Picosecond Time-Resolved Fluorescence Spectroscopy of (Z)-1-(2-Anthryl)-2-phenylethene and Its Model Compounds: Understanding the Photochemistry by Distinguishing between the s-cis and s-trans Rotamers Takashi Karatsu,* ,† Hajime Itoh, † Atsuko Nishigaki, ‡ Keijiro Fukui, † Akihide Kitamura,* ,† Shigeki Matsuo, § and Hiroaki Misawa § Department of Materials Technology, Faculty of Engineering, and Center for Frontier Electronics and Photonics, Chiba UniVersity, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan, and Faculty of Engineering, The UniVersity of Tokushima, Minamijosanjima-cho, Tokushima 770-8506, Japan ReceiVed: September 15, 1999; In Final Form: May 24, 2000 The photochemical reactions (Z-E isomerization and photocyclization) of the s-cis and s-trans rotamers of (Z)-1-(2-anthryl)-2-phenylethene (Z2APE) were investigated. The absorption, steady-state and picosecond time-resolved fluorescences, and transient absorption spectra were measured in order to achieve a better understanding of the rotamer photochemistry. The spectra were compared with those of the model compounds ((Z)-1-[2-(1-methylanthryl)]-2-phenylethene: Z1Me2APE and (Z)-1-[2-(3-methylanthryl)]-2-phenylethene: Z3Me2APE). It is confirmed that the s-trans rotamer undergoes only ZfE adiabatic isomerization and that the s-cis rotamer mainly photocyclizes to give the dihydrophenanthrene-type intermediate with a much faster rate constant (k cyc ) 2.3 × 10 10 s -1 ). The reason Z2APE does not give an aromatized photocyclization product (1,2-naphtho[a]anthracene, NA) is that the return of the dihydrophenanthrene-type intermediate to the Z isomer is much faster than the oxidation to produce NA. Introduction The compound 1-(2-anthryl)-2-phenylethene (2APE) has two rotational isomers with respect to the single bond that connects the anthryl group to the olefinic double bond. The s-trans- and s-cis-rotamers align their double bonds along the long and short axes of the anthryl moiety, respectively. The rotational isomer- ization of 2-anthrylethenes in the ground state and in the excited states is a subject of current interest. 1-24 Most of the studies have detailed the E isomer, because of its ease of preparation and its inactivity in the geometrical isomerization known as one- way ZfE isomerization. 23-29 This isomerization has been studied using steady-state fluorescence, 1,2 time-correlated single- photon counting (SPC), 3-9 time-gated fluorescence spectros- copy, 10-13 principal component analysis with self-modeling (PCA-SM), 3,4,14-17 and T-T absorption spectroscopy. 18-21 When compared with the extensive studies on the rotamers of the E isomers, only a few studies have been reported for the Z isomers. 22-24 In particular, the selective photochemistry of the two rotamers of (Z)-2APE (Z2APE) was elucidated by Saltiel’s group using the PCA-SM technique. 23,24 They showed that in toluene (1) the NEER (nonequilibration of excited rotamers) principle 1,30,31 holds for the singlet excited Z2APE, (2) that the 1 (Zs-trans)* rotamer undergoes efficient adiabatic Z fE one- way isomerization, but that the 1 (Zs-cis)* rotamer does not adiabatically isomerize to the E isomer (Scheme 1). In our previous reports, 18,19 we characterized the s-trans and s-cis rotamers of E2APE by using their model compounds, in which methyl groups restrict their conformations as previously reported for the analogous naphthalene derivatives. 15,32,33 We attempted to characterize the s-trans and s-cis rotamers of Z2APE along the same lines by comparing their properties with those of the model compounds: (Z)-1-[2-(1-methylanthryl)]-2- phenylethene (Z1Me2APE) and (Z)-1-[2-(3-methylanthryl)]-2- phenylethene (Z3Me2APE). The study of the Z2APE rotamer and its analogues is particularly difficult, because one-way ZfE * To whom correspondence should be addressed, e-mail: karatsu@ xtal.tf.chiba-u.ac.jp, FAX: +81-43-290-3039. † Faculty of Engineering, Chiba University. ‡ Center for Frontier Electronics and Photonics, Chiba University. § The University of Tokushima. SCHEME 1 6993 J. Phys. Chem. A 2000, 104, 6993-7001 10.1021/jp993305c CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000