Asymmetry in ground and excited states in styryls and methoxystyryls detected by NMR ( 13 C), absorption, fluorescence and fluorescence excitation spectroscopy A.V. Stanova a,⇑ , A.B. Ryabitsky b , V.M. Yashchuk a , O.D. Kachkovsky b , A.O. Gerasov b , Ya.O. Prostota b , O.V. Kropachev b a Taras Shevchenko National University of Kyiv, Physics Department, 2 Glushkova Prosp., 03022 Kyiv, Ukraine b Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska str., 03660 Kyiv, Ukraine article info Article history: Received 31 August 2010 Received in revised form 18 December 2010 Accepted 19 December 2010 Available online 25 December 2010 Keywords: Cyanine dyes Electron transitions Styryls Methoxystyryls Quantum-chemical calculations Spectroscopy abstract Combined quantum-chemical and spectral study of electron structure features of styryls and their oxyan- alogues containing benzothiazolium, benzooxazolium, indoleninium, pyridium, quinolinium residues has been fulfilled. It showed that asymmetry degree of molecular geometry and charge distribution in the chromophore of styryls and methoxystyryls considerably differ in the ground and excited states. It was established that two the lowest transitions in styryls are splitting and involve both donor levels, similarly to symmetrical cyanines. If compare with methoxystyryls the long-wave high intensive absorption band is shifted bathochromically due to considerable interaction between the donor quasi-local chromophores. In contrary, because of the low position of a lone electron pair of oxygen in methoxystyryls, only one donor quasi-local chromophore is effective, hence such unsymmetrical dyes absorb appreciably higher. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Unsymmetrical cyanine dyes, including styryls and their methoxyanalogues, continue to be used widely, in parallel with corresponding symmetrical dyes, in numerous applications; they are widely used objects due to their unique electronic and spectral properties which form a basis for the design of new effective mate- rials connected with the light conversion, i.e. spectral sensitization, molecular probing in biology, active and passive components for tuneable lasers, non-linear media exploring excited state absorp- tion, etc. [1–4]. Also, symmetrical and unsymmetrical cyanine mol- ecules remain convenient objects for developing of new theoretical concepts and quantum-chemical models [5–9]. Going from sym- metrical dyes to unsymmetrical ones causes significant changes in distribution of the total positive charge in the polymethine chro- mophore, molecular geometry and hence in spectral properties [1,4,10–12]. The styryls were usually considered as unsymmetrical cyanines with an extremely high degree of asymmetry because of the low basicity of the p-dimethylaminophenylene residue as a terminal group. Nevertheless, the nature of the lowest electron transition is similar to the nature of the one in symmetrical cyanines. Inser- tion of an alkyloxy group with a high electronegative oxygen atom instead of amino group into styryls is likely to lead to more consid- erable change in the molecule electron structure of the dyes and in the nature of the electron transitions. It is expected that the spec- tral effects should depend on the difference in donor strengths of other terminal groups. So, we use absorption and fluorescence spectra or, more correctly, electron transitions, for studying the asymmetry of the excited state, whereas the degree of the asym- metry in the ground state can be obtained from NMR spectra. Also the quantum-chemical calculations give the information about the asymmetry in the charge distribution and molecular geometry in both ground and excited states. This paper presents the results of the simultaneous quantum- chemical simulations and spectral measurements of the absorption spectra of unsymmetrical styryls and methoxystyrys. 2. Experimental 2.1. Objects and methodology The general structures of investigated styryls 1 and correspond- ing methoxystyryls 2 containing indoleninium (In), benzothiazoli- um (BT), benzooxazolium (Ox), pyridium (Py) and quinolinium (Qu) residues are presented in Fig. 1. Also, as reference dyes, the symmetrical cyanines 3–5 will be considered. Methods of the syn- 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.12.038 ⇑ Corresponding author. Tel.: +38 067 19 49 208. E-mail address: diakova_a@ukr.net (A.V. Stanova). Journal of Molecular Structure 988 (2011) 102–110 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc