Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 5160−5166
pubs.acs.org/JPCL
Ultrafast Broadband Transient Absorption and Circular Dichroism Reveal Relaxation of a Chiral Copolymer Mirko Scholz,† Marius Morgenroth,† Min Ju Cho,‡ Dong Hoon Choi,‡ Kawon Oum,*,† and Thomas Lenzer*,† †
Physical Chemistry, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany Department of Chemistry, Research Institute for Natural Sciences, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
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‡
S Supporting Information *
ABSTRACT: We present a study of the photoinduced dynamics of the chiral polyfluorene-phenylene copolymer PFPh in THF and in cholesteric thin films. After photoexcitation at 370 nm in THF, ultraviolet−visible−near-infrared (UV−vis−NIR) transient absorption spectra show fast subpicosecond to picosecond intrachain migration of singlet excitons, solvation dynamics, and an exciton lifetime of 410 ps. The PFPh thin film features also interchain singlet exciton migration and exhibits shorter (2.1 and 240 ps) and longer lifetime components (2800 ps, interchain recombination). Furthermore, a setup for ultrafast UV−vis broadband transient circular dichroism (TrCD) spectroscopy has been developed. Fast supramolecular relaxation processes are observed, which are linked to changes in the anisotropic polarizability and pitch length of the cholesteric film. Such combined ultrafast transient CD and absorption experiments hold promise to reveal not only details of relaxation processes in supramolecular arrangements but also structural rearrangements of chiral molecular systems featuring CD signals in the UV−vis region.
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successfully applied, e.g. by the groups of Kliger, Simon, Hache, Changenet and Dietzek,17−24 only a few attempts have been made to date to implement ultrafast CD spectroscopy with subpicosecond time resolution in a broadband fashion. A previous TrCD study of merocyanine nanorod aggregates using single-filament broadband probing, carried out by Fiebig and co-workers, reached a lower wavelength limit of ca. 350 nm in the UV.25 A newly developed setup of the Chergui group,26 employing a photoelastic modulator for polarizationswitching, is dedicated to the UV range (250−370 nm), and the upper wavelength range is currently limited by the spectral bandwidth of broadband UV femtosecond radiation employed for generating the circularly polarized pulses. First measurements were carried out for an enantiopure [Ru(bpy)3]2+ complex,26 following up on the seminal work of Kliger and co-workers using broadband CD probing with nanosecond time-resolution of the same system in the 1980s.27,28 Brixner and co-workers introduced the concept of polarization mirroring for ultrafast CD experiments,29 and preliminary experimental results for oxyhemoglobin in the range 540−610 nm have been provided.30 Key aspects of our present implementation of TrCD spectroscopy are the use of an intense multifilament supercontinuum, which covers a wide spectral range from the deeper UV into the visible region (260−700 nm), and the shot-to-shot recording of the
rganic homo- and copolymers based on polyfluorenes and other conjugated systems have gathered widespread attention because of promising applications in organic photovoltaics, organic light-emitting diodes (OLEDs), and organic lasers.1−4 Characterizing the excited-state dynamics of such systems is not only of general interest from the perspective of photophysics but also bears relevance to the understanding of the function of these polymers in such optoelectronic devices.5−7 Polymers featuring large circular dichroism (CD) response are of considerable interest, as they have particular relevance for applications in polymer photonics, such as LEDs with circularly polarized emission or color-tuning devices.8−12 Cholesteric stacking of rod-like polymer chains has been suggested as the main factor for obtaining large CD values above 1000 mdeg,13 and careful optimization of the annealing conditions of such films further improves the longrange order.2,11,14−16 In the current Letter, we address the photoinduced dynamics of a strongly CD-active polyfluorene copolymer with a phenyl ring as π-bridge in the repeating unit, denoted as PFPh. Broadband transient absorption spanning the ultraviolet−visible−near-infrared (UV−vis−NIR) range is applied to provide a clear picture of the intra- and interchain dynamics after photoexcitation for PFPh in solution and thin films. We also introduce a new general-purpose setup for ultrafast UV− vis transient circular dichroism (TrCD) spectroscopy and apply it to investigate the transient supramolecular response of the cholesteric PFPh thin film. While transient CD techniques based on single-wavelength pump−probe setups have been © XXXX American Chemical Society
Received: July 16, 2019 Accepted: August 20, 2019
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DOI: 10.1021/acs.jpclett.9b02061 J. Phys. Chem. Lett. 2019, 10, 5160−5166
Letter
The Journal of Physical Chemistry Letters supercontinuum fluctuations, which improves the signal-tonoise ratio. This way we are able to probe the dynamics of cholesteric PFPh thin films over a wide spectral range. In principle, such transient broadband CD probing can be extended into the deeper UV by employing even shorter seed wavelengths for the supercontinuum. We start with analyzing the steady-state optical properties of PFPh in different environments. Figure 1a shows the steady-
Next, we turn to the steady-state CD spectra of PFPh in Figure 1b. The CD signal of PFPh in THF is small (
