Interlayer-Sensitized Linear and Nonlinear Photoluminescence of Quasi-2D Hybrid Perovskites Using Aggregation-Induced Enhanced Emission Active Organic Cation Layers

Article abstract of DOI:10.1002/adfm.201909375

A concept of interlayer-sensitized photoluminescence (PL) of quasi-2D hybrid perovskite (PVK) with a π-conjugated optically interacting organic cation layer is introduced and demonstrated. A rod-shaped aggregation-induced enhanced emission (AIEE) organic

Full text of DOI:10.1002/adfm.201909375

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Interlayer-Sensitized Linear and Nonlinear Photoluminescence
of Quasi-2D Hybrid Perovskites Using Aggregation-Induced
Enhanced Emission Active Organic Cation Layers
Chang-Keun Lim, Melissa Maldonado, Robert Zalesny, Rashid Valiev, Hans Ågren,
Anderson S. L. Gomes, Jie Jiang, Ruth Pachter, and Paras N. Prasad*
superior charge carrier lifetime, photo-
A concept of interlayer-sensitized photoluminescence (PL) of quasi-2D hybrid
perovskite (PVK) with a π-conjugated optically interacting organic cation
layer is introduced and demonstrated. A rod-shaped aggregation-induced
enhanced emission (AIEE) organic cation (BPCSA+), well fitted into the lattice
size of 2D PVK layers, is designed and synthesized to prolong the exciton
lifetime in a condensed layer assembly in the PVK. The BPCSA+ promotes the
PL of this hybrid PVK up to 10-folds from that of a non-π-conjugated organic
cation (OA) 2D PVK. Notably, different from PL of OA 2D PVK, the increased
PL intensity of BPCSA 2D PVKs with an increase of the BPCSA ratio in the
PVK indicates a critical photon-harvesting contribution of BPCSA. The films
of BPCSA 2D PVKs are incredibly stable in ambient environments for more
than 4 months and even upon direct contact with water. Additionally, due to
the strong two-photon absorption property of BPCSA, the BPCSA 2D PVK dis-
plays superior emission properties upon two-photon excitation with a short
wavelength IR laser. Thus, the AIEE sensitization system for quasi-2D PVK
hybrid system can make a drastic improvement in performance as well as in
the stability of the PVK emitter and PVK based nonlinear optical devices.
luminescence (PL) quantum efficiency,
and defect tolerance,[2] their potential is not
only in the photovoltaics applications but
also as light emitter,[3] lasing media,[4] and
photo detection[5] devices. The absorption
and the emission wavelengths (i.e., energy
gap [Eg] between valence and conduction
band) of 3D PVKs are usually tunable by
substitutions of halide anions, metal cat-
ions, and organic or inorganic cations.[6]
Also, for the quasi-2D PVKs composed
of alternative structures of 2D PVK layers
and 2D interlayers of one-dimensionally
bulky organic cations, an increase of the
number of 2D PVK layers enlarges the Eg
sequentially.[6b,7] Hence, by the composi-
tional and structural versatility of PVKs,
we can achieve full-color PL in the visible
and near-IR range. Typically, long aliphatic
and small aromatic organic cations such
as butylammonium,[1c,d,3c,4b,5a,c,7] octylam-
monium,[8] dodecylammonium,[9] and phe
nylethylammonium[3a,b,d] have been incorporated into quasi-2D
PVKs as an insulator to confine photogenerated excitons in the
2D PVK quantum well. But, occasionally, some organic cat-
ions of extended π-conjugated molecules (e.g., naphthalene,[10]
1. Introduction
2D perovskites (PVKs) have been considered as appropriate
materials to upgrade water resistance and interfacial charge
transfer efficiency of PVK solar cells.[1] However, since they have
oligothiophene,[11] carbazole,[12] pyrene,[13] and polydiacetylene[14]
)
Dr. C.-K. Lim,[+] Prof. P. N. Prasad
Institute for Lasers, Photonics, and Biophotonics
Department of Chemistry
Prof. R. Zalesny
Department of Physical and Quantum Chemistry
Wroclaw University of Science and Technology
Wyb. Wyspian´skiego 27, PL-50370 Wrocław, Poland
University at Buffalo
State University of New York
Buffalo, NY 14260, USA
E-mail: pnprasad@buffalo.edu
Dr. R. Valiev
Department of General and Inorganic Chemistry
National Research Tomsk Polytechnic University
30 Lenin Avenue, Tomsk 634050, Russia
Dr. M. Maldonado, Prof. A. S. L. Gomes
Physics Department
Universidade Federal of Pernambuco
Recife, PE 50670-901, Brazil
Prof. H. Ågren
Department of Theoretical Chemistry and Biology
Royal Institute of Technology
S-10691 Stockholm, Sweden
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201909375.
