Altering the Emission Behavior with the Turn of a Thiophene Ring: The Photophysics of Condensed Ring Systems of Alternating Benzenes and Thiophenes Brigitte Wex,* ,‡,§ Bilal R. Kaafarani, | Evgeny O. Danilov, ‡ and Douglas C. Neckers* ,‡ Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403, and Department of Chemistry, American UniVersity of Beirut, Beirut 11-0236, Lebanon ReceiVed: August 27, 2006; In Final Form: October 31, 2006 Six aromatic compounds with embedded thiophenes differing in the number of rings (2-5) and thiophene orientation along the long axis of the molecule (syn, anti) were investigated. Photophysical properties, steady- state absorption, fluorescence, phosphorescence, lifetimes, quantum yields, and a comprehensive time-resolved spectroscopic analysis (femtosecond and nanosecond transient absorption spectroscopy) have been studied as a function of molecular structure. 1. Introduction Ladder-type, sulfur-containing ring compounds are an im- portant source of materials for optoelectronic device applica- tions, such as organic light-emitting diodes, organic field-effect transistors, and solar cells. 1-5 Fused, ladder-type materials have inherent advantages such as extended pi-frameworks, favorable stacking behavior in the solid state, conductivity, and high field- effect mobility. 6-11 While the rich excited-state properties of thiophene, oligo- and polymeric derivatives, have been exten- sively studied, 12-16 condensed S-containing aromatic systems have rarely been investigated systematically. 17-20 In particular, efforts to characterize the rich excited-state properties of these aromatic systems in the (sub)nanosecond time domain are scarce. 21-23 In this context, we have synthesized a series of well- defined, isomer-pure fused ring systems 11,24,25 and report herein the singlet- and triplet-state photophysical properties of a series of compounds composed of well-defined alternating benzene and thiophene moieties, Figure 1. We used steady-state absorp- tion, fluorescence, and phosphorescence spectroscopy, as well as femto- and nanosecond time-resolved spectroscopy. Compounds 3 and 4 contain one thiophene ring more than benzothiophene (1) in anti and syn orientation, respectively. Compounds 5 and 6 are derived from compound 2 with two additional thiophene moieties in the anti and syn orientation, respectively. We believe this to be the first report of the femtosecond (fs) transient absorption signals for this complete series of compounds 1-6. 2. Experimental Section 2.1. Materials. Benzothiophene (1) was distilled prior to use. Dibenzothiophene (2, sublimed, 99%) was used as received. Benzo[1,2-b:4,5-b′]dithiophenes (3), 9 benzo[1,2-b:5,4-b′]dithi- ophenes (4), 26 thieno[2,3-f:5,4-f ′]bis[1]benzothiophene (5), 24 and thieno[3,2-f:4,5-f ′]bis[1]benzothiophene (6) 25 were synthesized according to the detailed procedures published elsewhere. 2.2. Steady-State Spectroscopic Measurements. UV-vis absorption spectra were acquired on a Shimadzu UV-visible spectrophotometer (UV-2401 PC and Multispec-1501). Fluor- escence spectra were acquired in ethanol and in methylcyclo- hexane (MCH) on a SPEX Fluorolog3-11 using a 450 W xenon short arc and sample detection in a 90° geometry. Fluorescence lifetimes were determined using time-correlated single-photon counting (TCSPC) in MCH under ambient conditions as previously described. 27 Phosphorescence spectra were acquired using a SPEX 1934D3 phosphorimeter on a Fluorolog 3 spectrometer utilizing a UV xenon flash lamp (450 W Xe lamp, R928 Hamamatsu single-photon counting photomultiplier tube (PMT) detector). Emission detection in phosphorescence mode with a delay time g50 μs was carried out in a 90° geometry. Emission spectra were acquired in a matrix of diethyl ether, isopentane, ethyl alcohol (EPA, 5/5/2) using quartz NMR tubes (o.d. ) 5 mm) immersed in liquid nitrogen in a quartz dewar flask. Phosphorescence quantum yields were determined using biphenyl [Φp ) 0.24] as standard, 28 and phosphorescence lifetimes were determined using decay by delay method. 2.3. Transient absorption spectroscopy was carried out with sample solutions of 0.5-1.0 OD in MCH at the excitation wavelength. The solutions were filtered (PTFE filter, 0.45 μm) and checked for decomposition before and after laser excitation using UV-vis spectrometry. A detailed description of the * To whom correspondence should be addressed. E-mail: brigitte.wex@ lau.edu.lb (B.W.); neckers@photo.bgsu.edu (D.C.N.). ‡ Bowling Green State University. § Current address: Lebanese American University, Division of Natural Sciences, P.O. Box 36 Code 22, Byblos, Lebanon. | American University of Beirut. Figure 1. Fused thiophene compounds: [1]Benzothiophene (1), dibenzothiophene (2), benzo[1,2-b:4,5-b′]dithiophene (3), benzo[1,2- b:5,4-b′]dithiophene (4), thieno[2,3-f:5,4-f ′]bis[1]benzothiophene (5), and thieno[3,2-f:4,5-f ′]bis[1]benzothiophene (6). 13754 J. Phys. Chem. A 2006, 110, 13754-13758 10.1021/jp065548s CCC: $33.50 © 2006 American Chemical Society Published on Web 12/21/2006