Light harvesting and amplification of emission of donor perylene-acceptor perylene aggregates in aqueous medium

Article abstract of DOI:10.1002/chem.201304431

With the aid of rational design, we have synthesized a pair of water-soluble donor- and acceptor-type twisted perylene bisimide units, which together form aggregates upon lowering the pH of the medium, providing bright yellow fluorescence. The light-harvesting efficiency of the co-assembled system can be tuned by controlling the ratio of donor to acceptor and 98.1 % efficiency has been achieved. Light harvesting: Through the use of rational design, a pair of water-soluble donor- and acceptor-type twisted perylene bisimide units have been synthesized. Together, these form aggregates when the pH of the medium is lowered, emitting bright yellow fluorescence. The light-harvesting efficiency of the co-assembled system can be tuned by controlling the ratio of donor to acceptor and over 98 % efficiency has been achieved.

Full text of DOI:10.1002/chem.201304431

DOI: 10.1002/chem.201304431
Communication
&
Light Harvesting
Light Harvesting and Amplification of Emission of Donor
Perylene–Acceptor Perylene Aggregates in Aqueous Medium
Pradip K. Sukul,[a] Ayan Datta,[b] and Sudip Malik*[a]
ceived tremendous attention as functional supramolecular ma-
Abstract: With the aid of rational design, we have synthe-
terials.[12]
sized a pair of water-soluble donor- and acceptor-type
A few strategies have been undertaken to design light-har-
twisted perylene bisimide units, which together form ag-
vesting materials using perylene moieties as the acceptor unit
gregates upon lowering the pH of the medium, providing
and another moiety as the donor unit (for example, pyrene,[13]
bright yellow fluorescence. The light-harvesting efficiency
phthalocyanine,[14] porphyrin,[15] coumarin,[16] coronene,[17] etc.).
of the co-assembled system can be tuned by controlling
Most of these efforts were done in organic media because of
the ratio of donor to acceptor and 98.1% efficiency has
the hydrophobic nature of these derivatives.[13–16,18,19] In this
been achieved.
study, we demonstrate the dual nature of perylene moieties in
light-harvesting aggregates in aqueous medium. We have ra-
tionally designed a pair of water-soluble twisted perylene de-
Excitation energy transfer plays a crucial role in natural phe-
nomena such as photosynthesis.[1] In the natural photosynthe-
tic unit in plants, carotenoids, acting as antennas, absorb solar
radiation in the spectral region and transfer the resulting exci-
tation energy to chlorophyll through singlet–singlet energy
transfer.[2] The excitation energy then migrates from one chlo-
rophyll molecule to another, unidirectionally in an array, and
ultimately reaches the “special pair” in the reaction center
where charge separation occurs. The noncovalent approach
observed in the nature, has been widely established as a strat-
egy to construct artificial antenna moieties such as self-assem-
bled multiporphyrin systems.[3] In recent years, long-range or-
dering in well-defined aggregates of p-conjugated compo-
nents has been considered as a new type of light-harvesting
antenna as well as an energy-transfer system. Various ap-
proaches have been taken, with dendrimers,[4] polymers,[5] or-
ganic–inorganic hybrids,[6] mesoporous silica,[7] and porphyrin
arrays connected by covalent bonds[8] or by noncovalent self-
assembly[9–11] being used to mimic this natural energy-transfer
process.
rivatives (Figure 1a), namely PPy and PCl. Prior to carrying out
Owing to the presence of excellent thermal, chemical, and
photochemical inertness, as well as high fluorescence quantum
yield, perylene bisimide derivatives have been widely investi-
gated as an active component (n-type) in organic-based pho-
tovoltaic devices. Recently, these perylene derivatives have re-
Figure 1. a) Molecular structures of PPy and PCl derivatives. b) Spectral over-
lap of the emission of PPy (gray) and the absorption of PCl (black) in water
at pH 5.00.
[a] P. K. Sukul, Dr. S. Malik
photophysical investigations, the two derivatives of perylene
Polymer Science Unit
1
were characterized by H NMR, MS (MALDI-TOF), and FTIR (see
Indian Association for the Cultivation of Science
2A and 2B Raja S. C. Mullick Rd, Jadavpur, Kolkata-700032 (India)
E-mail: psusm2@iacs.res.in
the Supporting Information)[20] to establish the purity of the
samples. Owing to the presence of four carboxylic acid
groups[21] and the substituents at the bay position, both deriv-
atives are water soluble. Insertion of two pyrolidine units in
PPy at the bay position of the perylene moiety elevates the
electron density of the core, whereas the four chlorine atoms
in PCl reduce the electron density, hinting that these two moi-
eties may act as the donor and the acceptor, respectively,
[b] Dr. A. Datta
Department of Spectroscopy
Indian Association for the Cultivation of Science
2A and 2B Raja S. C. Mullick Rd, Jadavpur, Kolkata-700032 (India)
Supporting information (including full experimental details and compound
characterization) for this article is available on the WWW under http://
dx.doi.org/10.1002/chem.201304431.
Chem. Eur. J. 2014, 20, 3019 – 3022
3019
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
when they come close in space. There is a possibility of photo-
induced Fçrster resonant energy transfer (FRET) from PPy to
PCl derivatives that are noncovalently connected.
The emission spectrum of PPy (donor, 400–550 nm) has
good spectral overlap with the absorption bands of PCl (ac-
ceptor, 400–600 nm) at pH 5.00. The spectral overlap integral
(J(l)) between PPy and PCl is calculated from Equation 1:[22]
owing to low quantum yields. Neither PPy nor PCl show any
emission, whereas PPy*PCl emits bright yellow fluorescence at
pH 5.0, providing direct visual proof of energy transfer from
donor to acceptor. The brightness of the emission depends on
the amount of PCl present in the mixture and it reaches a maxi-
mum at 20 mol% of PCl. UV/Vis titration of the donor ([PPy]=
5ꢁ10ꢀ5 m) upon addition of different mol% of the acceptor
(PCl) reveals the gradual formation of a donor–acceptor band
at 695 nm (see Figure S2 in the Supporting Information).
1
Z
JðlÞ ¼
FDðlÞeAðlÞl4dl
ð1Þ
The feasibility of energy transfer of these mixed chromo-
phores with various compositions at pH 5.00 has been ob-
served at 350 nm (absorption range of PPy). The quenching of
the PPy emission between 400–500 nm is observed, with the
concomitant increase of PCl emission at 525–675 nm. It en-
sures the excitation energy transfer from the donor perylene
0
where l is the wavelength in nm, FD is the normalized fluo-
rescence intensity of the donor (area under the curve), eA is
the extinction coefficient (Mꢀ1 cmꢀ1) of the acceptor at l. The
estimated value of J(l) is 4.33ꢁ1016 mꢀ1 cmꢀ1 nm4. As the spec-
tral properties of these moieties satisfy the requirements of an
energy-transfer system, an efficient FRET between PPy and PCl
is envisaged, especially considering the proper spatial orienta-
tion between the chromophores. Furthermore, the well-sepa-
rated absorption bands of the donor and acceptor molecules
ensure that both PPy and PCl molecules are selectively excited
at 350 and 470 nm in mixed chromophore hybrids, a fact that
will help to analyze the energy-transfer efficiencies.
to acceptor perylene chromophore (Figure 3a), giving
a
Photographs of the aqueous solutions of PPy (green), PCl
(red), and mixed chromophores (PPy*PCl, brown) before and
after irradiation with UV light at 365 nm are presented in Fig-
ure 2a–c and Figure S1 (see the Supporting Information). Both
moieties exhibit weak fluorescence in neutral water (pH 7.4)
Figure 3. a) Fluorescence spectra of PPy in water with addition of different
mol% of PCl. [PPy]=2.5ꢁ10ꢀ5 m, [PCl]=2.5ꢁ10ꢀ5 m, lex =350 nm for PPy
and co-assembled aggregates, pathlength=10.00 mm, pH 5.00. Arrows indi-
cate spectral change upon increasing concentration PCl. b) Quantum-chemi-
cal-optimized structure of PPy*PCl complex in the 4:1 ratio.
unique example where perylene plays a dual role. Significantly,
the Job plot and the plot of emission intensity versus mol% of
PCl added to the system (Figures S5 and S6 in the Supporting
Information) suggest that the ratio of donor to acceptor is 4:1,
a
value that is reminiscent of natural Light Harvesting
Figure 2. Individual and mixed solutions under visible light and UV light: a)
PPy solution, b) PCl solution, c) mixed solution. [PPy]=2.5ꢁ10ꢀ5 m and
[PCl]=2.5ꢁ10ꢀ5 m, and for the mixed system, [PPy]:[PCl]=1:1. d) Solutions
of individual and mixed chromophores with different mol% of PCl under UV
light (lex =365 nm). The mol% of PCl is relative to PPy. For pure solutions,
[PPy]=5ꢁ10ꢀ5 m and [PCl]=5ꢁ10ꢀ5 m. lex =365 nm, path-
1 (PCP).[23,24] The structure of such a 4:1 donor/acceptor (D/A)
system has been modelled by optimizing the geometry of
a composite of four PPy molecules and one PCl molecule by
using the Drieding force field[25] in the Gaussian 09 Suite of
Programs.[26] Indeed, as shown in Figure 3b, four PPy mole-
length=10.00 mm, pH 5.00.
Chem. Eur. J. 2014, 20, 3019 – 3022
www.chemeurj.org
3020
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
cules are seen to be surrounding the PCl molecule, thereby
trapping it within a van der Waals cavity of radius ~12 ꢂ, a sit-
uation that facilitates efficient energy transfer. PCl emission in
the aggregates is attained owing to energy transfer within the
measurement time (<1 min) in the aggregates, suggesting
very fast dynamics of the molecules. The efficiency of FRET de-
pends on the Fçrster radius (R0), at which half of the excitation
energy of the donor is transferred to the acceptor. R0 is calcu-
lated from Equation 2:
1=6
R0 ¼ 0:211ðk2hꢀ4QDJðlÞÞ
ð2Þ
where k is the angular orientational factor between PPy and
PCl dipoles and equals 2/3 (assuming random orientation). h is
the refractive index of the medium. For an aqueous solution, h
should be 1.34. QD is the quantum yield of the donor in the ab-
sence of the acceptor. The Fçrster radius is thus estimated to
7.9 nm.
The fluorescence-lifetime decay curves of PPy*PCl (Figure S7
in the Supporting Information) become gradually steeper with
increasing PCl content, pointing to a gradual decrease of aver-
age lifetime of PPy owing to energy transfer. The efficiency of
energy transfer is calculated from the change in the average
lifetime of the donor (Table S1 in the Supporting Information)
and the highest efficiency is found to be 98.1%. When only
PCl is directly excited at 350 nm, this energy transfer is more
or less absent (Figure S8 in the Supporting Information). Inter-
estingly, if we critically examine PCl emission in the absence or
presence of PPy (Figure S9 in the Supporting Information), the
six-fold increase of emission intensity of PCl in the presence of
PPy reveals the amplification of emission intensity in our light-
harvesting system.
Figure 4. HR-TEM images of the solution dried under air: a) PPy (1ꢁ10ꢀ5 m),
b) PCl (1ꢁ10ꢀ5 m), c) PPy*PCl 1:1 co-assembled aggregates.
state is unprecedented[17] and it shows the remarkable efficien-
cy of novel supramolecular scaffolds for energy transfer.[20] So,
it is clear that owing to the close proximity of the donor and
acceptor molecules in the co-assembled aggregates, the
energy transfer occurs from the excited state of PPy to the
ground state of PCl, a transfer that can clearly be seen in the
solid state also.
