6102 J. Phys. Chem. A, Vol. 104, No. 25, 2000
Elisei et al.
f S2 transition, with 3,6-diOH substitution causing the largest
red-shift. The exception is 5,7-diOHFT.
The position of OH substitution and the number of OH groups
added can cause some dramatic differences in photophysical
behavior. These are caused by the relative state order changes
of the two lowest excited singlet and triplet states.
photosensitizers via a H-atom extraction mechanism or act via
formation of singlet oxygen in an environment where H-atom
abstraction was relatively noncompetitive (as in a benzene-like,
acetonitrile-like, or perhaps a water environment).
Acknowledgment. This work was supported by the Fun-
dac¸a˜o para o Desenvolvimento da Cieˆncia e Tecnologia (FCT),
Portugal, Grant PRAXIS/2/2.1/QUI/324/94. R.S.B. acknowl-
edges the G.G.P. XXI/BCC/3638/96 grant, J.C.L. and I.A. are
grateful for the PRAXIS 4/4.1/BPD/3410 and PRAXIS 4/4.1/
BD/3589 grants, respectively. F.E., F.O., and G.G.A. are
thankful for the grant from the Italian Consiglio Nazionale delle
Ricerche and for the support of the National Project by the
Ministero dell’Universita` e della Ricerca Scientifica e Tecno-
logica and the University of Perugia.
The 6-OH or 7-OH substitution (group 1) on FT does not
change the nature of the lowest triplet state (n, π*) or its lifetime
compared with FT. In all solvents these have a high φT (0.8-1,
including FT). The φ∆ is generally in the region of ∼0.6 in
benzene and decreases somewhat to 0.5 for 6-OH and still
lowers for 7-OH (0.3) in MeCN and TFE. There is a large
decrease in φ∆ in ETOH, as there is for τT, and these are caused
by an H-atom abstraction reaction from the solvent (triplet n,
π* state lowest) competing with energy transfer to oxygen.
The 3-OH or 3,6-diOH substitution (group 2) results in a
change in the configurational nature of the lowest triplet state
from n, π* (FT, 6-OHFT, and 7-OHFT, group 1) to π, π*. Also,
φT does not change from ∼1. Moreover, φ∆ is ∼0.6 (as for FT,
6-OHFT, and 7-OHFT) in all solvents including ETOH, which
is in marked contrast to the large decrease found for FT,
6-OHFT, and 7-OHFT. The latter result is explained by the fact
that there is essentially no H-atom abstraction reaction compet-
ing with energy transfer to oxygen for the 3-substituted cases
because of the change in the configurational origin of the lowest
triplet state.
The 5-OH or 5,7-diOH substitution results in a very marked
change in the photophysical (and φ∆) properties compared to
all other hydroxy substitutions. Essentially no triplet occupation
is observed by direct excitation, and φ∆ is essentially zero in
all solvents (four considered). Also, no fluorescence or phos-
phorescence are observed. The 5-methoxy compound also
showed parallel behavior but did exhibit a weak fluorescence.
The results are understandable on the basis of the energy
relationship among the two lowest singlet and triplet states of
n, π* and π, π* character, as well some proton transfer from
S2 for the hydroxy derivatives.
References and Notes
(1) Becker, R. S.; Mac¸anita, A. L. ReV. Port. Quim. 1995, 2, 30-44.
(2) Becker, R. S.; Chakrovorti, S.; Gartner, C. A.; Miguel, M. da G. J.
Chem. Soc., Faraday Trans. 1993, 89, 1007-1019.
(3) Maciejewski, A.; Szymanski, M.; Steer, R. P., J. Photochem.
Photobiol. A, 1996, 100, 43-52 and references therein.
(4) Scheeren, J. W.; Ooms, P. H. J.: Nivard, R. J. F. Synthesis 1973,
149-151.
(5) Still, W. C.; Kakin, M.; Mitra, A. In Rapid Chromatographic
Techniques for PreparatiVe Separations with Moderate Resolution; Co-
lumbia University: New York, 1978.
(6) Lima, J. C.; Abreu, I.; Santos, M. L.; Brouillard, R.; Mac¸anita, A.
L. Chem. Phys. Lett. 1998, 298, 189-195.
(7) Go¨rner, H.; Elisei, F.; Aloisi, G. G. J. Chem. Soc., Faraday Trans.
1992, 88, 29-34.
(8) Romani, A.; Elisei, F.; Masetti, F.; Favaro, G. J. Chem. Soc.,
Faraday Trans. 1992, 88, 2147-2154.
(9) Carmichael, I.; Hug, G. L. J. Chem. Phys. Ref. Data 1986, 15,
1-204.
(10) Murov, S. L.; Charmichael, I.; Hug, G. L. In Handbook of
Photochemistry; Marcel Dekker Inc., New York, 1993.
(11) Kumar, C. V.; Qin, L.; Das, P. K. J. Chem. Soc., Faraday Trans.
2 1984, 80, 783-793.
(12) Elisei, F.; Aloisi, G. G., Lattarini, C.; Latterini, L.; Dall’Acqua,
F.; Guiotto, A. Photochem. Photobiol. 1996, 64, 67-74.
(13) Schmidt, R.; Tanielian, C.; Dunsbach, R.; Wolff, C. J. Photochem.
Photobiol., A 1994, 79, 11-17.
(14) Zerner, M. C. Semiempirical Molecular Orbital Methods. In ReViews
in Computational Chemistry, 2; Lipkowitz, K. B., Boyd, D. B., Eds.; VCH
Publishers Inc.: New York, 1991; pp 313-365 and references therein.
(15) Becker, R. S. In Theory and Interpretation of Fluorescence and
Phosphorescence; Wiley-Interscience: New York, 1969; pp 163-167.
(16) Lima, J. C.; et al. To be published.
It is highly probable that the 5-OH and 5,7-diOH compounds
would not be good photosensitizers since they show no triplet-
state occupation and a quenched excited singlet-state behavior.
On the other hand, the best compounds as potential photosen-
sitizers via a singlet-oxygen mechanism (or possibly an electron-
transfer process as well) would be those of group 2, the 3-OH
and 3,6-diOH. The group 1 compounds could also be potential
(17) El Sayed, M. A. J. Chem. Phys. 1964, 41, 2462-2467 and
references therein.