Nanocomposites and nanomaterials

Detection of two relaxed forms in excited state
 in  2-aazulene cation by time-resolved spectroscopy

P. Pyryatinsky1, P. Lutsyk1,2,, J.Briks5 , A. Naumenko3 ,S. Vasylyuk3*, O. Kachkovsky4, A. Verbitsky1, A. Rozhin2
•	1Institute of Physics, National Academy of Sciences of Ukraine, 46, prospekt Nauky, 03680 Kyiv, Ukraine.
•	2Nanotechnology Research Group & Aston Institute of Photonic Technologies,
School of Engineering & Applied Science, Aston University, Aston Triangle, B4 7ET Birmingham, UK.
•	3 Faculty of Physics, Taras Shevchenko National University of Kyiv, 64/13, Volodymyrska Street, 01601, Kyiv, Ukraine
•	4Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 1, Murmanska str., 02660 Kyiv, Ukraine.
•	5 4Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5, Murmanska str., 02660 Kyiv, Ukraine
•	*Corresponding author: svitlanafvasylyuk@gmail.com

The 2-azaazulene (cyclohepta-[c]pyrrole) with its extended -electron system is one from deep colored heterocycles, especially, its alkylated salt, which absorb in visible spectral region [1]. Furthermore, 2-azaazulene residue can be used as terminal groups in the polymethine dyes; such significant extension of the conjugated system provides intense long-wavelength absorption (in near IR region) in the corresponding cationic cyanine dyes despite comparatively short polymethine chain [2-4]. The joint spectral and quantum-chemical study has established that the specific local orbitals exist in the 2-azaazulene and hence in the polymethine dyes with such terminal groups; just they can cause the specific spectral properties of the synthesized dyes: non-trivial fluorescence excitation anisotropy [5]. 
The joint spectral and quantum-chemical investigations of relaxation paths of symmetrical polymetine dyes are performed. They have shown existence of three relaxation paths: one symmetrical path with a conservation of the symmetry for the electronic structure and two unsymmetrical paths with various symmetry breaking mechanisms. All three components are experimentally detected by time-resolved fluorescent spectroscopy at low temperatures in polar medium. The fluorescent paths differ by their lifetime and spectral shape (band maxima) as well as strongly depend on rigidity and polarity. Reported studies give a significantly increased fundamental understanding of molecular electronic relaxations in excited state of symmetrical polymethine dyes.

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