C O M M U N I C A T I O N S
Figure 2. The UV/vis spectrum (A) and fluorescence spectrum (B) of 1 (toluene, 0.67 µM, 20 °C) on addition of trifluoroacetic acid (0.1 M): (a) black line,
0 mol, (b) red line, 0.5 µmol, (c) green line, 1.0 µmol, (d) yellow line, 3.0 µmol, and (e) blue line, 10.0 µmol.
signal at highest field is assigned to the two hydrogen atoms located
between the imine nitrogen atoms.
residue remaining at 800 °C confirms the retention of N. This
suggests that pristine heterosuperbenzenes without substituents are
likely to be highly thermally stable and particularly suitable for
many LED applications where thermally sensitive polymer materials
fail.
In conclusion the essential steps in the formation of intrinsically
nitrogen-doped graphite nanostructures in a controlled and system-
atic manner have been proven. The presence of imine N atoms has
rendered overall electron-accepting properties to 1 compared to its
all-C analogue. The preliminary emission studies undertaken on 1
suggest that the optoelectronic properties of this emerging hetero-
superbenzene family will be promising. Synthetic control of the
extent and position of N doping such as demonstrated here allows
molecular tuning of the optoelectronic properties of the resultant
material and provides the potential for ligand-based functionality.
1 formally combines superbenzene and bipyrimidine and ex-
perimentally exhibits desirable properties of both. The enhanced
π-electron mobility of 1 is evident from the polar nature of the
molecule ,which is supporting structurally opposing electronegative
nitrogen atoms and weakly donating tert-butyl groups. 1 is soluble
in a variety of organic solvents (polar and nonpolar). A UV-visible
spectrum recorded in toluene (unsubstituted graphenes are insoluble
in this solvent) shows the characteristic bands of hexa-peri-
benzocoronene (in dichlorobenzene) together with two weak bands
at 450 and 490 nm (Figure 2A(a)). There is also a characteristic
band at 372 nm that is not found in the all-benzene analogue.7 A
reduction in ꢀmax (355 nm, ꢀmax 140 000) compared to the hexa-
peri-benzocoronene (ꢀmax 177 000) is observed in agreement with
the depletion of π-electron density as a result of the imine nitrogen
atoms, thus rendering the molecule overall electron-acceptor
characteristics.
Acknowledgment. This research is funded by an Enterprise
Ireland Basic Research Award SC/00/168. We thank Dr. A. P.
Davey for his advice.
It has been reported that the linear extension of π-conjugation
on phenanthroline gives a vast increase in the fluorescence of the
resulting material.8 We observe a similar dramatic effect but the
extension to our π-system is supramolecular rather than linear. 1
exhibits strong green emission under both visible and UV light. A
very strong fluorescent band (quantum yield 4.00 ( 0.01) was
observed in toluene at 545 nm by exciting at 355 nm. The
unstructured nature of the band suggests strong interaction between
the solvent and the polarized compound in the excited state.
Protonation of the peripheral nitrogen atoms by the gradual addition
of acid quenches the fluorescence (Figure 2B) and changes the UV/
vis spectrum (Figure 2A). The decrease in intensity of the 355 nm
absorption band along with the disappearance of the 375 nm band
in the UV/vis spectrum indicates that nitrogen protonation has a
profound electronic effect on the π-electron density throughout the
system. The UV/vis spectrum is independent of concentration in
the region studied (10-1 to 10-7 M).
Supporting Information Available: Full experimental and spec-
troscopic details for all the compounds reported and HMQC, 13C NMR
data, ESI-mass spectrum, and thermogravimetric analysis of 1 (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Watson, M. D.; Fechtenkotter, A.; Mullen, K. Chem. ReV. 2001, 101,
1267-1300.
(2) Hyatt, J. A. Org. Prep. Proced. Int. 1991, 23, 460-463.
(3) Ogliaruso, M. A.; Romanelli, M. G.; Becker, E. I. Chem. ReV. 1965, 65,
261-367.
(4) Mueller-Westerhoff, U. T.; Zhou, M. J. Org. Chem. 1994, 59, 4988-
4992.
(5) des Abbayes, H.; Clement, J.; Laurent, P.; Tanguy, G.; Thilmont, N.
Organometallics 1988, 7, 2293-2299.
(6) Iyer, V.; Wehmeir, M.; Brand, J.; Keegstra, M.; Mu¨llen, K. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1604-1607.
(7) Hendel, W.; Khan, Z. H.; Schmidt, W. Tetrahedron 1986, 42, 1127-
1134.
Thermogravimetric analysis of 1 indicates compound stability
to around 450 °C. Decomposition at this temperature agrees with
mass loss of the flexible tert-butyl groups. Examination of the
(8) Joshi, H. S.; Jamshidi, R.; Tor, Y. Angew. Chem., Int. Ed. Engl. 1999,
38, 2721-2725.
JA017394U
9
J. AM. CHEM. SOC. VOL. 124, NO. 14, 2002 3487