Transient absorption spectroscopy of a heteroaromatic donor–acceptor-p-conjugated 2,2 0 -bipyridine dye S. Mosquera Vázquez a , A. Abbotto b , F. De Angelis c,d , P. Foggi a,d,⇑ , A. Lapini a , M. Lima a , M. Lobello c,d , A. Marcelli a , P. Tourón Touceda a a LENS, University of Florence, via Nello Carrara 1, I-50019 Sesto Fiorentino (FI), Italy b Department Sci. Mat., University of Milano Bicocca, UdR INSTM, via Cozzi 53, I-20125 Milan, Italy c Istituto CNR di Scienze e Tecnologie Molecolari (CNR-ISTM), via Elce di Sotto 8, I-06100 Perugia, Italy d Department of Chemistry, University of Perugia, via Elce di Sotto 8, I-06100 Perugia, Italy article info Article history: Available online 26 January 2011 Keywords: Energy transfer Charge transfer Excited states dynamics Transient absorption Ultrafast spectroscopy abstract We report on a study of the excited state dynamics of a heteroaromatic 4,4 0 -p-conjugated 2,2 0 -bipyridine dye, bearing conjugated p-excessive and p-deficient heteroaromatic rings as donor and acceptor substit- uents, by means of steady state and transient absorption spectroscopy in EtOH solutions and a room tem- perature. The ground state absorption shows an intense intramolecular charge transfer band in the visible region with a maximum at 515 nm. Excited state dynamics is measured by pumping at 480 nm at shorter wavelengths from the absorption maximum and probing at specific wavelengths corresponding to bleaching, stimulated emission and at the maximum of excited state absorption. The spectral evolution is also evaluated in the frequency domain as a function of the delay between pump and probe pulses. By means of SVD analysis two components with time constants around 2 ps and 150 ps are identified. In addition a relevant dynamical Stokes shift of the stimulated emission occurs in the first 60 ps which is attributed to solvent dynamics. Ab initio calculations allows to assign the ground spectral features and the excited state conformation. The geometry of the ground and of the first excited state are optimized both in the cisoid and in the transoid forms. The latter results more stable of about 0.4 eV. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction The symmetrical 2,2 0 -bipyridine (bpy) ligand is a common li- gand extensively used to chelate metal cations to form charged complexes [1,2]. Many metal-bpy complexes have been synthe- sized and investigated to be used for applications in non-linear op- tics (NLO)[3] and photovoltaics [4,5]. It is well established that it is necessary to use polar and polarizable bipyridine ligands with ex- tended p-conjugated frameworks to enhance the non-linear opti- cal response and to increase the molar extinction coefficient in the visible region. Molecular energy levels, structural and elec- tronic characteristics ruling materials properties closely depend on molecular symmetry and on the presence of polar p-donor (D) and p-acceptor (A) molecular subunits [6–8]. In order to better understand the link between electronic structure and non-linear optical responses it is necessary to combine results obtained by lin- ear and non-linear spectroscopic techniques and by ab initio calcu- lations. In addition by utilizing ultrashort pulses it is possible to characterize the dynamics of excited states which rule the charge transfer processes. We herein present the results of a spectroscop- ical study by steady state and sub-picosecond transient absorption spectroscopies (TAS) of a heteroaromatic p-conjugated bpy with a bpy-p-D-p-A symmetry (from now on indicated bpy-dye). The bpy-dye contains a pyrrole as a donor (D) and a pyridinium as a second acceptor (A) in addition to bpy while the spacers are ethe- nylic units (see Scheme 1). Ab initio calculations allowed us to identify the most stable structure and the nature of the first excited state. 2. Experimental and computational methods Synthesis and purification of the bpy-dye used in this investiga- tion are described elsewhere [9]. The dye purity was confirmed by 1 H NMR, 13 C NMR, HRMS-ESI and microanalysis. The solution was obtained by dissolving the solid in spectroscopic grade ethanol. UV–Vis absorption measurements were performed on a Perkin Elmer Lambda 950 UV/Vis/NIR Spectrophotometer. Fluorescence spectra were recorder on a standard spectrofluorimeter (Perkin El- mer LS55). For fluorescence quantum yield measurements, the absorbance at excitation wavelengths were kept below 0.1. Cou- marin 153 in EtOH (U = 0.26) and Coumarin 120 in MeOH (U = 0.51) were used as standards. 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.01.042 ⇑ Corresponding author at: LENS, University of Florence, via Nello Carrara 1, I-50019 Sesto Fiorentino (FI), Italy. Tel.: +39 055 4572497. E-mail address: foggi@lens.unifi.it (P. Foggi). Journal of Molecular Structure 993 (2011) 464–469 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc