DOI: 10.1002/chem.200802428 Ground- and Excited-State Tautomerization Rates in Porphycenes P. Fita, [a] N. Urban ´ ska, [b] C. Radzewicz,* [a, b] and J. Waluk* [b] Introduction A unique kind of a chemical reaction occurs when the reac- tant and the product are formally identical. Such systems offer an opportunity to investigate the phenomena resulting from quantum delocalization of the wave function over sym- metrical minima, such as tunneling splitting or transition from a coherent tunneling regime in an isolated molecule to a rate process in condensed phases. Monitoring a self-ex- change reaction is not a trivial task, since the standard tech- niques, which make use of different properties of the reac- tants and products, are not applicable. In this work, a proce- dure which overcomes this difficulty is described. It allows one to determine self-exchange reaction rates by means of ultrafast pump–probe polarization spectroscopy. This meth- odology is applied to the study of double hydrogen transfer in porphycene and its derivatives. With an appropriate choice of monitoring wavelengths, it is possible to determine the rates of tautomerization in both the ground and lowest excited electronic states. For various alkylated porphycenes studied here, the room-temperature reaction rates differ by more than three orders of magnitude, the reaction being much faster in the derivatives with stronger intramolecular hydrogen bonds. Substantial deuterium isotope effects sug- gest the importance of tunneling even at 293 K. The motion of two hydrogen atoms in the inner cavity composed of four nitrogen atoms has been studied intensely for porphyrin and its derivatives, mainly using NMR tech- niques. [1–5] The reaction involves a self-exchange-type inter- conversion between two chemically identical trans tautomers with protons located on the opposite pyrrole rings. Applica- tions of this phenomenon to information storage have been suggested. [6, 7] The investigations of the reaction mechanism address such basic notions as cooperativity (stepwise vs. con- certed transfer), role of tunneling, or mode selectivity. It is now accepted that the tautomerization proceeds in a step- wise fashion and involves thermally activated tunneling of a single hydrogen. [2–4] This leads to a cis tautomer, with pro- tons located on the adjacent pyrrole rings, from which the molecule either returns to the initial trans species or, through the transfer of a second hydrogen atom, is convert- ed into the other trans tautomer. Tautomerization in porphycene (1), a constitutional isomer of porphyrin, occurs much faster [8, 9] and, most proba- bly, according to a different mechanism. [10] Our recent stud- ies of the reaction in the lowest excited singlet state using fluorescence anisotropy suggested that for low temperatures a concerted trans–trans tunneling is promoted by a low-fre- quency mode, which simultaneously strengthens both NH···N hydrogen bonds. [11, 12] In this work, we present a novel technique for determina- tion of tautomerization rates in both ground and excited Keywords: femtochemistry · polari- zation spectroscopy · porphycenes · proton transfer · tautomerism Abstract: The rates of double hydrogen transfer in the ground and excited elec- tronic states have been measured for porphycene and its derivatives by using a new method based on pump–probe polarization spectroscopy. Changing the strength of two intramolecular hydrogen bonds by altering the NH···N distance leads to differences in the tautomerization rate exceeding three orders of magni- tude. The reaction is considerably slower in the lowest electronically excited state. A correlation was found between the tautomerization rates and 1 H chemical shifts of the internal protons. [a] Dr. P. Fita, Prof.Dr. C. Radzewicz Institute of Experimental Physics University of Warsaw, Hoz ˙a 69 00-681 Warsaw (Poland) Fax: (+ 48) 22-6256406 E-mail: radzewic@fuw.edu.pl [b] Dr. N. Urban ´ska, Prof. Dr. C. Radzewicz, Prof. Dr. J. Waluk Institute of Physical Chemistry Polish Academy of Sciences, Kasprzaka 44 01-224 Warsaw (Poland) Fax: (+ 48)-22-6311619 E-mail: waluk@ichf.edu.pl Chem. Eur. J. 2009, 15, 4851 – 4856  2009 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 4851 FULL PAPER