title: Mode specific excited state dynamics study of bis(phenylethynyl)benzene from ultrafast Raman loss spectroscopy.
abstract: Femtosecond transient absorption (fs-TA) and Ultrafast Raman Loss Spectroscopy (URLS) have been applied to reveal the excited state dynamics of bis(phenylethynyl)benzene (BPEB), a model system for one-dimensional molecular wires that have numerous applications in opto-electronics. It is known from the literature that in the ground state BPEB has a low torsional barrier, resulting in a mixed population of rotamers in solution at room temperature. For the excited state this torsional barrier had been calculated to be much higher. Our femtosecond TA measurements show a multi-exponential behaviour, related to the complex structural dynamics in the excited electronic state. Time-resolved, excited state URLS studies in different solvents reveal mode-dependent kinetics and picosecond vibrational relaxation dynamics of high frequency vibrations. After excitation, a gradual increase in intensity is observed for all Raman bands, which reflects the structural reorganization of Franck-Condon excited, non-planar rotamers to a planar conformation. It is argued that this excited state planarization is also responsible for its high fluorescence quantum yield. The time dependent peak positions of high frequency vibrations provide additional information: a rapid, sub-picosecond decrease in peak frequency, followed by a slower increase, indicates the extent of conjugation during different phases of excited state relaxation. The CC triple (-C≡C-) bond responds somewhat faster to structural reorganization than the CC double (>C=C<) bonds. This study deepens our understanding of the excited state of BPEB and analogous linear pi-conjugated systems and may thus contribute to the advancement of polymeric "molecular wires."
authors.affiliation: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India.
authors.forename: Khokan
authors.identifier: 0000000327612405
authors.initials: K
authors.lastname: Roy
authors.affiliation: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India.
authors.forename: Surajit
authors.identifier: 000000024239455X
authors.initials: S
authors.lastname: Kayal
authors.affiliation: On leave from LaserLaB, VU University Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, Netherlands.
authors.forename: Freek
authors.identifier: 0000000287567223
authors.initials: F
authors.lastname: Ariese
authors.affiliation: Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, United Kingdom.
authors.forename: Andrew
authors.initials: A
authors.lastname: Beeby
authors.affiliation: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India.
authors.forename: Siva
authors.identifier: 0000000285385004
authors.initials: S
authors.lastname: Umapathy