195
1.00
4700
4500
a
|
1
4300
T
i
Ly
4100
i
+
WAVELENGTH
hal
4.
(A)
oe
at00
4s 4" -D] HEXAHY DROFRANESOXY -P-TEAPHENTL
0.90 |
SOLVENT
ao
o
4 32000
EXCITING WAVELENGTH
3040A
)
s
oa
0027 G/L
.
4 28000 ~~
DECAY TIME 0.95 NSEC
WAVELENGTH (AVE)
Qo
Ww
0.09 MM
WAVELENGTH (CG)
-| 24000
2700 CM!
4 20000
PHOTON INTENSITY
+d
STOKES LOSS
J649A
3677A
16000
s
0.40
0,30 L
EMISSION
ABSORPTION 12000
0.20 |
0.10 L
0.00
21000
L
22000
i
23000
1
24000
7
25000
rn
26000
1
27000
L
28000
1
29000
k
30000
WAVE NUMBER
HaCiso-—{_)—{_)—{(_))—00Hs
31 000)
(CM 3
33000
33000
34000
35000
3e000
+
8000
4
4000
MOLAR EXTINCTION COEFFICIENT
=]
Qa
a
'
T
n
Oo
SLIT WIDTH
T
(ARBITRARY SCALE)
T
CONC.
BENZENE
37000
158.—Compound XIX, 4,4”-dihexahydrofarnesoxy-p-terphenyl, solvent benzene
systems of the similar size. Among the compounds
tested, XI is the longest molecule and, as expected,
has the shortest computed lifetime, 0.8 nsee.
It has been postulated by Suzuki‘® that the larger
readily noted in comparing the data for II with III,
VI with VII, and VII with IX, as shown in Table 67
and Figures 140, 141, and 143-146. If it were not for
steric crowding by the substituents with the hydrogen
mophor in the ground state. In support of this as-
atoms on the basic chromophor in VII, X (Figure
147) and XI (Figure 148), the natural fluorescence
become smaller as the chromophor becomes larger as
IV (Figure 142) can be considered as tetraphenyl-p-
the p-oligophenylene, the more planar is the chro-
sumption, the FWREvalues of the absorption spectra
shown in Table 67 for compoundsII, VI, and VIII.
Alkyl chains substituted in the para and meta posi-
lifetimes would be even shorter than those tabulated.
terphenyl and is an illustration of an extreme case of
steric hindrance. For this latter compound the quan-
tions of terminal rings are effective in increasing the
tum yield is very small, the intense transition that is
planarity of the chromophor or the strength of the
transition. For these reasons, these substituted com-
ward shorter wavelengths, and Stokes loss is very
solubility of the compound and yet do not affect the
pounds possess characteristics very similar to those
of the unsubstituted compounds. On the other hand,
if these same substituents are positioned on the ortho
position of the end rings or on meta or ortho positions
of phenylene rings, the planarity of the basic chromophoris disturbed, and its fluorescence characteristics are affected in the followmg manner: The long
wavelength absorption bond is shifted toward shorter
wavelengths, the value of enax Is reduced, the value
of FWRE of the absorption curve is increased, the
fluorescence spectrum becomes less structured, the
value of Stokes loss is increased, and the quantum
yield is reduced. These effects of steric hindrance are
the hallmark of the p-oligophenylenes is shifted tolarge. Surprisingly FWRE (Fl) is abnormally small.
This result may be explained by the fact that the six
phenyl rings make large angles with the central benzene rings so that there is no effective conjugation
between the rings and the fluorescence characteristics
correspond to a 1L, — 1A transition of the basic
chromophor, the central benzene ring.
The angle between successive phenyl rings can be
held fixed by means of a bridging group as in XII,
XIII, XIV, XV, and XVI, Figures 149-153, respec-
tively. This added planarity is responsible for the
structure in both the absorption and fluorescence
spectra, and their shift toward longer wavelengths, for
the increase in the value of emax, and for the de-