Dr. J. Jiang, Dr. R. Pachter
Air Force Research Laboratory
Wright-Patterson Air Force Base
Dayton, OH 45433, USA
[+]Present address: Department of Chemical and Materials Engineering,
Nazarbayev University, 53 Kabanbay Batyr Avenue, Nur-Sultan City,
010000, Kazakhstan
DOI: 10.1002/adfm.201909375
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have also been explored to improve the optical and conducting
features of PVKs. Considering the superior absorption coeffi-
cient per mass for π-conjugated molecules, they have a strong
potential as sensitizers to reinforce optical and optoelectronic
properties of PVKs. However, concentration quenching of their
PL could be a significant hurdle to realize an efficient sensitiza-
tion process in 2D PVKs.
2. Results and Discussion
The precursor of the AIEE LP cation, (Z)-2-([1,1′-biphenyl]-
4-yl)-3-(4-(3-aminopropoxy)phenyl)acrylonitrile (BPCSA, see
Figure 1a,b), was designed and synthesized. Among well-
known AIEE LP frameworks such as tetraphenylethene,[16]
cyanostilbene,[17] bis(styryl)anthracene,[18] and hydroxyphenylb-
enzoxazole,[19] we selected a rod-shaped cyanostilbene deriva-
tive that has only 1D bulkiness and has a diameter well fitted
to the 2D PVK lattice size, to facilitate the formation of a hybrid
2D PVK assembly. The propyl group at the top of the BPCSA
structure provides a degree of freedom for energy minimiza-
tion during cocrystallization between the 2D PVK compositions
and the LPs, and allows the terminal amine group, the pre-
cursor of the ammonium cation, to interact with the 2D PVK
layers. BPCSA was synthesized through Williamson etherifica-
tion, Knoevenagel condensation, and acid-catalyzed deprotec-
tion of the protected amine group, consecutively (Figure 1a).
As shown in Figure 1b, the synthesized BPCSA displayed a
strong sky-blue emission in the solid state upon 365 nm UV
illumination, while the solution of BPCSA in dimethyl sul-
foxide (DMSO) was almost nonfluorescent, thus displaying the
AIEE characteristic. The PL spectrum of the BPCSA powder
(λmax = 470 nm) was slightly red-shifted from that of its water
dispersion (λmax = 450 nm), which can be related to a degree of
J aggregation that induces band gap narrowing by a head-to-
tail dipole alignment. The absorption spectrum of the BPCSA
solution and the PL excitation spectra of its dispersion and its
powder form showed a further red-shift of their maximum
Here, we propose and demonstrate an efficient interlayer sen-
sitization system in quasi-2D hybrid PVK consisting of aggre-
gation-induced enhanced emission (AIEE) luminophores (LPs)
and multi-layered 2D PVK alternatively. AIEE LPs are a novel
class of LPs showing enhanced PL properties with an increase
in the concentration of LPs.[15] Due to the immunity of AIEE
LPs to concentration quenching observed in conventional LPs,
the lifetime of Frenkel excitons on the AIEE organic cation layer
can be prolonged, which can promote energy transfer to subse-
quently generate Wannier excitons in PVK layers. In the present
work, an organic cation of AIEE LP was successfully designed,
synthesized, and integrated into quasi-2D PVKs to demonstrate
efficient photosensitization. The AIEE 2D PVKs were synthe-
sized as nanoparticles as well as thin films with a variation of
the number of 2D PVK layers to tune the Eg of PVKs. They
showed superior PL intensities under single- and two-photon
excitations compared to those of nonsensitizing 2D PVKs with
insulating octylammonium cations with the same trends of
bandgap adjustments by stoichiometric control. Furthermore,
the films of AIEE 2D PVKs are incredibly stable in ambient
environment for more than several months and even against
direct contact with a drop of water on the sample surface.
(a)
1
2
BPCSA
BPCSA
Solution in DMSO
Abs.
(c)
(b)
Dispersion in water
Ex.
Em.
Powder
Ex.
Em.
300
400
500
600
Under 365 nm
Wavelength (nm)
Figure 1. a) Synthetic route of AIEE LP, BPCSA. b) PL image of BPCSA powder and solution in DMSO upon 365 nm illumination. c) Normalized
absorption, PL, and PL excitation spectra of BPCSA solution in DMSO, dispersion in water, and powder.
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wavelength from 350 to 380 nm, and then to 420 nm, respec-
tively. The appearance of a longer-wavelength PL excitation
band in the aggregation state (the dispersion and powder) of
BPCSA suggests that close stacking of the planarized LPs is
potentially accompanied with fluorescence-enhancing J aggre-
gation, often occurring in α-cyanostilbenes. Furthermore, the
degree of J aggregation in the powder is higher than that in the
water dispersion.[17a] With this AIEE LP (BPCSA), we explored
the interlayer-sensitized PL enhancement of quasi-2D PVKs.
