Alkyl substituent effects on J- or H-aggregate formation of bisazomethine dyes

Article abstract of DOI:10.1016/j.dyepig.2011.05.024

Bisazomethine dyes with terminal alkyl substituents of different chain lengths (BAR: R = 1, 2, 3, 4, 5 and 6) were synthesized and deposited on a glass substrate to investigate the effect of the alkyl chain length on aggregate formation. Methyl- and ethyl-substituted bisazomethine dyes (BA1 and BA2) formed J-aggregates in thin films (ca. 50 nm), whereas, propyl-, butyl-, pentyl- and hexyl-substituted derivatives (BA3, BA4, BA5 and BA6) formed H-aggregates in thin films (ca. 50 nm). The aggregate formation of the BARs changed drastically between ethyl- and propyl-substituents (BA2/BA3). However, no remarkable changes were observed in the surface morphologies of BA2 and BA3 films. It is suggested that the critical determinant of aggregate formation of BAR is the molecular packing in the film, which depends on the chain length of the terminal alkyl substituent.

Full text of DOI:10.1016/j.dyepig.2011.05.024

Dyes and Pigments 92 (2011) 783e788
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Alkyl substituent effects on J- or H-aggregate formation of bisazomethine dyes
,
b
c
b
b
Kenji Kinashi a , Kyun-Phyo Lee , Shinya Matsumoto , Kenji Ishida , Yasukiyo Ueda
*
a Graduate School of Science and Technology, Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan
b Graduate School of Engineering, Department of Chemical Science and Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan
c Graduate School of Environmental and Information Sciences, Yokohama National University, Hodogaya, Yokohama 240-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Bisazomethine dyes with terminal alkyl substituents of different chain lengths (BAR: R ¼ 1, 2, 3, 4, 5 and
6) were synthesized and deposited on a glass substrate to investigate the effect of the alkyl chain length
on aggregate formation. Methyl- and ethyl-substituted bisazomethine dyes (BA1 and BA2) formed J-
aggregates in thin films (ca. 50 nm), whereas, propyl-, butyl-, pentyl- and hexyl-substituted derivatives
(BA3, BA4, BA5 and BA6) formed H-aggregates in thin films (ca. 50 nm). The aggregate formation of the
BARs changed drastically between ethyl- and propyl-substituents (BA2/BA3). However, no remarkable
changes were observed in the surface morphologies of BA2 and BA3 films. It is suggested that the critical
determinant of aggregate formation of BAR is the molecular packing in the film, which depends on the
chain length of the terminal alkyl substituent.
Received 17 March 2011
Received in revised form
24 May 2011
Accepted 28 May 2011
Available online 12 June 2011
Keywords:
J-aggregates
H-aggregates
Bisazomethine dye
Functional dye
Vapor-deposited film
Optical property
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Thus, J-aggregate formation is irrelevant to polar and/or ionic dyes.
However, Matsumoto et al. found that large and homogeneous
Organic molecules can form crystals or aggregates, in which the
visible absorption bands in the solid state are generally split or
shifted from those of the monomeric molecules. Dye aggregates
have played important roles in both fundamental science and
technological applications such as optical memory, organic solar
cells and organic light-emitting diodes [1e3]. In general, they are
classified into two types, H- and J-aggregates [4e6]. H-aggregates
adopt face-to-face stacking structures, which are characterized by
a blue-shift in the absorption band. H-aggregates can effectively
contribute to the photocurrent in Schottky-type photovoltaic cells
where thin films are sandwiched between two different types of
metal electrodes [7]. J-aggregates are characterized by a sharp
absorption band (J-band) with red-shifted from that of isolated
molecules and have attracted much attention for both fundamental
studies and their large optical non-linearity and ultra-fast response
time for nonlinear optical materials. The specific structure of the
J-aggregates is a head-to-tail stacking between adjacent molecules.
Intriguingly, most studies on J- or H-aggregates described them as
compounds with dipole moments [8e10] and ionic features [11,12].
films of J-aggregates can be formed in vapor-deposited films of
N,N0-bis[(4-(N,N-diethylamino)benzylidene]diaminofumaronitrile
(BA2), even though it is a non-polar (and non-ionic) dye. The
packing structure of BA2 is consistent with the J-aggregate films
consisting of a stable single crystal phase and a metastable crys-
talline phase [13]. BA2 also forms J-aggregates in vapor-deposited
films depending on the film thickness (>100 nm), despite its
symmetric and non-ionic molecular structure. The J-aggregate film
was extremely stable such that no significant change in absorption
spectra was observed even after one year at ambient conditions or
after thermal treatment at 375 K for 1 h. However, despite enor-
mous effort, sufficient understanding of the substituent effects on
aggregate formations has not yet been obtained. Therefore, studies
on the controlling dye aggregate formation and the formation
mechanism of these aggregates in thin films are of great impor-
tance for practical applications as well as for basic research of
assembled dye molecules. Recently, some interesting results have
been reported by Kim et al. regarding the effect of phenyl substi-
tution on the J-aggregation of bisazomethine dyes [14]. The
absorption spectra of these dyes showed a variety of spectral
changes in the visible region and some of the synthesized dyes
were found to form J-aggregates in vapor-deposited films. There-
fore, bisazomethine dyes are excellent candidates for the further
* Corresponding author.