In summary, a perylene-based light-harvesting system with
high efficiency in aqueous medium has been described. The
system consists of an array of pyrolidine-substituted perylene
(light collector) and chlorine-substituted perylene (energy col-
lector). The observed amplification of emission in both aque-
ous solution and in the solid state may allow future applica-
tions in the field of responsive supramolecular systems and
photovoltaic devices.
To investigate the effect of pH on the energy transfer of the
system (PPy*PCl 20 mol%), pH titration has been performed
from pH 9.0 to pH 4.0 (Figure S10 in the Supporting Informa-
tion). Emission intensity gradually increased up to pH 5.0 as
lowering the pH induces closer proximity of the donor and ac-
ceptor molecules. Below pH 5.0, a steep decrease of emission
intensity was observed along with the formation of precipitate,
providing the optimum pH of 5.0ꢁ0.1 for efficient energy
transfer. The HR-TEM images are very interesting as they indi-
cate that only pure donor (PPy) or acceptor (PCl) solutions
form micellar aggregates. Conversely, in the mixed state, they
produce one-dimensional fibers upon lowering the pH of the
medium (Figure 4) owing to the strong donor–acceptor inter-
action.
Acknowledgements
P.K.S. is indebted to the Council of Scientific and Industrial Re-
search (CSIR), India, for his fellowship. We thank the unit of
nanoscience (IACS), DST, Govt. of India, and the MALDI-TOF fa-
cility of IACS.
Efficient energy transfer in solid films is also an important re-
quirement for photonic- and electronic-device applications. For
this purpose, the fluorescence has been measured in the film
state made from the solution by drop casting. The film state
showed a similar trend to the solution state (Figure S11 in the
Supporting Information).[20] Particularly, PPy*PCl 2 mol% film
revealed significant quenching of PPy emission and enhanced
fluorescence intensity of PCl owing to energy transfer com-
pared with direct excitation of the acceptor, indicating this is
a very efficient light-harvesting system in the solid state too.
This amplified emission through energy transfer in the film
Keywords: donor–acceptors
·
energy transfer
·
light
harvesting · perylene diimide · self-assembly
[1] a) W. Kꢃhlbrandt, D. N. Wang, Y. Fujiyoshi, Nature 1994, 367, 614–621;
b) G. McDermott, S. M. Prince, A. A. Freer, A. M. Hawthornthwaite-Law-
less, M. Z. Papiz, R. J. Cogdell, N. W. Isaacs, Nature 1995, 374, 517–521.
[2] W. Kꢃhlbrandt, Nature 1995, 374, 497–498.
[3] a) J. Seth, V. Palaniappan, T. E. Johnson, S. Prathapan, J. S. Lindsey, D. F.
Bocian, J. Am. Chem. Soc. 1994, 116, 10578–10592; b) J.-S. Hsiao, B. P.
Krueger, R. W. Wagner, T. E. Johnson, J. K. Delaney, D. C. Mauzerall, G. R.
Chem. Eur. J. 2014, 20, 3019 – 3022
www.chemeurj.org
3021
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Fleming, J. S. Lindsey, D. F. Bocian, R. J. Donohoe, J. Am. Chem. Soc.
1996, 118, 11181–11193; c) A. Nakano, A. Osuka, I. Yamazaki, T. Yamaza-
ki, Y. Nishimura, Angew. Chem. 1998, 110, 3172–3176; Angew. Chem. Int.
Ed. 1998, 37, 3023–3027.
[15] a) R. S. Loewe, K. Tomizaki, W. J. Youngblood, Z. Bo, J. S. Lindsey, J.
Mater. Chem. 2002, 12, 3438–3451; b) G. M. Hasselman, D. F. Watson,
J. R. Stromberg, D. F. Bocian, D. Holten, J. S. Lindsey, G. J. Meyer, J. Phys.
Chem. B 2006, 110, 25430–25440.
[16] J. M. Serin, D. W. Brousmiche, J. M. J. Frꢄchet, J. Am. Chem. Soc. 2002,
124, 11848–11849.
[4] a) C. Devadoss, P. Bharathi, J. S. Moore, J. Am. Chem. Soc. 1996, 118,
9635–9644; b) D.-L. Jiang, T. Aida, Nature 1997, 388, 454–456; c) S. L.
Gilat, A. Adronov, J. M. J. Freꢄchet, Angew. Chem. 1999, 111, 1519–1524;
Angew. Chem. Int. Ed. 1999, 38, 1422–1427; d) A. Adronov, J. M. Frꢄchet,
Chem. Commun. 2000, 1701–1710; e) A. Adronov, S. L. Gilat, J. M. J. Frꢄ-
chet, K. Ohta, F. V. R. Neuwahl, G. R. Fleming, J. Am. Chem. Soc. 2000,
122, 1175–1185.
[17] K. V. Rao, K. K. R. Datta, M. Eswaramoorthy, S. J. George, Angew. Chem.
2011, 123, 1211–1216; Angew. Chem. Int. Ed. 2011, 50, 1179–1184.
[18] a) K. Sugiyasu, N. Fujita, S. Shinkai, Angew. Chem. 2004, 116, 1249–
1253; Angew. Chem. Int. Ed. 2004, 43, 1229–1233; b) E. H. A. Beckers,
S. C. J. Meskers, A. P. H. J. Schenning, Z. Chen, F. Wꢃrthner, P. Marsal, D.
Beljonne, J. Cornil, R. A. J. Janssen, J. Am. Chem. Soc. 2006, 128, 649–
657; c) M. R. Wasielewski, Acc. Chem. Res. 2009, 42, 1910–1921; d) X.
Zhang, S. Rehm, M. M. S. Sempere, F. Wꢃrthner, Nat. Chem. 2009, 1,
623–629; e) F. J. C. Guirao, K. Ohkubo, S. Fukuzumi, A. S. -Santos, F. F.
Lꢅzaro, J. Org. Chem. 2009, 74, 5871–5880; f) M. Supur, Y. Yamada, S.
Fukuzumi, J. Mater. Chem. 2012, 22, 12547–12552; g) Y. Du, L. Jiang, J.
Zhou, G. Qi, X. Li, Y. Yang, Org. Lett. 2012, 14, 3052–3055.
[5] S. E. Webber, Chem. Rev. 1990, 90, 1469–1482.
[6] a) R. Haldar, K. V. Rao, S. J. George, T. K. Maji, Chem. Eur. J. 2012, 18,
5848–5852; b) V. M. Suresh, S. J. George, T. K. Maji, Adv. Funct. Mater.
2013, 23, 5585–5590.
[7] a) N. Mizoshita, Y. Goto, T. Tani, S. Inagaki, Adv. Mater. 2009, 21, 4798–
4801; b) Y. Maegawa, N. Mizoshita, T. Tani, S. Inagaki, J. Mater. Chem.
2010, 20, 4399–4403.
[19] B. Rybtchinski, L. E. Sinks, M. R. Wasielewski, J. Am. Chem. Soc. 2004,
126, 12268–12269.
[20] See the Supporting Information for details.
[21] a) P. K. Sukul, D. Asthana, P. Mukhopadhyay, D. Summa, L. Muccioli, C.
Zannoni, D. Beljonne, A. E. Rowan, S. Malik, Chem. Commun. 2011, 47,
11858–11860; b) P. K. Sukul, P. K. Singh, S. K. Maji, S. Malik, J. Mater.
Chem. B 2013, 1, 153–156.
[22] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/
Plenum Publishers, New York, 1999.
[23] E. Hofmann, P. M. Wrench, F. P. Sharples, R. G. Hiller, W. Welte, K. Dieder-
ichs, Science 1996, 272, 1788–1791.