The quasi-2D PVKs were synthesized under stoichiometric
control of Pb2+ and a monovalent cation iodide for 2D PVK
layers (methylammonium [MA] or formamidinium [FA] or
cesium [Cs+], and the BPCSA cation [BPCSA+] iodide) to tune
the layer structures (Figure 2a). As conceptually shown in
Figure 2b, BPCSA in this hybrid 2D PVKs transfers its excita-
tion energy after absorption of light to the 2D PVK layers that
exhibit the typical PVK PL. On this hypothesis, the PL should
have the PVK emission characteristics but predominantly the
BPCSA excitation characteristics. BPCSA 2D PVK nanoparti-
cles were synthesized with MA, FA, and Cs+ as the cations for
the 2D PVK layers, by a simple precipitation method. Briefly,
appropriate amounts of BPCSA, PbI2, MAI/FAI/Cs+, and hydri-
odic acid (HI) for two layers of 2D PVK (n = 2) were mixed
and dissolved in dimethylformamide (DMF) with heating up
to 60 °C, and then small portion of the mixture was injected
into a poor solvent (toluene) for PVK, with vigorous stirring
at room temperature (RT). The resulting suspensions showed
strong red PL at a slightly different emission maximum wave-
length, and a long PL lifetime component (about 200–300 ns)
dependent on the cations (see Figure S1, Supporting Informa-
tion), which are well-matched with common PL characteristics
of lead iodide PVKs. Meanwhile, their PL properties are far
different from the PL wavelength of 450 nm (see Figure 1c)
and lifetime of 57 ns (see Figure S2, Supporting Information)
for the BPCSA aggregates, which suggests that the 2D PVK
layers are the main PL centers in these hybrid BPCSA 2D
PVKs. Among these hybrid 2D PVKs, MA-incorporated PVK
was selected for further studies because of its relatively higher
stability (Figure S1, Supporting Information). For comparison,
a conventional quasi-2D PVK (n = 2) was synthesized by the
same precipitation method but using now a non-π-conjugated
octylammonium (OA) as a reference (Figure 2c). The OA 2D
PVK displayed further red-shift (λmax = 705 nm) and 2.5-fold
weaker PL than those of BPCSA 2D PVK (Figure 2d). Notice-
ably, the excitation spectrum of BPCSA 2D PVK almost entirely
overlapped with that of the BPCSA powder, while the excitation
spectrum of OA 2D PVK showed a pronounced peak at 605 nm
attributable to layered 2D PVK absorption (Figure 2e). These
results establish that the photoexcitation of BPCSA dominantly
contributes to the PL of BPCSA 2D PVK, which proves efficient
AIEE sensitization for the quasi-2D PVK. For further study,
BPCSA 2D PVK with three of 2D PVK layers (n = 3) was also
synthesized, and its photoinduced excitation/emission proper-
ties were studied. As shown in Figure 2f, with an increase of
the number of 2D PVK layers, the PL red-shifted in the same
manner as for typical quasi-2D PVKs; however, the primary
excitation band still perfectly overlapped with that of BPCSA
2D PVK (n = 2) and with the BPCSA powder, which again veri-
fied the existence of AIEE-sensitized PL in this hybrid 2D PVK.
The transmission electron microscopy (TEM) image of BPCSA
2D PVK (n = 2) mostly showed spherical nanoparticles with a
50–150 nm diameter (Figure 2g).
Since the nanosuspensions could not provide long-term sta-
bility for further studies (based on PL intensity, the half-life is
Figure 2. a) Schematic representation of BPCSA 2D PVK. Energy diagram and estimated photoinduced excitation/emission property in b) BPCSA and
c) OA 2D PVKs. d) PL spectra of BPCSA 2D PVK (n = 2, (BPCSA)2MAPb2I7) and OA 2D PVK (n = 2, (OA)2MAPb2I7) nanosuspensions. e) PL excita-
tion spectra of BPCSA 2D PVK (n = 2, (BPCSA)2MAPb2I7) nanosuspension, OA 2D PVK (n = 2, (OA)2MAPb2I7) nanosuspension, and BPCSA powder.
f) Normalized PL and PL excitation spectra of BPCSA 2D PVKs nanosuspension with two different numbers of 2D PVK layers (n = 2 and 3). g) TEM
image of BPCSA 2D PVK (n = 2) nanoparticles. Inset shows picture of PL of their nanosuspension upon 365 nm excitation.