E-mail address: kinashi@kit.ac.jp (K. Kinashi).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.024

784
K. Kinashi et al. / Dyes and Pigments 92 (2011) 783e788
study of the correlation between substituent effects and the
aggregation mechanism.
3.1.1. N,N-dipentylaniline
70.3% yield. Product: yellow oil. FT-IR (cmꢀ1): 3986, 3647, 3412,
3093, 3061, 3024, 2958, 2873, 2735, 2649, 2596, 2495, 2470, 2449,
2390, 2358, 2264, 2186, 1909, 1806, 1752, 1714, 1607, 1569, 1539,
1519, 1505, 1464, 1432, 1494, 1386, 1357, 1316, 1288, 1255, 1231,
1213, 1185, 1148, 1116, 1040, 998, 987, 963, 886, 858, 815, 743, 724,
690. TOF-MS calcd (found): m/z 233.3 (233.4) [Mþ]. 1H NMR
In this study, based on the spectroscopic evidence, the influence
of the chain length of the terminal substituent on the aggregate
formation was determined in order to understand the mechanism
of J- and H-aggregate formation, and to guide the design of novel
J- and H-aggregate organic dyes.
(500 MHz, CDCl3):
d
0.91 (t, 6H, J ¼ 6.9 Hz, CH3), 1.21e1.37 [m, 8H,
(CH2)2], 1.38 (sextet, 4H, J ¼ 7.0, CH2CH3), 3.35 (t, 4H, J ¼ 8.0, NCH2),
6.60 (d, 2H, J ¼ 7.0 Hz, 2,6Ph), 6.62 (t, 1H, J ¼ 7.5 Hz, 4Ph), 7.18 (t, 2H,
J ¼ 7.5 Hz, 3,5Ph).
2. Experimental
Bisazomethine dyes (BA) with different alkyl chain lengths were
synthesized. BAs with different alkyl chains are abbreviated as BAR
where R ¼ 1, 2, 3, 4, 5 and 6 to represent methyl-, ethyl-, propyl-,
butyl-, pentyl- and hexyl-substituents, respectively. The synthetic
procedure of BA dyes is shown in Scheme 1. BA1 and BA2 were
synthesized from p-N,N-dialkylaminobenzaldehyde, BA3 and BA4
were synthesized from N,N-dialkylaniline, and BA5 and BA6 were
synthesized from aniline. p-N,N-Dimethyl- and p-N,N-dieth-
ylaminobenzaldehyde were purchased from Nacalai Tesque, Inc.
N,N-Dipropyl- and N,N-dibutylaniline were purchased from Tokyo
Kasei Co., Ltd. They were used without further purification. The
chemical structures of the synthesized compounds were charac-
terized using 1H NMR [Bruker Advance500, CDCl3 solvent, tetra-
methylsilane (TMS) internal standard], FT-IR (Jasco FT/IR-600 plus)
and TOF-MS (PerSeptive Biosystems Mariner) spectroscopies.
Melting points (mp) were determined using a melting point
apparatus (Yanaco MP-500P) with a digital thermometer. UVevi-
sible absorption and photoluminescence (PL) spectra were recor-
ded on JASCO V-670 and Hitachi F-2500 spectrophotometers,
respectively. Measurements of absorption and PL spectra in solu-
tion were performed in a quartz cell with a path length of 1 cm. The
films were prepared on a glass substrate from a quartz crucible
heated by a tungsten filament in a vacuum of 10ꢀ4 Pa. The depo-
sition rate was 5 nm/s and the substrate temperature was main-
tained at 40 ꢁC. The film thickness was controlled at 50 nm. The
morphologies of the films were characterized using an atomic force
microscope (AFM, SPI-3700, Seiko Instruments).
3.1.2. N,N-dihexylaniline
74.3% yield. Product: yellow oil. FT-IR (cmꢀ1): 3986, 3647, 3412,
3093, 3061, 3024, 2958, 2873, 2735, 2649, 2596, 2495, 2470, 2449,
2390, 2358, 2264, 2186, 1909, 1806, 1752, 1714, 1607, 1569, 1539,
1519, 1505, 1464, 1432, 1494, 1386, 1357, 1316, 1288, 1255, 1231,
1213, 1185, 1148, 1116, 1040, 998, 987, 963, 886, 858, 815, 743, 724,
690. TOF-MS calcd (found): m/z 262.3 (262.2) [Mþ]. 1H NMR
(500 MHz, CDCl3):
(CH2)3], 3.35 (t, 4H, J ¼ 8.0, NCH2), 6.60 (d, 2H, J ¼ 7.0 Hz, 2,6Ph),
6.62 (d, 1H, J ¼ 7.5 Hz, 4Ph), 7.18 (t, 2H, J ¼ 7.5 Hz, 3,5Ph).
d
0.91 (t, 6H, J ¼ 6.9 Hz, CH3), 1.22e1.39 [m, 12H,
3.2. p-N,N-dialkylaminobenzaldehydes
POCl3 (22 mmol) was slowly added to anhydrous DMF
(18.4 mmol) in an ice bath (ꢀ10 ꢁC) for 30 min. To this reaction
mixture N,N-dialkylaniline (18.4 mmol) was added, and the
resulting mixture was carefully heated at 60 ꢁC for 2 h. After
hydrolysis for 2 h under vigorous stirring at r.t. using an aqueous
solution of 2 N NaOH, the crude product was extracted with EtOAc.
The organic layer was dried over anhydrous MgSO4 and evaporated
in vacuo before purification by column chromatography in silica gel
(eluent: CHCl3) to produce an oil. This synthetic procedure was
carried out according to the VilsmeiereHaack reaction [15]. Inter-
mediates with different alkyl chains (propyl, butyl, pentyl and
hexyl) were synthesized using the same procedure. Specific details
for each compound are given below.
3.2.1. p-N,N-dipropylaminobenzaldehyde
13.2% yield. Product: colorless oil. FT-IR (cmꢀ1), 2904, 2814,
2795, 2730, 2713, 2695, 1608, 1567, 1537, 1516, 1435, 1405, 1384,
1354, 1315, 1294, 1265, 1241, 1186, 1143, 1120, 1100, 1034, 976, 816,
797, 765, 749. TOF-MS calcd (found): m/z 205.3 (206.2) [Mþ]. 1H
3. Synthesis
3.1. N,N-dialkylanilines
NMR (500 MHz, CDCl3):
d
0.96 (t, 6H, J ¼ 7.5 Hz, CH3), 1.65 (sextet,
A mixture of fresh aniline (21.5 mmol), iodoalkane (4.51 mmol),
and K2CO3 (4.51 mmol) in EtOH (20 mL) was refluxed at 75 ꢁC for
18 h. The suspension was filtered, and the resulting solid was
washed with CH2Cl2. The filtered solution was extracted with water,
and the organic layer was dried over anhydrous MgSO4 and
concentrated in vacuo. Purification by column chromatography on
silica gel (eluent: CHCl3) produced an oil. Products with different
alkyl chains (pentyl and hexyl) were synthesized using the same
procedure. Specific details for each compound are given below.
4H, J ¼ 8.0 Hz, CH2CH3), 3.32 (t, 4H, J ¼ 8.0 Hz, NCH2), 6.64 (d, 2H,
J ¼ 9.0 Hz, 3Ph), 7.70 (d, 2H, J ¼ 8.5 Hz, 2Ph), 9.70 (s, 1H, CHO).
3.2.2. p-N,N-dibutylaminobenzaldehyde
10.1% yield. Product: colorless oil. FT-IR (cmꢀ1), 2904, 2814,
2795, 2730, 2713, 2695, 1609, 1570, 1540, 1518, 1434, 1403, 1388,
1356, 1314, 1286, 1263, 1223, 1186, 1147, 1122, 1107, 1017, 926, 813,
757, 727, 634. FAB-MS calcd (found): m/z 233.4 (233.2) [Mþ]. 1H
NMR (500 MHz, CDCl3):
d
0.91 (t, 6H, J ¼ 7.0 Hz, CH3), 1.59 (sextet,
4H, J ¼ 7.5 Hz, CH2CH3), 1.66 (quintet, 4H, J ¼ 7.0 Hz, NCH2CH2), 3.20
(t, 2H, J ¼ 7.5, NCH2), 6.66 (d, 2H, J ¼ 9.0 Hz, 3Ph), 7.70 (d, 2H,
J ¼ 9.0 Hz, 2Ph), 9.70 (s, 1H, CHO).
3.2.3. p-N,N-dipentylaminobenzaldehyde
52.0% yield. Product: colorless oil. FT-IR (cmꢀ1), 2904, 2814,
2795, 2730, 2713, 2695, 1607, 1569, 1539, 1519, 1505, 1464, 1432,
1494, 1386, 1357, 1316, 1288, 1255, 1231, 1213, 1185, 1148, 1116, 1040,
998, 987, 963, 886, 858, 815, 743, 724, 690. FAB-MS calcd (found):
m/z 289.2 (289.2) [Mþ]. 1H NMR (500 MHz, CDCl3):
d 0.91 (t, 6H,
Scheme 1. Synthetic procedure for BAR.
J ¼ 7.0 Hz, CH3), 1.21e1.35 [m, 8H, (CH2)2], 1.59 (sextet, 4H,

K. Kinashi et al. / Dyes and Pigments 92 (2011) 783e788
785
J ¼ 7.5 Hz, CH2CH3), 3.20 (t, 4H, J ¼ 7.5, NCH2), 6.68 (d, 2H, J ¼ 9.0 Hz,
3Ph), 7.71 (d, 2H, J ¼ 9.0 Hz, 2Ph), 9.70 (s, 1H, CHO).
1388, 1356, 1314, 1286, 1263, 1223, 1186, 1147, 1122, 1107, 1017, 926,
813, 757, 727, 634. FAB-MS calcd (found): m/z 538.8 (539.4) [Mþ].
