Singlet-Singlet Annihilation Leading to a Charge-Transfer ...

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Singlet–Singlet AnnihilationScheme 1. Molecular structures of PI-(pPh) 1 -PI (1) and PI-(pPh) 2 -PI (2); R= n-octyl, Ar = 4-n-octylphenyl.Results and DiscussionSteady-State MeasurementsFigure 1 displays the normalised steady-state absorption andemission spectra of PI-(pPh) 1 -PI and PI-(pPh) 2 -PI in methylcyclohexane(MCH). The fluorescence quantum yields measuredin degassed conditions are compiled in Table 1. The extinctioncoefficient of the model compound PI-(pPh) 1 measured in tolueneis 52000 m 1 cm 1 at 515 nm.Both PI-(pPh) 1 -PI and PI-(pPh) 2 -PI have a large offset in theabsorption energy of the two constituents. The blue parts(350–450 nm) of their spectra originate from the dominantbackbone absorption (pPh units), whereas the bands centredat 520 nm are due to the S 0 –S 1 transition of the PI subsystem.While the features and positionof the lowest (S 0 !S 1 ) absorptionband of PI-(pPh) 1 , [17] PI-(pPh) 1 -PI and PI-(pPh) 2 -PI inMCH are nearly identical, the relativeintensity compared to thatof the pPh band reflects theratio of PI and pPh units(Figure 1). The PI absorptionband appears slightly unstructuredwhen compared to theS 0 !S 1 of the phenyl-substitutedchromophore (C1P1), suggestingsome ground-state delocalisationover the neighbouring pPhunit. [15] Although the maximum and the vibronic structure ofthe absorption band attributed to the pPh moiety is situatedat the same wavelength for PI-(pPh) 1 -PI and its single-chromophoreanalogue PI-(pPh) 1 , this band is shifted over 20 nm tolonger wavelength in PI-(pPh) 2 -PI. In addition, in PI-(pPh) 2 -PI,the fine structure is partially lost, suggesting increased conjugationbetween the pPh subsystems. In the emission spectrain MCH, the maxima (560 nm) and the vibronic shoulders aresimilar to previously studied PI-substituted pPh compounds.Again, the maximum and vibronic characteristics of the emissionspectra in MCH are only weakly shifted with respect tothose of C1P1 in toluene. The loss of fine structure in the absorptionspectra and the persistence of the fine structure inthe emission spectra suggest an increased planarity of thepPh n moiety versus the PI moiety in the singlet excited state.We recently demonstrated that PI-(pPh) 1 dissolved in polarsolvents shows an efficient photoinduced charge transfer fromthe backbone to the end-cap; similar behaviour is expected forPI-(pPh) 1 -PI and PI-(pPh) 2 -PI. [17] The decrease of the quantumyield of the latter compounds in tetrahydrofuran (THF; see theSupporting Information, Table 1 for details) points to an improvedefficiency of the radiationless deactivation of the relaxedsinglet excited state. This effect is probably related tothe dipolar character of the excited state suggested by a systematicredshift of the emission spectra with less pronouncedvibronic features in this solvent (see the Supporting Information,Figure 1 for details).Time-Resolved Experiments on PI-(pPh) 2 -PIFigure 1. Normalised absorption and emission spectra (excitation at 495 nm)of PI-(pPh) 1 -PI (b) and PI-(pPh) 2 -PI (c) in MCH.Table 1. Fluorescence quantum yields, absorption and emission maximaof the compounds PI-(pPh) 1 , PI-(pPh) 1 -PI and PI-(pPh) 2 -PI in MCH.Compound PI-(pPh) 1 PI-(pPh) 1 -PI PI-(pPh) 2 -PIQuantum yield 0.85 0.78 0.72Absorption maximum [nm] 510 515 512Emission maximum [nm] 560 560 562By single-photon-timing (SPT) experiments, the fluorescenceintensity of PI-(pPh) 2 -PI was found to decay monoexponentiallyover the entire wavelength range of the emission spectrumwith a time constant of 2.8 ns (3.5 ” 10 8 s 1 ). Combined withthe fluorescence quantum yield of 0.72, this decay yields a fluorescentrate constant of 2.5”10 8 s 1 . The slightly faster decaycompared to C1P1 (2.38 ” 10 8 s 1 , F=1 in toluene) at thesame emission maximum suggests a stronger transition dipolemoment. [15]Time-resolved transient absorption spectra obtained uponexcitation at 495 nm with two different excitation powers(60 mW and 400 mW) are depicted in Figure 2. The spectroscop-ChemPhysChem 2007, 8, 1386 – 1393 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org 1387
F. C. De Schryver et al.ic features characteristic of PI can be recognised by the twobands rising instantaneously after excitation and decaying concurrentlyon a nanosecond time scale (see Figures 2 A and C).The negative part in the 500–550 nm region is due to groundstatedepletion mainly resembling the steady-state absorption,whereas the stronger negative band at 540–650 nm is associatedwith stimulated emission fully matching the stationaryemission spectra. The positive part (650–730 nm) with a molarextinction coefficient comparable to the value for the depletionband (when the ratio of the two signals is assessed) correspondsto the S 1 –S n absorption band within the PI chromophore.The transient spectrum between 540 nm and 750 nm isa sum of the excited-state absorption (positive contribution)and stimulated emission of the locally excited (LE) state (negativecontribution). [11] One should keep in mind that the excited-stateabsorption band might extend to lower wavelengthsthan the positive band, where it cancels to some the extentthe band due to induced emission. Therefore, in the calculationof the overlap with the fluorescence spectrum (seebelow), a correction for the stimulated emission componentthat affects this band had to be taken into account.The important point is that a faster decay of these bands isclearly observed in the high-excitation-power experiments (seeFigures 2 C and D). By global kinetic analysis of these decaytraces (section across the wavelength axis—Figure 2 D) in additionto the component found in SPT, three extra decay-timecomponents were found: a component of (0.5–2) ps attributedto intramolecular vibrational redistribution (IVR), [12] a 4.2 pscomponent attributed to a vibrational relaxation (VR) processand another decay-time constant of 130 ps found[12, 18]only in the high-excitation-power experiments. Upon changingthe excitation power from 60 mW to 400 mW, the spectral amplitudeshape of both the 4.2 ps and 2.8 ns components remainsunaltered. When plotting the amplitude of the 130 pscomponent versus wavelength, the resulting shape exhibits anegative value in the 500–570 nm region and a positive valuein the red part of the spectrum at 620–730 nm (data notshown). Hence, the amplitude of this component resemblesthe spectral position of the ground-state depletion band andstimulated emission at short wavelengths and of the excitedstateabsorption band at long wavelengths. On the basis ofthe comparison with the kinetics of PI-(pPh) 1 and the powerFigure 2. Three-dimensional plot of the femtosecond transient absorption spectra of PI-(pPh) 2 -PI in MCH recorded with excitation powers of 60 mW (A) and400 mW (C), and the time-resolved monochromatic transient absorption traces recorded in a 50 ps time window and the corresponding fits for the two excitationpowers 60 mW (B) and 400 mW (D); DOD is the change in optical density.1388 www.chemphyschem.org 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2007, 8, 1386 – 1393
Page 1: DOI: 10.1002/cphc.200700136Singlet-
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