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about 20 h), we decided to prepare thin films of BPCSA 2D
PVKs and OA 2D PVKs. The films were prepared on indium
tin oxide (ITO)-deposited glass substrates by spin-coating of the
precursor solution including lead iodide, MA iodide, BPCSA
(or OA), and HI (at various stoichiometric ratio for quasi-2D
PVK with n = 2, 3, 4 2D PVK layers). The chemical formula
is, thus, BPCSA(or OA)2MAn-1PbnI3n+1). Followed by thermal
annealing at 100 °C for 10 min, we obtained brown trans-
parent films (Figure 3a). Upon 365 nm illumination, both of
the BPCSA/OA 2D PVKs films displayed decreased red PL with
an increase in the number of 2D PVK layers (Figure 3a). But
the increased number of 2D PVK layers induced a significant
red-shift of the PL toward invisible near-IR region (Figure 3b–d;
Figure S3, Supporting Information). At the same time, for both
of the BPCSA and OA 2D PVK films, the longer edge of the
absorption spectra also shifted to a longer wavelength with the
increase of the number of 2D PVK layers (Figure S3, Supporting
Information). Again, this spectral red-shift with the increase of
the 2D layers is well accorded with the typical quasi-2D PVK
characteristic. However, the PL intensity of BPCSA 2D PVKs
decreased with an increase of the number of 2D PVK layers,
while that of OA 2D PVKs clearly showed an opposite trend
(Figure 3b,c). These trends strongly support that the AIEE LPs
efficiently sensitize the PL of their quasi-2D PVKs by strong
light absorbance. In BPCSA 2D PVKs, the quantitative ratio of
BPCSA in the PVK decreases with an increase of the number
of 2D PVK layers; thus, the PL intensities are proportional to
amounts of the AIEE LPs (BPCSAs). Meanwhile, in OA 2D
(a)
(e)
BPCSA 2D PVK
4x105
n=2
n=2
IBP/IOA=9.5
(d)
n=3
n=4
3x105
(b)
2x105
1x105
OA 2D PVK
BPCSA 2D PVK
n=3
IBP/IOA=4.2
0
8.0x104
OA 2D PVK
n=2
n=3
6.0x104
n=4
n=4
4.0x104
IBP/IOA=3.0
(c)
2.0x104
0.0
500
600
700
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 3. a) Photograph of BPCSA 2D PVKs (n = 2, 3, 4) and OA 2D PVKs (n = 2, 3, 4) films on ITO glass substrates under room light and 365 nm UV
light. PL spectra of b) BPCSA 2D PVKs films and c) OA 2D PVKs films. d) Direct comparison of PL spectra between BPCSA 2D PVKs and OA 2D PVKs
at the same 2D PVK layers. e) PL images of BPCSA 2D PVK (n = 2) film before adding water, with water, and after removal of water.
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PVKs, the PL intensity of PVKs is proportional to the number
of 2D layers. These results represent that light absorption in
the BPCSA layer in BPCSA 2D PVKs, but that of the 2D PVK
layer in OA 2D PVKs, are the primary contributors to the PL
intensity of these quasi-2D hybrid PVKs (Figure S4, Supporting
Information). As a result of the sensitized and nonsensitized
system, the difference of the PL intensity between BPCSA 2D
PVKs and OA 2D PVKs increased up to about 10-fold with a
decrease of the 2D PVK layers.
The stability issue in PVK materials, mainly attributable
to the high water solubility of their composition, is a signifi-
cant problem in PVK research, and the quasi-2D PVKs with a
hydrophobic organic cation layer is a leading solution for this
issue.[1] In this regard, the BPCSA layer in our study is a poten-
tial candidate for the hydrophobic layer. Although the stability
of the BPCSA 2D PVK nanosuspensions was not satisfactory,
the films are incredibly stable in ambient environment (average
temperature: 25 °C, average relative humidity: 50%) for more
than 4 months, without any inert gas blowing or encapsulation.
We anticipate, in the nanosuspension, a solvated or suspended
environment can have a dynamic character that can cause
energy minimizing disassemble of the crystal lattice, while in a
rigid environment of the film, the compositions are locked up
after annealing. In addition, hydrophobic BPCSA substantially
blocks penetration of humidity into the PVKs. To prove water
resistance of the films, we added a water drop on the film.
As shown in Figure 3e, we could not observe any significant
change in the PL properties. Thus, we suggest that our AIEE
LP, BPCSA, can sensitize as well as protect the quasi-2D PVKs
from the moisture, when it is incorporated into the PVK as a
bulky OAs layer.
