to examine the effects different imino substituents exert on
the photophysical properties.
derivatives. Upon excitation into the absorption band
at 350 nm of 5, a weak, high energy emission band is
also observed near 470 nm in CH2Cl2 and methanol,
likely due to fluorescence from the enol tautomer. The
emission energy of compound 6 (λem = 600-622 nm,
Table 2) is similar to that of 1, 3, and 4 and varies little
with solvent, although the quantum efficiency is much
lower (Φ = 0.02-0.12). Likewise, the emission energy
of benzannulated compound 7 is minimally affected by
solvent and the quantum efficiencies are significantly
diminished (Φ = 0.03-0.04). The emission lifetime
cannot be fit to a single exponential, suggesting the
presence of conformational isomers for the keto tau-
tomer of 7. Despite having lower quantum yields, the
emission lifetimes for 5-7 are comparable to values for
1, indicating that a significant fraction of nonradiative
decay occurs prior to tautomerization to the lumines-
cent species.
In summary, a new class of proton-transfer dyes based
on the 1,3-bis(imino)isoindole diol motif has been intro-
duced. The ESIPT nature of the luminescence was sup-
ported by inhibition of emission upon alkylation of the
dihydroxylated BPI. Several derivatives of the parent
ESIPT structure were prepared by nucleophilic addition
of alkyl or aryl amines to substituted 1,2-dicyanobenzene.
Modification of either the indole or imino substitutents
shifts the equilibrium between keto and enol forms of the
molecule in both the ground and the excited state. The
ability to alter the thermodynamics of proton transfer by
such changes of the 1,3-bis(imino)isoindole can be used to
investigate the structure-property relationships that con-
trol the excited-state proton transfer process with the long-
term goal of developing guiding principles for ESIPT-
luminophore design. A large variety of new ESIPT dyes
can also be envisioned by tailoring the substituents around
the 1,3-bis(imino)isoindoline diol core.
Figure 5. Absorption (empty shape) and emission (filled shape)
spectra of compounds 5 (black, square), 6 (red, circle), and 7
(blue, triangle) in CH2Cl2 at room temperature.
The room-temperature absorption and emission spectra
of 5-7 in CH2Cl2 are shown in Figure 5. Compound 5
(R1 = n-C12H25) displays a band at low energy (λmax
=
495 nm, ε ≈ 1 Â 104 M-1 cm-1) that is assigned to the keto
tautomer. The transition occurs at higher energy than that
for the keto tautomers of 1, 3, and 4, presumably due to the
lack of conjugation with the imine nitrogen. The absorp-
tion profile of 6 (R1 = 4-t-Bu-phenyl) is dominated by the
higher energy, enol tautomer (λmax = 365 nm, ε ≈ 1.8 Â 104
M-1 cm-1) in CH2Cl2 and toluene, whereas the keto tauto-
mer is favored in methanol (λ=475-610 nm). The spectrum
of the benzannulated derivative 7 (R1 = 1-isoquinoyl) shows
a red-shifted, enol transition (λ = 350-500 nm, εmax ≈ 3 Â
104 M-1 cm-1) that does not vary significantly wth solvent.
As shown in Figure 5, the emission from compound 5 is
blue-shifted (λem = 550-575 nm) relative to the other
Acknowledgment. The authors would like to thank
Universal Display Corporation and the Department of
Energy for their financial support of this work.
ꢀ
(10) (a) Mordzinski, A.; Grabowska, A. Chem. Phys. Lett. 1982, 90,
ꢀ
€
€
ꢀ
122. (b) Mordzinski, A.; Grabowska, A.; Kuhnle, W.; Krowczynski, A.
Chem. Phys. Lett. 1983, 101, 291. (c) Mutai, T.; Tomoda, H.; Ohkawa,
T.; Yabe, Y.; Araki, K. Angew. Chem., Int. Ed. 2008, 47, 9522. (d) Sliwa,
M.; Mouton, N.; Ruckebusch, C.; Poisson, L.; Idrissi, A.; Aloise, S.;
Potier, L.; Dubois, J.; Poizat, O.; Buntinx, G. Photochem. Photobiol. Sci.
2010, 9, 661.
(11) (a) Gruzinskii, V. V.; Staneva, T. G. Zhurnal Prikladnoi Spectro-
skopii 1975, 23, 820. (b) Sultanova, N.; Staneva, T. Proc. SPIE-Int. Soc.
Opt. Eng. 2003, 5226, 99. (c) Wakita, J.; Inoue, S.; Kawanishi, N.; Ando,
S. Macromolecules 2010, 43, 3594.
Supporting Information Available. Experimental pro-
cedures and full spectroscopic data for all compounds.
This material is available free of charge via the Internet at
Org. Lett., Vol. 13, No. 7, 2011
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