is not good enough for precise discussion due to disorder,
nonplanarity of dibenzophosphaborin ring is obviously
strengthened compared to that of 2a derived from steric
repulsion of the mesityl group on the phosphorus atom (see
the Supporting Information).
the nitrogen atom because the planar structure around the
nitrogen atom of azaborine is not widely affected by its
substituents.
Phosphaborins 2a and 2b showed different fluorescence
spectra (Figure 3). The fluorescence spectra of 2a were
Density functional calculations8 (B3LYP/6-31G(d) level)
were performed on model compounds 2c and 2d.9 The
optimized structure for 2c reproduced the molecular structure
of 2a almost completely. The frontier molecular orbitals are
delocalized over the phosphaborin rings, but the HOMO and
LUMO have the significant coefficients of a lone pair on
the phosphorus atom and a vacant 2p orbital on the boron
atom, respectively. Natural bond orbital (NBO) analysis was
performed on 2c in order to reveal further insight into the
electronic structure of phosphaborin.10 NBO analysis showed
that the highest occupied natural localized molecular orbital
corresponds to the phosphorus lone pair and the lowest
unoccupied natural localized molecular orbital is the vacant
2p orbital on the boron atom. The electronic excitation of
phosphaborin is probably derived from ICT from the
phosphorus atom to the boron atom. Second-order perturba-
tion analysis indicates strong electron donation from πCC to
2pB*, while the delocalization of the phosphorus lone pair
to πCC* is weak. These data are in contrast to a structural
and theoretical investigation on the electronic structure of
the azaborine that the donation from the nitrogen lone pair
to πCC* is stronger than the πCC-2pB* interaction.11
Figure 3. Fluorescence spectra of 2a (298 K, cyclohexane,
excitation at 368 nm, (orange) 1.0 × 10-3 M, (green) 1.0 × 10-4
M, (blue) 1.0 × 10-5 M) and 2b (298 K, cyclohexane, 8.0 × 10-5
M, excitation at 393 nm).
The UV-vis spectra showed absorption maxima in the
near-ultraviolet region (λmax 368 nm for 2a, 393 nm for 2b).
These absorptions are derived from the ICT as predicted by
TD-DFT calculations on model compounds (2c, λmax 365
nm; 2d, λmax 382 nm).12 The red shift of the absorption in
2b is explained by planarization around the phosphorus atom
induced by the bulky mesityl group of 2b and the consequent
elevation of the HOMO level.13 The UV-vis absorption
maxima of azaborine do not depend on the substituent of
concentration-dependent in the range of 1.0 to 0.010 mM in
cyclohexane solution. In the diluted solution, the intensity
of the broad emission near 560 nm was weakened and the
sharp emission at 410 nm increased. The fluorescence of 2b
(φFL 0.1614), however, was independent of its concentration.
Our interpretation of the broad emission observed near 560
nm is that 2a forms an excimer upon excitation when in a
concentrated solution; 2b does not appear to form an excimer
because of steric repulsion around the phosphorus atom,
resulting in monomer-derived emission. In the case of
azaborine, the emission is independent of concentrations
regardless of the reduced bulkiness of substituents on the
nitrogen atom. Thus, the excimer formation of 2a is
considered a specific characteristic to phosphaborin, which
has a more active lone pair than azaborine.
The monomer emission of 2a does not show significant
Stokes shift (40 nm), but 2b exhibits rather large Stokes shift
(115 nm). Structural relaxation from excited-state induces
Stokes shift, and 2b should experience greater structural
change around phosphorus atom than 2a upon photoexcita-
tion. Both crystallographic analysis and theoretical calcula-
tions indicate the phosphorus atom of 2b deviates from
phosphaborin ring much more than that of 2a. The phos-
phorus atom of 2b seems to move toward the mean plane
defined by the five atoms of the phosphaborin ring other
than phosphorus atom upon photoexcitation, resulting in a
larger Stokes shift.
(8) Gaussian 03, Revision B.05: Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.,
Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;
Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda,
R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,
V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D.
K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,
Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
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Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and
Pople, J. A. Gaussian, Inc., Wallingford CT, 2004.
(9) To reduce the computational time, 2c and 2d, which have phenyl
groups instead of mesityl groups on boron atoms, were used. These
simplifications rarely affected the structure of phosphaborin rings.
(10) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899-926.
(11) Kranz, M.; Hampel, F.; Clark, T. Chem. Commun. 1992, 1247-
1248.
(12) (a) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218-8224. (b) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys.
Lett. 1996, 256, 454-464. (c) Casida, M. E.; Jamorski, C.; Casida, K. C.;
Salahub, D. R. J. Chem. Phys. 1998, 108, 4439-4449.
(13) Sasaki, S.; Sutoh, K.; Murakami, F.; Yoshifuji, M. J. Am. Chem.
Soc. 2002, 124, 14830-14831.
(14) The fluorescence quantum yield (φFL) of 2b was determined at 298
K using 9,10-diphenylanthracene as a standard in cyclohexane. The φFL of
2a could not be determined due to the excimer formation and weakness of
emission.
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