1H NMR (500 MHz, CDCl3):
d
0.98 (t, 12H, J ¼ 7.0 Hz, CH3), 1.38
3.2.4. p-N,N-dihexylaminobenzaldehyde
(sextet, 8H, J ¼ 7.0 Hz, CH3CH2), 1.63 (quintet, 8H, J ¼ 7.0 Hz,
NCH2CH2), 3.37 (t, 8H, J ¼ 7.0 Hz, NCH2), 6.66 (d, 4H, J ¼ 9.0 Hz, 3Ph),
7.81 (d, 4H, J ¼ 9.0 Hz, 2Ph), 8.54 (s, 2H, PhCH).
12.3% yield. Product: colorless oil. FT-IR (cmꢀ1), 2904, 2814,
2795, 2730, 2713, 2695, 1607, 1569, 1539, 1519, 1505, 1464, 1432,
1494, 1386, 1357, 1316, 1288, 1255, 1231, 1213, 1185, 1148, 1116, 1040,
998, 987, 963, 886, 858, 815, 743, 724, 690. FAB-MS calcd (found):
3.3.5. N,N0-bis[(4-(N,N-dipentylamino)benzylidene]
diaminofumaronitrile (BA5)
m/z 288.5 (289.2) [Mþ]. 1H NMR (500 MHz, CDCl3):
d 0.91 (t, 6H,
J ¼ 7.0 Hz, CH3), 1.22e1.35 [m, 8H, (CH2)3], 1.59 (sextet, 4H,
J ¼ 7.5 Hz, CH2CH3), 3.20 (t, 8H, J ¼ 7.5, NCH2), 6.68 (d, 2H, J ¼ 9.0 Hz,
3Ph), 7.71 (d, 2H, J ¼ 9.0 Hz, 2Ph), 9.70 (s, 1H, CHO).
14.8% yield. Product: red-violet crystals; mp 209 ꢁC. FT-IR (cmꢀ1
)
2952, 2925, 2855, 2208, 1607, 1569, 1539, 1519, 1505, 1464, 1432,
1494, 1386,1357,1316,1288,1255,1231,1200,1185,1148,1116,1040,
998, 987, 963, 815, 743, 724, 690. FAB-MS calcd (found): m/z 594.8
3.3. N,N0-bis[(4-(N,N-dialkylamino)benzylidene]
diaminofumaronitriles (BAR)
(595.8) [Mþ]. 1H NMR (500 MHz, CDCl3)
: 0.91 (t, 12H, J ¼ 6.9 Hz,
d
CH3), 1.21e1.39 [m, 32H, (CH2)2], 1.59 (sextet, 8H, J ¼ 8.0 Hz,
CH2CH3), 3.35 (t, 8H, J ¼ 8.0, NCH2), 6.65 (d, 4H, J ¼ 9.5 Hz, 3Ph), 7.81
(d, 4H, J ¼ 7.5 Hz, 2Ph), 8.54 (s, 2H, PhCH).
p-N,N-Dialkylaminobenzaldehyde (18.5 mmol) was added to
a benzene solution (100 mL) containing diaminomaleonitrile
(18.5 mmol) and several drops of piperidine that had been vigor-
ously stirred for 30 min. The mixture was then refluxed by stirring
for 2 h. Water was removed using a Dean-Stark trap. The reaction
mixture was then cooled to room temperature. The precipitated
crude product was separated by filtration and washed with cold
toluene, then purified by column chromatography on silica gel
using hexane/chloroform (1/1, v/v) and recrystallized from chlo-
roform. The coupling reaction was carried out according to
a previous study [16]. Products with different alkyl chains (methyl,
ethyl, propyl, butyl, pentyl and hexyl) were synthesized using the
same procedure. Specific details for each compound are given
below.
3.3.6. N,N0-bis[(4-(N,N-dihexylamino)benzylidene]
diaminofumaronitrile (BA6)
13.5% yield. Product: red-violet crystals; mp 160 ꢁC. FT-IR
(cmꢀ1), 2952, 2925, 2855, 2208, 1607, 1569, 1539, 1519, 1505,
1464, 1432, 1494, 1386, 1357, 1316, 1288, 1255, 1231, 1200, 1185,
1148, 1116, 1040, 998, 987, 963, 815, 743, 724, 690. FAB-MS calcd
(found): m/z 651.0 (651.5) [Mþ]. 1H NMR (500 MHz, CDCl3)
d: 0.91
(t, 12H, J ¼ 6.9 Hz, CH3), 1.25e1.38 [m, 32H, (CH2)3], 1.59 (sextet, 8H,
J ¼ 8.0 Hz, CH2CH3), 3.35 (t, 8H, J ¼ 8.0, NCH2), 6.65 (d, 4H,
J ¼ 9.5 Hz, 3Ph), 7.81 (d, 4H, J ¼ 7.5 Hz, 2Ph), 8.54 (s, 2H, PhCH).
4. Results and discussion
3.3.1. N,N’-bis[(4-(N,N-dimethylamino)benzylidene]
diaminofumaronitrile (BA1)
The UVevisible absorption and PL spectra of the analogs of BA in
chloroform (8.75 ꢂ 10ꢀ6 M) are shown in Fig. 1. The absorption
maxima (lmax) were observed in the range of 534e550 nm with
molar extinction coefficients (3max) in the range of 97,200ꢀ134,000.
