Ê
From Fig. 5 one can also see that the central wavelengths of
two-photon pumped lasing bands are at 625±630 nm, with a
width at half maximum of y25 nm. There is a tendency for a
red shift when the donor strength is increased, for example, the
central wavelengths for PSPI and DEASPI are at y625 nm,
while those of HEASPI and ASPI are both at 630 nm.
BLA value of ASPI of 0.0217 A, which has a lasing ef®ciency of
7.1%. From such an analysis, it can be concluded that large
Dr1,2 and a small BLA value are the factors that contribute to a
TPP up-conversion lasing ef®ciency.
Conclusions
(
1) A series of new stilbene-type derivatives with various
Results and discussion
terminal electron donor groups and the same pyridinium
terminal acceptor residue have been synthesized. Strong two-
photon pumped (1064 nm) lasing at around 625±630 nm with a
half-bandwidth of y25 nm was observed in DMF solution.
The largest up-conversion ef®ciency was as high as 10.7% at
2.14 mJ input energy. To our knowledge, DEASPI, PSPI and
HEASPI are among the few laser dyes with such a high up-
conversion ef®ciency.
The two-photon-pumped lasing ef®ciencies of the six stilbene-
type chromophores measured in DMF at a concentration of
d ~0.05 M are summarized in Table 6. The ef®ciencies range
0
from 0.13% for CSPI to 10.7% for DEASPI, i.e. varying by
about two orders of magnitude. We consider that such large
differences must originate from fundamental variations in the
chromophoric molecular structure.
Using PM3 semi-empirical method, we calculated two
intrinsic parameters of the chromophores, namely charge
density distribution and bond length alternation (BLA) and
found that both of them are related to TPP emission ef®ciency.
The charge density on each atom in the styrylpyridine
skeleton for the various dyes is shown in Table 5. Charge
density distribution can re¯ect the extent of resonance
interaction between donor and acceptor, which in turn
indicates the extent of intramolecular charge transfer. We
(2) The electron donor strength of different substituted
1
amino groups could be ordered by measuring the H NMR
chemical shift values in appropriate model compounds, giving
the sequences: carbazolylvdiphenylaminovpyrrolidino#
diethylaminovethyl(hydroxyethyl)amino#methyl(hydroxye-
thyl)amino. The in¯uence of donor strength upon two-photon
pumped emission behavior shows the variety the charge density
distribution and BLA.
(3) Charge density distribution (de®ned by Dr1,2) and bond
length alternation (BLA) in the chromophoric skeleton were
calculated by the PM3 semi-empirical method. The experi-
mental and theoretical results demonstrated that both Dr1,2
and BLA have a signi®cant effect on the two-photon up-
conversion lasing ef®ciency, which increases with an increase in
the Dr1,2 value and a decrease in BLA.
divide the structure of the skeleton into three parts: r
1 2
, r and
the conjugating bridge. Here, r describes the average charge
1
densities on atoms C10 to C14, while r
corresponding average value for atoms C2 to C6 (excluding
C4 and N1). The difference between r and r , Dr1,2, is thus a
2
describes the
1
2
measure of the donor±acceptor interaction, or intramolecular
charge transfer character. From Table 6, one can see that Dr1,2
in the skeleton is negligible (0.0074 e), but once the skeleton has
an attached donor±acceptor pair, then Dr1,2 becomes sig-
ni®cant, and this change will in¯uence two-photon pumped up-
conversion ef®ciency.
Acknowledgements
This work was supported by a grant for the State Key Program
of China and by the National Science Foundation of China.
The calculation programs were designed by Dr Zhao Xian.
An alternative molecular parameter that may affect emission
1
ef®ciency is bond length alternation (BLA). BLA is de®ned
8
here as the average of the difference in length between adjacent
single and double bonds. The bond lengths in the skeleton for
the various dyes are present in Table 5. From Table 5, one can
see that when the skeleton is end-capped with the donor±
acceptor pair, all of the bond lengths change; at same time,
bond length alternation (BLA) occurs in some single and
double bonds in the skeleton. The calculated BLA is in the
order HEASPIvDEASPIvPSPIvASPIvDPASPIvnaked
skeletonvCSPI, shown in Table 6. The decrease in BLA for
the chromophoric skeleton can be explained as originating
from resonance interaction between the donor and acceptor
end groups. The decrease of BLA means that p-charge
delocalization is greater.
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12
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1
Ê
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J. Mater. Chem., 2000, 10, 2698±2703
2703