We note that our results are consistent with density func-
tional theory (DFT) calculations,[20] where, for example, the
excitation energy for the BPCSA molecule was calculated as
3.47 eV (see computational details in ref. [19]). This agrees
well with the measured absorption peak energy of 3.54 eV for
BPCSA in solution. In addition, the band alignment for the
quasi-2D PVK, namely of BPCSA2MAPb2I7, was calculated as
type I, consistent with no experimental evidence of a charge
transfer exciton that would demonstrate type II band alignment
in which the valence and the conduction bands in the inorganic
layer are lower (or higher) than the HOMO and the LUMO
levels in the organic cation, respectively. In the case of type I
band alignment, the electrons and the holes are confined in
either the organic or the inorganic parts. The calculated absorp-
tion spectrum for BPCSA2MAPb2I7 can be considered qualita-
tively consistent with the experimental PL excitation spectrum,
and a discrepancy of 0.5 eV in the prediction of the wavelength
can be attributed, in part, to the level of theory applied to the
large material system.
For the applications of BPCSA 2D PVKs to nonlinear optical
devices, we explored third-order nonlinear optical properties
of their suspension and films. When a short-wavelength IR
(1 µm) laser was focused into the nanosuspension of BPCSA
2D PVK (n = 2) and OA 2D PVK (n = 2), interestingly, we
observed about a 10-fold stronger PL from the BPCSA 2D PVK
than that from OA 2D PVK (Figure 4a). This further promotion
of two-photon-excited PL intensity in BPCSA 2D PVK is attrib-
utable to the superior two-photon absorption (TPA) capability
of π-conjugated organic materials to the TPA of inorganic
materials of the same mass. The nonlinearity of TPA results
in strong enhancement of the PL intensity in BPCSA 2D PVK
upon two-photon excitation than the enhancement upon single-
photon excitation (Figure 2c). To verify the nonlinear optical
character of the PL intensity, we measured the PL spectra under
1 µm laser illumination as a function of the excitation laser
power. A two-photon process should show a quadratic depend-
ence on the excitation intensity. As shown in Figure 4b, up to
1 mW cm–2 pump power, no significant PL was observed from
the BPCSA 2D PVK (n = 2) nanosuspension. However, for illu-
mination intensities of 2 mW cm–2 or higher, the PL intensity
increased nonlinearly with an increase of excitation laser power.
When plotted on a log–log scale, the power dependence fits
well by a line with slope 1.96 (Figure 4b), consistent with a two-
photon process.[21]
To explore the third-order nonlinear optical property of the
BPCSA 2D PVK thin film, we measured open- and closed-
aperture Z-scans of the film using an 800 nm femtosecond
laser with 100 fs pulses at 1 kHz with intensities varying from
50 to 300 GW cm–2 to obtain TPA and nonlinear refraction
coefficient, respectively. We determined the thickness by cross-
sectional SEM imaging before the Z-scan. In the SEM image,
we observed an evenly coated 147 nm thick BPCSA 2D PVK
film (see Figure 4C). We regarded the film thickness as 150 nm
and measured the Z-scan. The result yielded a TPA curve
(Figure 4d) with the TPA coefficient, β = 11.25 2.0 cm GW–1,
and a nonlinear Kerr coefficient with a positive (self-focusing)
sign (see Figure 4e) and n2 = 1.2 0.3 × 10−13 cm2 W–1. The
values calculated using the equations described in reference
articles[22] are constant in this intensity regime and arise only
from contributions of the imaginary and the real part of the
third-order nonlinear coefficient χ(3), respectively. The value of
n2 was corrected for the nonlinearity of the substrate, following
a reference work by Zheng et al.,[23] since there was no non-
linear absorption in the substrate, at these intensities, no cor-
rection was required for the β value.
To shed light on the electronic structure and TPA of the iso-
lated (i.e., nonaggregated) BPCSA precursor and its cation, we
performed DFT calculations using the CAM-B3LYP functional
and the cc-pVDZ basis set. As demonstrated by the results
shown in Table S1, Supporting Information (see also Figure S5,
Supporting Information, for a clear comparison between one-
and TPA spectra of both species), both BPCSA and BPCSA+
exhibit a strong one-photon transition to the first excited state
and much weaker transitions to higher electronic excited
states. However, the orbital character of the first allowed tran-
sition is different for the two species, that is, the S0→S1 elec-
tronic excitation is either dominated by the HOMO→LUMO
(BPCSA) or HOMO→LUMO+1 (BPCSA+) one-electron orbital
transition. There are also remarkable differences in the cor-
responding two-photon transition strengths of BPCSA and
BPCSA+. Namely, the two-photon S0→S1 transition strength
of BPCSA+ is much larger than the corresponding value for
BPCSA. It is, thus, beneficial to use the BPCSA cation in the
active organic layer, as it has much larger two-photon activity
than its neutral precursor. The estimated two-photon cross sec-
tion for the S0→S1 electronic transition of BPCSA+ is roughly
40 GM, or even higher considering that the CAM-B3LYP
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Figure 4. a) PL spectra of BPCSA 2D PVK (n = 2, (BPCSA)2MAPb2I7) and OA 2D PVK (n = 2, (OA)2MAPb2I7) nanosuspensions under 1 µm laser
illumination. b) PL spectra of BPCSA 2D PVK nanosuspension upon 1 µm laser illumination at various power and log–log plot of excitation power of
1 µm laser versus PL intensity of the BPCSA 2D PVK (n = 2) nanosuspension. c) Cross-sectional SEM image of BPCSA 2D PVK-coated ITO glass.
