other hand, sonication of NPs to prepare the seeds and addition of those into
the initial solution gave NFs. The kinetic pathways of this system are summarized as aschematic illustration in Figure 3b. The morphologies of each nanostructure were clearlyvisualized by atomic-force microscopy (Figure 3c–e) and living polymerization nature
was also confirmed. Due to the different π-π stacking geometry, the absorption spectra ofthe three states are different from each other.

Figure 3. (a) Molecular structure of ZnP. (b) Differentiation of a metastable supramolecularassembly. Energy landscape of ZnP deduced from the mechanistic studies. AFM images of (c) NF,(d) NP, and (e) NS. Reproduced with permission from Ref. 10. Copyright 2017 Macmillan
Publishers Ltd.

Noncontact evaluation of charge transporting property for supramolecularpolymers of porphyrin chromophores.
The different π-π stacking behaviors of ZnP supramolecular polymer system
motivated us to investigate the nanostructure-dependence on charge carrier transporting
property. FP-TRMC measurements of dropcast samples revealed that NF showed thelargest maximum transient conductivity (Σμ)max of 2.6 × 10−5 cm2 V−1 s−1, while othertwo forms provided ca. four times smaller values (Figure 4a, Table 1). TAS
measurements at the identical excitation condition of FP-TRMC disclosed the sign ofgeneration of porphyrin radical cations, where kinetic traces of FP-TRMC and TAS are
correlated each other (Figure 4b–d). These observation indicated that holes are
dominantly transported along H/J- stacked-porphyrin cores under photoexcitation inambient condition. By using the absorption coefficient of Zn porphyrin•+ at 600–650 nmwith the observed maximum TAS intensity, we evaluated the max values as 3 × 10−2, 6 ×

10−2, and 6 × 10−2 for the dropcast samples of NF, NP, and NS, respectively (Table 1).
Thus, the hole mobility was calculated as 9 × 10−4, 1 × 10−4, and 1 × 10−4, respectively,by combining the above max and (Σμ)max values. The obtained values suggest that strong
H-stacking of porphyrin cores in NFs is favorable for hole transport, while porphyrins inboth NPs and NSs adopt J-aggregation and thus resulted in similar performance in localscale hole transports.

Figure 4. (a) Conductivity transients observed for NF (green), NP (magenta), and NS (orange) upon
photoexcitation at 355 nm with photon density of 9.1 × 1015 photons cm–2 pulse–1. (b) Normalized
conductivity transients (green) and transient absorption at 620 nm (black) for NF. (c) Normalized
conductivity transients (magenta) and transient absorption at 615 nm (black) for NP. (d) Normalized
conductivity transients (orange) and transient absorption at 640 nm (black) for NS.

Table 1. Summary of (Σμ)max and max values evaluated by FP-TRMC and TAS measurements,
together with calculated μhole values.
Entry(Σμ)max / cm2 V–1 s–φmax / –μhole / cm2 V–1 s–1

1NF

2.6 × 10–5

3 × 10–2

9 × 10–4

NP

6.4 × 10–6

6 × 10–2

1 × 10–4

NS

7.0 × 10–6

6 × 10–2

1 × 10–4

Conclusion
Combination of FP-TRMC and TAS measurements revealed the strong impact offolded/unfolded structural difference of NDI-appended foldamer P1 on the intrinsic
electron mobility and carrier lifetime of the materials. Tuning of the structure of theappended π-conjugated cores would be a subject worthy of investigations for further

developing the supramolecular electronics.[24,25] Moreover, inspired by the H-bonding directed assembly, an amide-appended porphyrin system with supramolecular living
polymerization capability was studied. Depending on the nanostructures––nanofiber,
nanoparticle, and nanosheet––the porphyrin chromophores adopt different stacking
configurations. The contactless FP-TRMC/TAS method unveiled the different holemobility depending on H/J- stacking self-assembly modes. Control of competitive
intermolecular interactions including π-π stacking, H-bonding, and van der Waals would
tailor the supramolecular materials with desired charge carrier transporting natures.
Acknowledgement
This research was supported by International Polyurethane Technology Foundation. S.S
and S.G. acknowledge DST and JSPS for the grants of Bilateral Joint Research Projects(DST/INT/JSPS/P-218/2016). S.S. and M.T. acknowledge JSPS for Grant-in-Aid for
Scientific Research on Innovative Areas “π-System figuration: control of electron andstructural dynamism for innovative functions” (Numbers 26102009, 26102011, and15K21721). T.S. acknowledges JSPS for Grant-in-Aid for Young Scientists (B) (Number
26810049) and (A) (Number 17H04880). K.S. acknowledges JSPS for Grant-in-Aid for
Scientific Research on Innovative Areas “Dynamical ordering of biomolecular systems
for creation of integrated functions” (Number 16H00787) and financial support from the
Sekisui Chemical Grant Program.
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