0.92% yield. Product: red-violet crystals; mp > 500 ꢁC. FT-IR
(cmꢀ1): 2967, 2929, 2899, 2869, 2204, 1281, 1729, 1688, 1611,
1571, 1525, 1482, 1432, 1411, 1392, 1367, 1336,1317, 1266, 1230, 1183,
1147, 1116, 1059, 999, 959, 940, 806, 782, 764, 746, 726. TOF-MS
calcd (found): m/z 370.4 (371.3) [Mþ]. 1H NMR (500 MHz, CDCl3):
The absorption peaks were assigned to
pꢀp
* electronic transitions
between the ground state and the excited state of the chromophore
moiety [14]. The lmax with large 3max was suggested that an intra-
molecular charge- transfer chromophoric system in which the
arylamine moiety was the donor and the central dicyanoethylene
moiety was the acceptor. In addition, the lmax gradually shifted to
a longer wavelength (bathochromic shift) with an increase in the
substituent a chain length, which depended on the electron
donating ability of the donor moiety. PL maxima (Pmax) of the BARs
were observed in the region of 585e600 nm by excitation with the
wavelength of absorption maxima. The BARs are a red-light-
d
3.10 (s,12H, CH3), 6.71 (d, 4H, J ¼ 4 Hz, 3Ph), 7.85 (d, 4H, J ¼ 8.5 Hz,
2Ph), 8.59 (s, 2H, PhCH).
3.3.2. N,N0-bis[(4-(N,N-diethylamino)benzylidene]
diaminofumaronitrile (BA2)
74.4% yield. Product: redeviolet crystals; mp 278 ꢁC. FT-IR
(cmꢀ1):2967, 2929, 2899, 2869, 2204, 1607, 1561, 1513, 1436, 1410,
1377, 1349, 1316, 1266, 1195, 1139, 1118, 1036, 1076, 1010, 961, 820,
795, 727. TOF-MS calcd (found): m/z 426.6 (449.3) [Naþ]. 1H NMR
(500 MHz, CDCl3):
d
1.23 (t, 12H, J ¼ 7.0 Hz, CH3), 3.46 (q, 8H,
J ¼ 7.0 Hz, CH2), 6.68 (d, 4H, J ¼ 9.0 Hz, 3Ph), 7.82 (d, 4H, J ¼ 8.5 Hz,
2Ph), 8.55 (s, 2H, PhCH).
BA1
BA2
BA3
BA4
BA5
BA6
1
1
3.3.3. N,N0-bis[(4-(N,N-dipropylamino)benzylidene]
diaminofumaronitrile (BA3)
10.5% yield. Product: red-violet crystals; mp 213 ꢁC. FT-IR (cmꢀ1
)
2959, 2931, 2872, 2206, 1608, 1567, 1537, 1516, 1435, 1405, 1384,
1354, 1315, 1294, 1265, 1241, 1186, 1143, 1120, 1100, 1034, 976, 816,
797, 765, 749. TOF-MS calcd (found): m/z 482.7 (483.3) [Mþ]. 1H
NMR (500 MHz, CDCl3):
8H, J ¼ 7.5 Hz, CH3CH2), 3.20 (t, 8H, J ¼ 7.5, NCH2), 6.68 (d, 4H,
J ¼ 9.0 Hz, 3Ph), 7.82 (d, 4H, J ¼ 8.5 Hz, 2Ph), 8.55 (s, 2H, PhCH).
d
0.91 (t, 12H, J ¼ 7.0 Hz, CH3), 1.59 (sextet,
0
0
3.3.4. N,N0-bis[(4-(N,N-dibutylamino)benzylidene]
diaminofumaronitrile (BA4)
34.3% yield. Product: red-violet crystals; mp 235 ꢁC. FT-IR
(cmꢀ1) 2950, 2928, 2866, 2210, 1609, 1570, 1540, 1518, 1434, 1403,
400
500
600
700
Wavelength /nm
Fig. 1. Absorption spectra and PL spectra in chloroform (8.75 ꢂ 10ꢀ6 M) of BAR.

786
K. Kinashi et al. / Dyes and Pigments 92 (2011) 783e788
emitting materials with Stokes shifts (SS) of approximately 49 nm
on average due to their symmetrical donor-accepter conjugated
structures [17], although their fluorescent quantum efficiencies are
very low [16,18]. In solution, the obtained photoluminescence
results indicated that the alkyl substituents had no significant
influence on the electronic states in the visible region.
the photoluminescence. In the case of a J-aggregate, emission
occurs from the excited state, which is located at a lower energy
than the corresponding monomer emission. In contrast, in the
H-aggregates where the excited state is higher than the corre-
sponding monomer excited state, there is usually a quenching of
fluorescence observed in the emission spectrum of the aggregate.
After excitation to the higher excited state, there is a rapid internal
conversion to the lower excited state. Since the transition from the
lower excited state to the ground state is not allowed, the system
returns to the ground state via nonradiative decay or intersystem
crossing through the triplet excited state [20]. Experimentally, the
detected fluorescence shows a bathochromic shift compared to the
fluorescence of the monomer. Therefore, the bathochromic shift in
H-aggregates is smaller than that of J-aggregates and is not
consistent with the above exiton theory.
