from the in-plane to the out-of-plane orbitals explains the
differences between the intramolecular ESHT of photolabile
and photoprotecting compounds. For o-NBA, the photo-
tautomerization is photochemically allowed and thermally
forbidden, similar to the [2+2] cycloaddition textbook
example. The back hydrogen transfer is prevented by a barrier,
and the new tautomer reacts further to yield the final product.15
The electronic relocation also makes the reverse tautomerization
of o-nitrotoluene slow16 and the phototautomerization of
o-acylbenzaldehyde irreversible.17 In contrast, in the aromatic
photoprotectors the number of in-plane and out-of-plane
electrons remains constant during the tautomerization. As a
consequence, the back hydrogen transfer is barrierless and
exothermic, laying the ground for the photostability.
Fig. 4 Energy profiles for decay of the (p,p*) state (blue line) after
258 nm excitation. Inset: orbitals involved in the excitation.
We thank MICINN (Spain) for grants CTQ2008-00696 and
HA2006-0096 (LB) and a Juan-de-la-Cierva contract (IC);
AGAUR (Spain) for doctoral (FF) and B.-de-Pinos (AM)
´
fellowships; CNCT (Mexico) for a doctoral fellowship (VL);
COST Action CUSPFEL; Deutsche Forschungsgemeinschaft
(Germany) for grant GI 349/1-2 (PG).
Notes and references
1 M. Rini, B. Z. Magnes, E. Pines and E. T. J. Nibbering, Science,
2003, 301, 349; C. Tanner, C. Manca and S. Leutwyler, Science,
2003, 302, 1736; T. Schultz, E. Samoylova, W. Radloff,
I. V. Hertel, A. L. Sobolewski and W. Domcke, Science, 2004,
306, 1765.
2 A. Douhal, F. Lahmani and A. H. Zewail, Chem. Phys., 1996, 207,
477.
3 G. Ciamician and P. Silber, Ber. Dtsch. Chem. Ges., 1901, 34, 2040;
R. W. Yip and D. K. Sharma, Res. Chem. Intermed., 1989, 11, 109.
4 M. V. George and J. C. Scaiano, J. Phys. Chem., 1980, 84, 492;
S. Kuberski and J. Gebicki, J. Mol. Struct., 1992, 275, 105.
5 S. Laimgruber, W. J. Schreier, T. Schrader, F. Koller, W. Zinth
and P. Gilch, Angew. Chem., Int. Ed., 2005, 44, 7901.
6 B. Heinz, T. Schmierer, S. Laimgruber and P. Gilch, J. Photochem.
Photobiol., A, 2008, 199, 274.
Fig. 5 Energy profiles along the CI cascade on the ketene state (red
line, path 2) and access from the bright (p,p*) state (blue line).
decay path has not been optimized because of the high density
of states (see ESIw), but internal conversion to S1 is consistent
with the detection of a weakly fluorescent (n,p*) state after the
decay of the (p,p*) state.6 The hydrogen transfer from
(n,p*)Min probably competes with population of the triplet
manifold from this minimum.7
7 S. Laimgruber, T. Schmierer, P. Gilch, K. Kiewisch and
J. Neugebauer, Phys. Chem. Chem. Phys., 2008, 10, 3872.
8 V. Leyva, I. Corral, T. Schmierer, B. Heinz, F. Feixas, A. Migani,
L. Blancafort, P. Gilch and L. Gonzalez, J. Phys. Chem. A, 2008,
´
For the direct tautomerization mechanism along the CI
cascade (path 2, see Fig. 5) we have optimized three CIs
corresponding to points d, c, and b. They are crossings of
the ketene state with a higher lying reactant state ((S3/S2)X),
the aldehyde state ((S2/S1)X), and the intermediate (n,p*) state
((S1/S0)X-Ket). They lie along the hydrogen transfer coordinate,
where the energy of the ketene state decreases steeply. This
path goes through the syn-Ket conformer and continues to
anti-Ket after rotation of the newly formed OH bond. The
barrier to access the cascade from (p,p*)Min has been estimated
by locating the crossing between the (p,p*) and ketene states
with a linear interpolation (e in Fig. 5). The resulting value of
0.8 eV is an upper limit to the actual value (see ESIw), but it
suggests that internal conversion to S1 and stepwise ESHT
along path 1 will be dominant after excitation at 258 nm.
Tunneling could contribute to path 2 because (p,p*)Min is
prearranged for the hydrogen transfer, but the cascade will
be more relevant for higher excitation energies.
112, 5046; T. Schmierer, W. J. Schreier, F. O. Koller,
T. E. Schrader and P. Gilch, Phys. Chem. Chem. Phys., 2009, 11,
11596.
9 T. P. Causgrove and R. B. Dyer, Chem. Phys., 2006, 323, 2.
10 P. Klan, A. P. Pelliccioli, T. Pospisil and J. Wirz, Photochem.
Photobiol. Sci., 2002, 1, 920.
11 S. Lochbrunner, T. Schultz, M. Schmitt, J. P. Shaffer,
M. Z. Zgierski and A. Stolow, J. Chem. Phys., 2001, 114, 2519;
J. D. Coe and T. J. Martinez, Mol. Phys., 2008, 106, 537;
A. Migani, L. Blancafort, M. A. Robb and A. D. DeBellis,
J. Am. Chem. Soc., 2008, 130, 6932.
12 M. Wiechmann, H. Port, F. Laermer, W. Frey and T. Elsaesser,
Chem. Phys. Lett., 1990, 165, 28; M. J. Paterson, M. A. Robb,
L. Blancafort and A. D. DeBellis, J. Am. Chem. Soc., 2004, 126,
2912; A. L. Sobolewski, W. Domcke and C. Hattig, J. Phys. Chem. A,
2006, 110, 6301.
13 H. J. Kuhn, S. E. Braslavsky and R. Schmidt, Pure Appl. Chem.,
2004, 76, 2105.
14 V. Leyva, I. Corral, F. Feixas, A. Migani, L. Blancafort,
´ ´ ´
J. Gonzalez-Vazquez and L. Gonzalez, manuscript under revision.
15 M. L. Donten, P. Hamm and J. VandeVondele, J. Phys. Chem. B,
2011, 115, 1073–1083.
16 T. Schmierer, S. Laimgruber, K. Haiser, K. Kiewisch,
J. Neugebauer and P. Gilch, Phys. Chem. Chem. Phys., 2010, 12,
15653.
17 J. C. Netto-Ferreira and J. C. Scaiano, Can. J. Chem., 1993, 71,
1209.
To conclude, the global mechanistic picture for the ultrafast,
irreversible phototautomerization of o-NBA consists of a
stepwise path involving the (n,p*) state and a direct path with
early access to the ketene state. The relocation of two electrons
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6383–6385 6385