Gold and platinum were sputtered on the 2D PVK film. d) The result from the open-aperture Z-scan measurement of BPCSA 2D PVK (n = 2) thin film
(150 nm thick) at ≈193GW cm–1. e) Closed-aperture measurement for the 2D PVK film as in panel (d), with an intensity of ≈197GW cm–2.
functional systematically underestimates this property for
organic chromophores. One can anticipate that after TPA to the
higher lying electronic states, the molecular system will expe-
rience an ultrafast internal conversion decay to the S1 level.
The lifetime of this level is very consequential for excited state
energy injection into to the PVK part of the hybrid system, and
we have, therefore, estimated its lifetime for free BPSCA using
newly developed algorithms, namely as the inverted sum of the
radiative decay rate (kr), the nonradiative (internal conversion)
decay rate (kIC) and the intersystem singlet–triplet transfer rate
(kISC). The radiative rate constant (kr) was estimated using the
Strickler–Berg equation, which includes the oscillator strength
and the de-excitation energy; the energy to the ground state
internal conversion (kIC) and intersystem crossing (kISC) rate
constants were calculated in the Herzberg–Teller approxima-
tion using a formulation accounting for all vibrational degrees
of freedom, including the calculated first- and second-order
non-adiabatic coupling elements (for kIC) and spin–orbit cou-
pling and nuclear derivatives of the spin–orbit coupling ele-
ments (for kISC). The basic electronic structure theories were
in this case time-dependent DFT and variational-perturbation
theory (CASSCF and XMC-QDPT2 methods). An account of
these methods as well as of detailed results for BPSCA and four
related organic molecules and their cations will be given in a
separate report. These rates are highly dependent on the pre-
cise energetics of the ground, excited singlet, and triplet states
and can only be associated with qualitative significance—the
prediction is 3.0 × 108, 1.0, and 10.0 for the kr, kIC, and kISC
constants, respectively. This gives an estimate of the S1 level
lifetime as long as 3 ns, thus into the nanoscale range, in fact
not far from the expected natural decay rate of the compound.
The overwhelming dominance of the radiative rate is a conse-
quence of strong one-photon emission oscillator strength for
the S1 level, of the stiff skeleton of the BPCSA molecule and
a relatively large HOMO-LUMO gap (small kIC constant). A
large singlet–triplet gap (small kISC) together with a significant
two-photon excitation cross section makes BPSCA very well
designed for effective IR energy injection to the PVK system.
3. Conclusion
In summary, we have introduced the concept of interlayer
sensitized PL of a quasi-2D hybrid PVK containing an AIEE
organic cation layer. A rod-shaped AIEE organic cation, well
fitted into the lattice of 2D PVK layers, was designed and syn-
thesized to maximize the energy-sensitizing property. The
produced cation precursor, BPCSA, emitted strong fluores-
cence in the solid state, while its solution was almost nonflu-
orescent, which is representative of the AIEE characteristic.
When BPCSA was incorporated into quasi-2D PVKs as a bulky
organic layer in between 2D PVK layers, it enhanced the PL of
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6.995 (d, 2H, J = 11 Hz), 4.752 (br, 1H), 4.108 (t, 2H, J = 6 Hz), 3.342 (q,
2H, J = 6.5 Hz), 2.023 (t, 2H, J = 6.5 Hz) 1.440 (s, 9H).
PVK up to 10-folds compared to that of a non-π-conjugated OA
containing 2D PVK. The PL excitation wavelength of BPCSA
2D PVKs well-matched with that of the BPCSA solid, but the
PL was of typical quasi-2D PVKs, establishing sensitization by
BPCSA to produce strong PL from 2D PVKs. Notably, the PL
intensity of BPCSA 2D PVKs increased with a decrease of the
number of 2D PVK layers, while that of OA 2D PVKs showed
exactly an opposite trend, indicating the critical contribution of
the amount of BPCSA for generating PL. The films of BPCSA
2D PVKs are incredibly stable in ambient environments for
more than 4 months and even upon direct contact of water.
Furthermore, the BPCSA 2D PVK displayed superior emission
properties upon two-photon excitation by short-wavelength IR
laser. Therefore, the AIEE sensitization system for quasi-2D
PVK can reinforce the PL properties upon linear/nonlinear
optical excitations for PVK as an emitter and for PVK based
nonlinear optical devices.
Synthesis of tert-butyl(Z)-(3-(4-(2-([1,1′-biphenyl]-4-yl)-2-cyanovinyl)
phenoxy)propyl)carbamate (2) (Figure 1a):
1 (1.5 g, 5.4 mmol),
4-biphenylacetonitrile (1.24 g, 6.4 mmol), and tert-butyl alcohol (90 mL)
were added to a 150 mL round-bottomed flask and then were heated
to 60 °C with stirring. Tetrabutylammonium hydroxide (1 M solution
in methanol, 1 mL) was added dropwise into the reaction mixture and
the mixture was stirred at 60 °C overnight. The resulting precipitate
was filtered and washed with tert-butyl alcohol. A pale-yellow crystals
of 2 (69% yield) were obtained by recrystallization from hot methanol
1
solution. H NMR (500 MHz, DMSO-d6), δ 8.021 (s, 1H), 7.961 (d, 2H,
J = 8.5 Hz), 7.813 (s, 4H), 7.731 (d, 2H, J = 7.5 Hz), 7.488 (t, 2H, J =
7.5 Hz), 7.392 (t, 1H, J = 7.5 Hz), 7.094 (d, 2H, J = 8 Hz), 6.911 (br,
1H), 4.067 (t, 2H, J = 6.5 Hz), 3.088 (q, 2H, J = 6.5 Hz), 1.850 (t, 2H, J =
6 Hz), 1.370 (s, 9H).
Synthesis of (Z)-2-([1,1′-biphenyl]-4-yl)-3-(4-(3-aminopropoxy)phenyl)
acrylonitrile (BPCSA) (Figure 1a): 2 (1.68 g, 3.7 mmol) and 20 mL
dichloromethane were added to a 100 mL round-bottomed flask and then
10 mL trifluoroacetic acid was added dropwise into the reaction mixture
with stirring at RT. The mixture was stirred at RT for 2 h and the solvent
was evaporated. A saturated aqueous solution of sodium bicarbonate
was slowly added to the resulting mixture to form precipitation of the
products. The precipitates were filtered and washed with water. A pale-
yellow powder of BPCSA (85% yield) was obtained by recrystallization
from hot methanol solution. 1H NMR (500 MHz, DMSO-d6), δ 8.025 (s,
1H), 7.962 (d, 2H, J = 8.5 Hz), 7.814 (s, 4H), 7.731 (d, 2H, J = 7.5 Hz),
7.489 (t, 2H, J = 7.5 Hz), 7.394 (t, 1H, J = 7.5 Hz), 7.111 (d, 2H, J =
8.5 Hz), 4.135 (t, 2H, J = 6.5 Hz), 3.649 (br, 2H), 2.768 (t, 2H, J =
6.5 Hz), 1.859 (qui, 2H, J = 6.5 Hz). 13C NMR (75 MHz, DMSO-d6), δ
160.271, 142.200, 139.35, 139.011, 133.135, 133.181, 129.044, 127.900,
127.304, 126.633, 126.434, 126.022, 118.361, 144.988, 106.731, 65.019,
36.631, 27.687.
Synthesis of Quasi-2D PVK Nanosuspension: In a general procedure,
0.01 mmol PbI2, x mmol MAI (or FAI or CsI), y mmol BPCSA (or OA), z
mmol HI, and 200 µL DMF were added into a vial and all the chemicals
were dissolved with heating (up to 80 °C) and stirring. For n = 2 (or 3),
the x, y, and z were respectively 0.005 (or 0.0066), 0.007 (or 0.0033), and
0.008 (or 0.004). After 20 min stirring at 80 °C, 10 µL of the mixture was
injected into 5 mL toluene at RT with vigorous stirring, and then the
stirring was stopped immediately to obtain nanosuspension of BPCSA
(or OA) 2D PVKs.
Preparation of Quasi-2D PVK Films: The films were fabricated on
the prepatterned ITO substrates, which were cleaned sequentially with
the Hellmanex III solution, acetone, methanol, and isopropyl alcohol,
with 10 min ultrasonication in each solvent. To prepare PVK precursor
solutions, 0.05 mmol PbI2, x mmol MAI, y mmol BPCSA (or OA), z mmol
HI, and 200 µL DMF were added into a vial and dissolved with heating
(up to 80 °C) and stirring. The x, y, and z were 0.027, 0.033, and 0.04
for n = 2; 0.038, 0.013, and 0.015 for n = 3; and 0.04, 0.01, and 0.012 for
n = 4, respectively. After 30 min stirring at 80 °C, the PVK precursor
solution was spin-coated at 3000 rpm for 30 s and then annealed at
100 °C for 10 min to remove residual solvent and to crystallize the film.
