COMMUNICATION
6 mol% of [RuH2(CO)ACHTUNTRGNEG(UN PPh3)3]. After silica-gel separation,
tetraalkylated PBI 2a was obtained in 94% yield
(Scheme 2). The alkylation of N-(2,6-diisopropyl)phenylPBI
1b needed longer reaction times due to its lower solubility,
but afforded the desired product 2b in almost quantitative
yield. After tetraalkylation, solubility in organic solvents is
significantly enhanced: 2a is soluble in ethyl acetate and 3a
is soluble in warm hexane. A particularly attractive feature
is facile introduction of the alkoxylsilyl side chain (6a) that
serves as a docking group for inorganic substrates (see
below).
UV/Vis absorption and fluorescence spectra are shown in
Figure 1. As expected, the alkylation does not significantly
change the absorption and emission properties in the solu-
tion state, but somewhat lowers molar extinction coefficients
of the lowest energy absorption band. Fluorescence quan-
tum yields in toluene are almost the same as the parent
PBIs 1a and 1b. Importantly, alkylated PBIs fluoresce even
in the solid state with intense red emission.[7] For example, a
powder sample of 3a emits at 635 nm with a much enhanced
quantum yield of 0.59 (Table 1). The fluorescence spectra in
the solid state are broad and substantially red-shifted, sug-
gesting emission from aggregates of PBI. In sharp contrast
to the solution state, quantum yields in the solid state heavi-
ly depend on the introduced alkyl groups, implying the im-
portance of alkyl substituents that influence the crystal
packing. Absorption spectra in the solid state were mea-
sured for microdispersion of PBIs (Figure 1c).[8] PBI 1a ex-
hibits a broad and blue-shifted spectrum, which is typical for
H-type aggregates. For 3a and 4a, the absorption features in
solution including the vibronic bands were preserved in the
solid sate, but 3a showed 12 nm of red-shift. These results
indicate smaller intermolecular electronic interaction in
crystal than parent PBI 1a by the introduced alkyl substitu-
ents.
Figure 1. a) UV/vis absorption spectra in toluene. b) Emission spectra in
toluene and solid state emission spectra. c) Solid-state absorption spectra
of dispersion of PBIs in water.
The structure of PBI 4a has been clearly elucidated by
the X-ray analysis (Figure 2 and Figure S31).[9] Introduction
of four alkyl groups does not result in any distortion of the
flat perylene core. This is in sharp contrast to bay-area sub-
stitution, which often induces severe twist in perylene. Each
molecule of 4a aligns in a parallel manner without stacking.
Obviously, the interaction between alkyl groups is essential
to form this type of aggregation. In contrast, PBI 2a exhibits
the herringbone arrangement
Using trimethoxysilyl groups, 6a can be chemisorbed on a
glass surface via the following process: 1) recovery of the
OH groups on the glass surface, 2) dropping a toluene solu-
tion of 6a on the surface; 3) removal of the physisorbed
PBI. We were able to obtain a fluorescence image of single
molecules of 6a adsorbed on glass surface. The fluorescence
is still observable, even though the molecule is exposed to
with distinct p-stacking of an
interplanar distance of about
3.6 ꢂ.[10] Two molecules are
crossed with each other by 668,
resulting from steric hindrance
of bulky silylethyl substituents.
Although it was difficult to
obtain a nice single crystal of
3a, the synchrotron powder X-
ray diffraction (SXRD) re-
vealed a high crystallinity of 3a
with the triclinic symmetry.
Table 1. Photophysical properties of alkylated PBIs.
PBI e [mÀ1 cmÀ1 lmax [nm][a] lem [nm][b] Stokes shift [cmÀ1
]
]
Ff (solution) lem [nm] (solid) Ff (solid)[c]
1a
2a
3a
4a
5a
1b
2b
3b
8.43ꢃ104
5.97ꢃ104
6.42ꢃ104
5.30ꢃ104
5.19ꢃ104
8.50ꢃ104
6.29ꢃ104
6.80ꢃ104
526
522
521
521
525
527
524
523
533
533
533
533
534
534
534
533
250
414
380
451
339
249
357
358
1.0
676
640
635
644
634
–
0.07
0.41
0.59
0.24
0.08
0.01
0.02
0.09
0.91
0.94
0.95
0.92
0.97
0.94
0.94
584
591
[a] The longest absorption maxima. [b] Excited at the longest absorption maxima. [c] Excited at 470 nm. Abso-
lute quantum yields were determined by a calibrated integrating sphere system within Æ3% errors. Powder
samples were prepared by recrystallization of alkylated PBIs from CHCl3/CH3CN.
Chem. Eur. J. 2009, 15, 7530 – 7533
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7531