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.”

Understanding Ultrafast Dynamics of Conformation Specific Photo-Excitation: A Femtosecond Transient Absorption and Ultrafast Raman Loss Study

“…29 Our previous report on time-resolved TA and ultra-fast Raman loss spectroscopy (URLS) of BPEB upon 307 nm photo-excitation concerns the multi-exponential kinetic behavior observed in different solvents. 34 The longest lifetime component obtained from femtosecond TA was consistent with the fluorescence lifetime (~500-600 ps) reported in the literature. 29,34 The time dependent integrated peak area/intensity of three major Raman bands (2126 cm -1 , 1573 cm -1 and 1130 cm -1 modes) showed a biphasic increase, as concluded from transient URLS measurements for the same excitation wavelength.…”

“…Bis(phenylethynyl)-benzene was prepared via the Sonogashira coupling reaction and the purity was tested as reported earlier. 34 The ground state absorption spectrum of BPEB was recorded using a Shimadzu UV-Vis Spectrophotometer, model UV-2600 at 20 µM concentration in acetonitrile in a 1-cm path length quartz cuvette. The absorption was processed using UVprobe-2.50 software which is supplied as standard with the spectrophotometer.…”

Correlating Pressure‐Induced Emission Modulation with Linker Rotation in a Photoluminescent MOF

“…This is a further illustration of the flexibility of the MOF framework, and how this can influence the photoluminescence properties. [26] This behaviour is in contrast to a recently reported two-fold interpenetrated tetraphenylethylene-based Zr MOF, which showed changes in luminescence when pressurised up to 20 MPa as a consequence of a breathing effect, whereby the two interpenetrating nets were compressed towards one another and the unit cell contracted dramatically. [27] Finally, on decreasing pressure, the pressure-induced spectral shifts were reversible, indicating the transition from a mixed twisted:planar structure at ambient pressure, to a 100 % twisted structure at 2.1 GPa, is fully reversible.…”

Femtosecond stimulated Raman line shapes: Dependence on resonance conditions of pump and probe pulses

“…In this work, we investigate the relationship between the GS-FSRS line shapes and Raman pump and probe wavelengths. Different from previous reports [38,53], the line shapes are also examined at shorter wavelengths resonant with the blue side of the electronic absorption band which fills a current knowledge gap. R6G in methanol is used as the model system because its absorption band (maximum at ∼530 nm) lies within the wavelength tunable range (ca.…”

“…In an electronic excited state, transient vibrational population complicates the Raman line shapes to a greater extent. The resonance with excited-state absorption (ESA) and stimulated emission (SE) bands provides more channels for "dynamic" resonance Raman during the study of a broad range of photoactive systems [20,35,39,[50][51][52][53][54]. Some theoretical work has been performed for the ES Raman line shapes [18,20,46,50].…”