The films of BAR analogs also showed a bright red PL laid in the
wavelength region 620ꢀ650 nm by excitation with at their
absorption maxima. SS between the PL peak in the film and the
absorption peak in solution were 43 and 15 nm for BA1 and BA2,
respectively. It is notable that the PL band is a mirror image of the
low-energy edge of the J-band absorption. The small SS of the BA2
film is clearly consistent with the existence of J-aggregates. The
interaction energy between monomeric transition states lowers the
overall energy of the cooperative state; hence, the absorption and
PL spectra of the J-aggregates show a bathochromic shift with
a smaller SS relative to that in solution.
Fig. 2 shows the absorption and PL spectra of BARs in vapor-
deposited films. The absorption peaks for BAR (R ¼ 1ꢀ6) films
appeared at 588, 637, 481, 480, 491 and 485 nm, respectively. These
absorption spectral changes of BAR films showed significant
differences, despite their similar electronic states. Therefore, the
spectral changes are correlated with the solid-state structure and
are considered to depend on the alkyl chain length. A shoulder
peak, which was overlapped with the absorption spectrum in
solution, was assigned to the original BAR monomers. In the cases
of the methyl- and ethyl-substituted BAR (BA1 and BA2) films, large
bathochromic shifts were observed from the solution to the films.
The intense new peaks with bathochromic shifts are a photo-
physical effect characteristic of J-aggregate formation, in which the
interaction strength exceeds the monomeric dephasing processes
[19], and indicates a head-to-tail configuration between the adja-
cent molecules in the films. Furthermore, the broad absorption in
the short-wavelength region is assigned to the electronic transition
of a short axis for the molecular conjugation length.
In contrast, the propyl-, butyl-, pentyl- and hexyl-substituted
BAR (BA3, BA4, BA5 and BA6) films were found to exhibit a short-
wavelength shift (hypsochromic shift). The broad spectral features
with the hypsochromic shift are a photophysical effect character-
istic of H-aggregate formation. The H-aggregate is formed with
face-to-face stacking, with the molecules adopting a slipped-
On the other hand, the absorption spectra of BA3, BA4, BA5, and
BA6 films were shifted to higher energies compared with those in
solution and the SS were 165, 145, 135 and 139 nm, respectively.
The spectral shifts originating from H-aggregation are consistent
with the results of Labarthet’s study [21]. H-type molecular
aggregation, an assembly in which the chromophores are arranged
in such a way that their dipoles are parallel to each other, has been
found mainly in LangmuireBlodgett or self-assembled films
[21e24].
parallel structure due to the neighboring weak
molecular interaction.
pep intra-
Although the spectroscopic consequences of aggregation can be
identified in the absorption spectra, there is also a consequence on
All spectroscopic data of BA analogs both in solution and film are
listed in Table 1. From the spectroscopic features, a remarkable
difference was observed between the BA2 and BA3 films. The strong
intermolecular interactions owing to the alkyl chain length account
for the differences in J- and H-aggregate formation.
Abs., solution
Abs., films
PL., films
a
b
1
1
1
1
The surface morphologies of BAR films were examined by AFM
to observe the differences between J- and H-aggregates, as shown
in Fig. 3. The grain sizes in BA1, BA2, BA3, BA4 and BA5 films were
found to be in the range of approximately 0.3e1.0
grains in the BA6 films were extremely large, with the blockish
grains in the range of 2e3 m. It is suggested that BA6 crystals grew
mm. However, the
0
400
0
700
0
0
500
600
400
500
600
700
Wavelength /nm
Wavelength /nm
m
larger since the melting point of BA6 is low, about 160 ꢁC, compared
to that of the others (BA1, 2, 3, 4 and 5: >500, 278, 213, 235, and
209 ꢁC, respectively). From the observations, intriguingly, a signifi-
cant difference could not be found in the surface morphologies of
BA2 and BA3. Therefore, the details of the film structure should be
investigated further.
We have summarized the results of the spectroscopic and the
surface morphological features. There was no significant change in
the surface morphologies between BA2 and BA3, whereas
a remarkable difference was observed in BA2 and BA3 films spec-
troscopically. In other words, it was suggested that the critical
determinant of aggregate formation of BAR is the molecular
packing in thin films, which depends on the chain length of the
terminal alkyl substituent. Previously, Ueda et al. reported on
the aggregates of merocyanine dyes in vapor-deposited film [25].
The merocyanine dyes substituted with different alkyl chains
formed either J- or H-aggregates, depending on the degree of steric
hindrance between alkyl chains. These unique results were found
with asymmetrical neutral molecules. A similar phenomenon has
c
d
1
1
1
1
0
0
0
0
400
500
600
700
400
500
600
700
Wavelength /nm
Wavelength /nm
f
e
1
1
1
1
0
0
700
0
0
400
500
600
700
400
500
600
Wavelength /nm
Wavelength /nm
Fig. 2. Absorption spectra and PL spectra in chloroform (8.75 ꢂ 10ꢀ6 M) of (a) BA1,
(b) BA2, (c) BA3, (d) BA4, (e) BA5 and (f) BA6 films.