[24] T. Polꢆvka, H. A. Frank, Acc. Chem. Res. 2010, 43, 1125–1134.
[25] S. L. Mayo, B. D. Olafson, W. A. Goddard III, J. Phys. Chem. 1990, 94,
8897–8909.
[8] J. Li, A. Ambroise, S. I. Yang, J. R. Diers, J. Seth, C. R. Wack, D. F. Bocian,
D. Holten, J. S. Lindsey, J. Am. Chem. Soc. 1999, 121, 8927–8940.
[9] a) R. Takahashi, Y. Kobuke, J. Am. Chem. Soc. 2003, 125, 2372–2373; b) T.
van der Boom, R. T. Hayes, Y. Zhao, P. J. Bushard, E. A. Weiss, M. R. Wasie-
lewski, J. Am. Chem. Soc. 2002, 124, 9582–9590.
[10] a) A. Ajayaghosh, S. J. George, J. Am. Chem. Soc. 2001, 123, 5148–5149;
b) A. Ajayaghosh, S. J. George, V. K. Praveen, Angew. Chem. 2003, 115,
346–349; Angew. Chem. Int. Ed. 2003, 42, 332–335; c) A. Ajayaghosh, C.
Vijayakumar, V. K. Praveen, S. S. Babu, R. Varghese, J. Am. Chem. Soc.
2006, 128, 7174–7175; d) V. K. Praveen, S. J. George, R. Varghese, C. Vi-
jayakumar, A. Ajayaghosh, J. Am. Chem. Soc. 2006, 128, 7542–7550;
e) A. Ajayaghosh, V. K. Praveen, C. Vijayakumar, S. J. George, Angew.
Chem. 2007, 119, 6376–6381; Angew. Chem. Int. Ed. 2007, 46, 6260–
6265.
[26] Gaussian 09, Revision A.1, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Men-
nucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,
A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Ko-
bayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyen-
gar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg,
S. Dapprich, A. D. Daniels, ꢇ. Farkas, J. B. Foresman, J. V. Ortiz, J. Cio-
slowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
[11] a) T. Nakashima, N. Kimizuka, Adv. Mater. 2002, 14, 1113–1116; b) D. W.
Brousmiche, J. M. Serin, J. M. J. Frꢄchet, G. S. He, T.-C. Lin, S. J. Chung,
P. N. Prasad, J. Am. Chem. Soc. 2003, 125, 1448–1449; c) F. J. M. Hoeben,
L. M. Herz, C. Daniel, P. Jonkheijm, A. P. H. J. Schenning, C. Silva, S. C. J.
Meskers, D. Beljonne, R. T. Phillips, R. H. Friend, E. W. Meijer, Angew.
Chem. 2004, 116, 2010–2013; Angew. Chem. Int. Ed. 2004, 43, 1976–
1979; d) L. Chen, S. Revel, K. Morris, D. J. Adams, Chem. Commun. 2010,
46, 4267–4269; e) Y. Ren, W. H. Kan, V. Thangadurai, T. Baumgartner,
Angew. Chem. 2012, 124, 4031–4035; Angew. Chem. Int. Ed. 2012, 51,
3964–3968.
[12] a) F. Wꢃrthner, Chem. Commun. 2004, 1564; b) T. Seki, X. Lin, S. Yagai,
Asian J. Org. Chem. 2013, 2, 708–724.
[13] A. Sautter, B. K. Kaletas¸, D. G. Schmid, R. Dobrawa, M. Zimine, G. Jung,
I. H. M. v. Stokkum, L. D. Cola, R. M. Williams, F. Wꢃrthner, J. Am. Chem.
Soc. 2005, 127, 6719–6729.
[14] X. Li, L. E. Sinks, B. Rybtchinski, M. R. Wasielewski, J. Am. Chem. Soc.
2004, 126, 10810–10811.
Received: November 12, 2013
Published online on February 23, 2014
Chem. Eur. J. 2014, 20, 3019 – 3022
www.chemeurj.org
3022
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim