Humboldt University of Berlin.

Free University of Berlin.

Russian Academy of Sciences.

University of Bremen.

Present address: Institute of Physical Chemistry, Friedrich-Schiller-University of Jena, Helmholtzweg 4, 07743 Jena, Germany.

Present address: Max-Planck-Institute of Colloids and Interfaces, Department of Interfaces, Am Mühlenberg 1, 14476 Potsdam-Golm, Germany.

Present address: PicoQuant GmbH, Rudower Chaussee 29, 12489 Berlin, Germany.

The results of steady-state and time-resolved absorption and fluorescence experiments as well as quantum mechanical density functional theory (DFT) calculations of metal-free and Zn(II) mononuclear and dinuclear (sharing a common benzene ring) phthalocyanines are presented. A detailed comparison between measured and calculated absorption spectra of all compounds is done, showing a good agreement between theory and experiment. The NH tautomerization for phthalocyanines with an extended π-electron system was shown for the first time at room temperature. The photophysical properties of all possible NH tautomers of metal-free dinuclear Pc have been fully characterized. In the first tautomer, Pc, both pairs of hydrogen atoms are parallel to the connection line of two Pc units. The maximum of the lowest-energy Q absorption band, λ_abs, in Pc is located at 832 nm, whereas the spectral position of the fluorescence maximum lies at λ_fl = 837 nm. The second NH tautomer, Pc (λ_abs = 853 nm, λ_fl = 860 nm), presents the two pairs of hydrogen atoms perpendicularly orientated to the covalent axis, and the third one, Pc^mix (λ_abs = 864 nm, λ_fl = 872 nm), contributing in a minor extend to the absorption and fluorescence spectra of the metal-free dinuclear phthalocyanine, has one perpendicular and one parallel pair of hydrogen atoms. Obviously, only one configuration exists in the case of the Zn(II)-containing dinuclear phthalocyanine (λ_abs = 845 nm, λ_fl = 852 nm).