Electronic-Structure Calculations: The geometries of BPCSA and
BPCSA+ were optimized based on DFT, employing the B3LYP functional
and the cc-pVDZ basis set. In doing so, we neglected environmental
effects, that is, we considered molecules in vacuo. The minima on
potential energy hypersurface were confirmed by evaluation of Hessian.
Subsequently, the minimum-energy geometries were used for electronic-
structure calculations using the CAM-B3LYP functional and cc-pVDZ
basis set. All calculations were performed using GAMESS US program.
4. Experimental Section
Material: 4-Hydroxybenzaldehyde (98%), 3-(Boc-amino)propyl
bromide (≥96%), 4-biphenylacetonitrile (97%), trifluoroacetic acid
(CF3COOH, 99%), tetrabutylammonium hydroxide solution (1.0 M in
methanol), potassium carbonate (anhydrous, ACS reagent, ≥99%),
sodium bicarbonate (ACS reagent, ≥99.7%), lead(II) iodide (99.999%,
PVK grade), MA iodide (MAI, anhydrous, ≥99%), FA iodide (FAI,
anhydrous, ≥99%), cesium iodide (CsI, 99.999%), octylamine (OA,
99%), HI (57 wt% in H2O, stabilized, 99.95%), acetone (≥99.9%), tert-
butanol (≥99.5%), dichloromethane (CH2Cl2, anhydrous, ≥99.8%), DMF
(anhydrous, 99.8%), and toluene (anhydrous, 99.8%) were obtained
from Sigma-Aldrich. Acetone (certified ACS), hexane (ACS reagent grade,
≥98.5%), hexane (95%, HPLC grade), ethyl acetate (certified ACS), and
methanol (certified ACS) were purchased from Fisher Scientific. All
materials were used as received, without further purifications.
Instruments: 1H NMR spectra were recorded on a Varian Inova-500
500 MHz spectrometer and referenced to tetramethylsilane as the
internal standard, and 13C NMR spectra were recorded on a Varian
Mercury-300 75 MHz spectrometer. The following abbreviations are
used to explain the multiplicities: s = singlet, d = doublet, t = triplet,
q = quartet, qui = quintet, and br = broad. The steady-state UV–vis
absorption spectra were measured by Shimadzu UV-3600 Plus UV–
vis-near NIR spectrophotometer. The steady-state PL spectra were
measured using the HORIBA Scientific Fluorolog-3 spectrofluorometer
equipped with xenon lamp. The lifetime was measured using HORIBA
Scientific EasyLife X lifetime fluorescence spectrometer equipped with a
370 nm LED for excitation and 480 nm long-pass filter to minimize IRF.
SEM images were taken using a Carl Zeiss AURIGA CrossBeam system
at 2 kV accelerating voltage. The cross-sectional images were obtained
by milling using the focused ion beam in the same microscope. The
tilt angle was set at 54° and corrected by 36° when taking images. TEM
images were obtained by using a JEOL-2100 microscope operating at
200 kV.
Synthesis of tert-butyl(3-(4-formylphenoxy)propyl)carbamate (1) (Figure 1a):
4-Hydroxybenzaldehyde (0.855 g,
7 mmol), 3-(Boc-amino)propyl
bromide (2 g, 8.4 mmol), potassium carbonate (1.94 g, 14 mmol),
and acetone (50 mL) were added to a 100 mL round-bottomed flask and
then refluxed for 18 h. Then the reaction mixture was cooled to RT and
diluted with 50 mL water to form precipitation. The precipitates were
filtered and washed with water, and then dissolved in dichloromethane
and dried with sodium sulfate. After removing the sodium sulfate
by filtration, the solution of the product was condensed by solvent
evaporation and then purified with silica gel column chromatography
(ethyl acetate/hexane = 1:2) to obtain 1 as a colorless liquid (82% yield).
1H NMR (500 MHz, CDCl3), δ 9.885 (s, 1H), 7.831 (d, 2H, J = 11 Hz),
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
©
Adv. Funct. Mater. 2020, 1909375
1909375 (7 of 8)
2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Acknowledgements
The authors acknowledge the support from the Air Force Office of
Scientific Research (no. FA9550-18-1-0042 and FA9550-18-1-0032).
R.Z. thanks National Science Centre (Poland) for financial support
(grant no. 2018/30/E/ST4/00457).
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Conflict of Interest
The authors declare no conflict of interest.
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Keywords
aggregation-induced emission, energy transfer, nonlinear optics,
photoluminescence, quasi-2D perovskites
Received: November 10, 2019
Revised: February 8, 2020
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©
Adv. Funct. Mater. 2020, 1909375
1909375 (8 of 8)
2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim