Direct Measurements of Intersystem Crossing Rates and Triplet Decays of Luminescent Conjugated Oligomers in Solutions ² Hsin-Liang Chen, ‡ Yi-Fang Huang, § Chao-Ping Hsu, | Tsong-Shin Lim, § Li-Chung Kuo, ⊥ Man-kit. Leung, ⊥ Teng-Chih Chao, ⊥ Ken-Tsung Wong, ⊥ Show-An Chen, # and Wunshain Fann *‡,§ Department of Physics, National Taiwan UniVersity, Taipei 106, Taiwan, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan, Institute of Chemistry, Academia Sinica, Taipei 106, Taiwan, Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, and Department of Chemical Engineering, National Tsing Hua UniVersity, Hsinchu 300, Taiwan ReceiVed: May 25, 2007; In Final Form: August 3, 2007 Photothermal calorimetry and fluorescence spectroscopy were used to determine the relaxations of the photoexcited singlet state of two PPV and polyfluorene oligomers, (E,E)-1,4-bis[(2-benzyloxy)styryl]benzene (PVDOP) and ter(9,9′-spirobifluorene) (TSBF). The decay rates of different S 1 relaxation channels, which include intersystem crossing (ISC), radiative, and nonradiative decay can be determined by the combination of photoacoustic calorimetry (PAC) and the time-correlated single photon counting (TCSPC) technique. The triplet state energy level is determined by the phosphorescence (Ph) spectra recorded at 77 K. The ISC yields are ∼3% and 6% for PVDOP and TSBF, respectively. The T 1 to S 0 transition decay rate is acquired by PAC and photothermal beam deflection (PBD) measurements. The triplet state decay rate is 17 and 21 ms -1 at room temperature. The Ph intensity decay at 77 K shows that the triplet state lifetime increases by 4 orders of magnitude, as compared to room temperature. Introduction There is extensive interest in using conjugated polymer for flexible electronic and electro-optic devices such as thin-film transistors, 1-3 polymer light emitting diodes (PLEDs), 4-6 and solar cells. 7,8 Among the different conjugated polymers, the derivatives of poly(p-phenylenevinylene) (PPV) and polyfluo- rene (PF) play significant roles in the development of the PLEDs. PPV and PF have been extensively studied both experimentally and theoretically. 9-17 However, the photophysi- cal behaviors of conjugated polymers are strongly influenced by the conjugated length distribution. The problem can be simplified by studying the model oligomers with similar building blocks of the polymers, thus eliminating the effect of conjugated length distribution on photophysics. 23 Except for the singlets (the excitons created by light excitation directly), molecules can also transfer from the lowest energy vibrational state S 1 to the first triplet state T 1 and undergo a spin conversion intersystem crossing (ISC). Phosphorescence (Ph), emitted from T 1 to the singlet ground state with a longer wavelength and much longer lifetime (microseconds to mil- liseconds) relative to fluorescence, has a much smaller rate constant. The transition from T 1 to singlet the ground state is effectively inhibited, thus the triplet exciton is believed to reduce the luminescence quantum yield, especially the electrolumines- cence quantum yield of polymeric light-emitting diodes (PLED). Because presumably in PLED 75% of the population is in the triplet state, understanding their properties are important. The concentrations in solution provide a tuning knob to vary the intermolecular interactions. This is important for studying the photophysics properties. For example, the triplet decays in films are dominated by triplet-triplet annihilation. In solution, we can vary the concentration, so the decay can go from triplet- triplet dominated to T 1 -S 0 decay. Because the ground state typically has the singlet characteristic for conjugated polymers and oligomers, direct optical absorption can only access singlet excited states. The decay from S 1 to S 0 has been studied by various experimental and theoretical methods. The T 1 state can also be populated by optical excitation through intersystem crossing. In general, the ISC yield depends on the chain length and can be enhanced in the presence of heavy atoms. 20,24 The subsequent triplet state decay from T 1 to S 0 is generally nonradiative at room temperature. The T 1 to S 0 transition generally exhibits a much longer lifetime and nonradiative characteristics at room temperature. Intersystem crossing provides a way to reach the triplet state in optical excitation experiments. Unon optical excitation, the triplet exciton can only be generated through intersystem crossing. The techniques include photoinduced absorption (PA) spectroscopy, photoacoustic calorimetry (PAC), single-molecule spectroscopy (SMS), singlet oxygen quenching, and femtosec- ond ground state recovery methods that have been applied to investigate the ISC and triplet energy of the conjugated oligomers and polymers. PA spectroscopy can be used to obtain the T 1 to T n transition energy gap. However, it is not sufficient to obtain ISC yields and T 1 values by only using the PA method; combination with other measurements is needed. For example, magnetic resonance spectroscopy needs to be used to decide the T 1 energy, 37 whereas either varying the excitation power 37 or PA frequency-modulation spectroscopy is needed to get the ISC yield. 19-21 In addition, a well-known ISC yield from a ² Part of the “Sheng Hsien Lin Festschrift”. ‡ Department of Physics, National Taiwan University. § Institute of Atomic and Molecular Sciences, Academia Sinica. | Institute of Chemistry, Academia Sinica. ⊥ Department of Chemistry, National Taiwan University. # National Tsing Hua University. 9424 J. Phys. Chem. A 2007, 111, 9424-9430 10.1021/jp0740651 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/16/2007