K. Kinashi et al. / Dyes and Pigments 92 (2011) 783e788
787
Table 1
Substituent effects on the absorption and fluorescence spectra of BAR in solution and in films.
solution
max
l
=nmð
3
max=Mꢀ1 cmꢀ1
Þ
P
=nm
SSsolution/nma
lfmilmax=nm
Pfmilmax=nm
SSflim/nma
Dl/nmb
J/Hc
solution
max
BA 1
BA 2
BA 3
BA 4
BA 5
BA 6
534 (128,000)
544 (140,000)
548 (107,000)
550 (134,000)
552 (118,000)
550 (97,200)
585
593
594
598
603
600
51
51
46
48
51
50
588
637
481
480
491
485
631
652
646
625
625
624
43
15
165
145
134
139
54
93
ꢀ67
ꢀ70
ꢀ61
ꢀ65
J
J
H
H
H
H
a
Stokes shift (SS) ¼ Pmax
ꢀ
lmax.
film
max
solution
.
max
b
c
Dl
¼
l
ꢀ
l
J/H: J-aggregates/H-aggregates.
also been found in other molecules such as a porphyrin with
a cationic surfactant [26], and a pseudoisocyanine with a poly-
electrolyte [27], which formed either J- and H-aggregates.
absorption spectra of J- and H-aggregates were emphasized
because the contribution of the number of molecules is much
greater than that of the bulk phase within the two-dimensionally-
restricted film thickness. In other words, it can be understood that
crystal growth with specific spectral shifts dose not necessary
correlate with defined J- and H-aggregates.
To summarize, in the BA1 and BA2 films, the head-to-tail
intermolecular stacking structure due to the strong dipoleedipole
interaction was oriented parallel to the two-dimensionally-
restricted film. Judging from the spectral characteristics, J-bands
which resemble typical J-aggregates, were recognized. However, in
this case, it may be more appropriate to call the specific spectra
“J-like bands”. In the case of BA3, BA4, BA5 and BA6, the stacking
plane was oriented obliquely, face-to-face, in the two-dimension-
ally-restricted film owing to the long alkyl chain lengths. As a result,
face-to-face stacking H-aggregates as in LangmuireBlodgett films
appeared. Therefore, the J- and H-aggregates changed only due to
differences in the alkyl chain length.
Consequently, the BARs in this study adopted either J- and
H-aggregate structures in the films despite being symmetrical and
neutral molecules, and hence they have characteristics similar to
the above referenced compounds. In addition, it is more important
that the specific spectral properties of BAR films could be found
even in an extremely thin film (ca. 50 nm). Two-dimensionally-
restricted crystal growth dominated since the J- and H-aggregates
could not grow with restricted film thickness. It was suggested that
the slip angle in the molecular stacking is an important factor for
aggregate formation. Namely, the number of the molecules was
a dominant contributor to the absorption intensities in the
restricted film thickness. Therefore, it was suggested that the
In the future, to confirm these conclusions, crystallographic
analysis (i.e., X-ray structural analysis, electron diffraction) is
necessary to understand the mechanism of J- and H-aggregate
formation. Crystallographic features are more important than
spectroscopic features for identification of J- and H-aggregate
formation.
5. Conclusions
Bisazomethine dyes with different alkyl substituents were
successfully synthesized, and their optical characteristics and
surface morphologies were investigated. From this spectroscopic
study, chloroform solutions of the synthesized BAR analogs
exhibited similar spectral shapes and peak positions. However, in
the films, BA1 and BA2 exhibited bathochromic shifts, whereas BA3,
BA4, BA5 and BA6 exhibited hypsochromic shifts under the same
conditions. The J- and H-aggregates in the films could be controlled
by adjusting of the number of eCH2e units. Thus, it was possible to
control J- and H-aggregate formation by the careful design of
molecular structures. Unfortunately, the mechanism of J- and
H-aggregate formation could not be identified from the spectro-
scopic and surface morphological studies along. Their crystallo-
graphic features will provide more information about the
differences between J- and H-aggregates. This will be the subject of
future studies.
References
[1] Spano FC, Siddiqui S. Excitonevibrational coupling in pinwheel aggregates of
p-conjugated molecules. Chemical Physics Letters 1999;314:481e7.
[2] Borsenberger PM, Weiss DS. Organic photoreceptors for imaging systems.
New York: Marcel Dekker; 1993.
[3] Jelly EE. Spectral absorption and fluorescence of dyes in the molecular state.
Nature 1936;138:1009e10.
[4] West W, Pearce S. The dimeric state of cyanine dyes. The Journal of Physical
Chemistry 1965;69:1894e903.
Fig. 3. AFM images (10 ꢂ 10
BA5 and (f) BA6 films.
m
m2 surface area) of (a) BA1, (b) BA2, (c) BA3, (d) BA4, (e)

788
K. Kinashi et al. / Dyes and Pigments 92 (2011) 783e788
[5] Maiti NC, Mazumdar M, Periasamy N. J- and H-aggregates of porphyr-
[16] Kinashi K, Kotake T, Ono Y, Ishida K, Ueda Y. Photoswitching of diarylethene
using bisazomethine dye. Optical Materials 2009;31(11):1711e4.
[17] Higashiguchi K, Matsuda K, Kobatake S, Yamada T, Kawai T, Irie M. Fatigue
mechanism of photochromic fatigue mechanism of photochromic
1,2-Bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene. Bulletin of the Chem-
ical Society of Japan 2000;73(10):2389e94.
inꢀsurfactant complexes: time-resolved fluorescence and other spectroscopic
studies. The Journal of Physical Chemistry B 1998;102:1528e38.
[6] Yu J, Chen Z, Sone M, Miyata S, Li M, Watanabe T. Red-light-emitting organic
electroluminescent devices with bisanil dye as emitter. Japanese Journal of
Applied Physics 2001;40:3201e5.
[7] Saito K. H-aggregate formation in Squarylium LangmuireBlodgett films. The
Journal of Physical Chemistry B 2001;105:4235e8.
[8] Berlepsch HV, Bottcher C, Dahne L. Structure of J-aggregates of pseudoiso-
cyanine dye in aqueous solution. The Journal of Physical Chemistry B 2000;
104(37):8792e9.
[9] Ikegami K, Mingotaud C, Lan M. Intramolecular charge transfer in mer-
ocyanine dye molecules enhanced by formation of J-aggregates. Thin Solid
Films 2001;393:193e8.
[10] Matsumoto S, Shirai K, Kobayashi K, Wada T, Shiro M. J-aggregate structures
in crystals of three bisazomethine dyes. Zeitschrift für Kristallographie 2004;
219(4):239e43.
[18] Kim S-H, Yoon S-H, Kim S-H, Han E-M. Red electroluminescent azomethine
dyes derived from diaminomaleonitrile. Dyes and Pigments 2005;64:45e8.
[19] Lemaistre JP. A two-dimensional model for energy transfer in pure and mixed
J-aggregates. Chemical Physics 2007;333:186e93.
[20] Kasha M, Rawls HR, Ashraf El-Bayoumi M. The exciton model in molecular
spectroscopy. Pure Appl. Chem. 1965;11:371e92.
[21] Labarthet FL, Freiberg S, Pellerin C, Pézolet M, Natansohn A, Rochon P. Spec-
troscopic and optical characterization of a series of azobenzene-containing
side-chain liquid crystalline polymers. Macromolecules 2000;33(18):6815e23.
[22] Menzel H, Weichart B, Schmidt A, Paul S, Knoll W, Stumpe J, et al. Small-angle
X-ray scattering and ultraviolet-visible spectroscopy studies on the structure
and structural changes in LangmuireBlodgett films of polyglutamates with
azobenzene moieties tethered by alkyl spacers of different length. Langmuir
1994;10(6):1926e33.
[11] Fujieda T, Ohta K, Wakabayashi N, Higuchi S. H-aggregation of methyl
orange at the interface between the water phase and oil phase in a water-in-
oil microemulsion. Journal of Colloid and Interface Science 1997;185(2):
332e4.
[23] Ricceri R, Neto C, Abbotto A, Facchetti A, Pagani GA. Morphological charac-
[12] Tani K, Matsuzaki K, Kodama Y, Fukita M, Kodaira T, Horiuchi H, et al. Pho-
tophysical property of the J-aggregate thin film of an oxacyanine dye prepared
by the spin-coating method and enhancement of its photostability by use of
polydimethylsilane. Journal of Photochemistry and Photobiology A: Chemistry
2008;199:150e5.
terization of
H
aggregates in LangmuirꢀBlodgett films of pyr-
idiniumꢀdicyanomethanide dyes. Langmuir 1999;15(6):2149e51.
[24] Moon JH, Choi JU, Kim JH, Chung H, Hahn JH, Kim SB, et al. Self-assembly of
non-linear optical chromophores through ionic interactions. Journal of
Materials Chemistry 1996;6(3):365e8.
[13] Matsumoto S, Kobayashi T, Aoyama T, Wada T. J-Aggregates in vapor depos-
[25] Ueda Y, Nitta K. Formation and characterization of aggregates of merocyanine
dyes in vacuum-deposited thin film. Japanese Journal of Applied Physics
2001;40:6951e5.
ited films of
1910e1.
a bisazomethine dye. Chemical Communications 2003;15:
[14] Kim B-S, Kashibuchi D, Son Y-A, Kim S-H, Matsumoto S. Effect of phenyl ring
substitution on J-aggregate formation ability of novel bisazomethine dyes in
vapour-deposited films. Dyes and Pigments 2011;90:56e64.
[26] Koti ASR, Periasamy N. Cyanine induced aggregation in meso-tetra-
kis(4-sulfonatophenyl)porphyrin anions. Journal of Material Chemistry 2002;
12:2312e7.
[15] Raimundo JM, Blanchard P, Gallego-Planas N, Mercier N, Ledoux-Rak I,
Hierle R, et al. Design and synthesis of pushepull chromophores for second-
[27] Sayama K, Tsukagoshi S, Hara K, Ohga Y, Shinpou A, Abe Y, et al. Photo-
electrochemical properties of J aggregates of benzothiazole merocyanine dyes
on a nanostructured TiO2 film. The Journal of Physical Chemistry B 2002;
106(6):1363e71.
order nonlinear optics derived from rigidified thiophene-based
gating spacers. The Journal of Organic Chemistry 2002;67:205e18.
p-conju-