Investigation of molecular dimers by ensemble and single molecule spectroscopy Christ, Th.; Petzke, F.; Bordat, P.; Herrmann, Andreas; Reuther, E.; Müllen, K.; Basché, Th. Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Christ, T., Petzke, F., Bordat, P., Herrmann, A., Reuther, E., Müllen, K., & Basché, T. (2002). Investigation of molecular dimers by ensemble and single molecule spectroscopy. Journal of Luminescence, 98, 23-33. CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. AbstractWe have investigated molecular dimers with different electronic coupling strengths by bulk and single molecule spectroscopy. In one of the dimers the two monomers (perylene-monoimide) are directly connected via a single bond while in the other one they are separated by the benzil motif. The close proximity of the monomers in the first case gives rise to excitonic band splitting which is clearly observable in the bulk absorption spectra. For the benzil structure the electronic interactions are governed by F .orster-type energy hopping between the monomers. Fluorescence intensity trajectories at the single molecule level show one-step and two-step bleaching behaviour which appears to be very similar for both dimers. However, emission spectra recorded simultaneously with the trajectories indicate spectral changes which allow to distinguish between weakly and strongly coupled dimers. In the latter case the spectral shape changes significantly when excitonic coupling has been lifted because of photochemical transformation of one of the monomers. r
Optical fluorescence spectra of 9-cyanoanthracene (9CA), , and 10, in 2-methylbutane (2MB) were measured at several temperatures between 120 and 298 K. The 9CA spectra were subjected to a Franck-Condon (FC) analysis based upon a two-state model with eleven normal modes. On the basis of these results a simultaneous FC and band-shape analysis of the temperaturedependent spectra was carried out for CBA and DCB to determine the effective SI torsional potential. The S1 double-minimum torsional potential of the symmetrical derivative DCB in 2MB has a local barrier at cp = 90° of AV = 290 cm-l, very similar to 9,9'-bianthryl (BA). On the other hand, a strong increase of the local barrier (AV= 680 cm-I) can be deduced for the symmetry-disturbed derivative CBA from the model calculations.The minimum angle wn was found to be 63O and 62O for DCB and CBA, respectively. The fluorescence quantum yields and lifetimes were determined for BA, CBA, and DCB in apolar solvents. The comparison of the calculated radiative transition rates (estimated from absorption spectra) with the experimental values confirms the validity of the Condon approximation. No evidence for an emission from a charge-transfer (CT) state could be found for CBA and DCB in 2MB.
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Optical fluorescence spectra of 9,9′‐bianthryl (BA) in polyisobutene (PIB) were measured as a function of the excitation wavelength at various temperatures between 210 and 293 K. Irradiation at the red edge of the absorption spectra selectively excites distinct conformers with respect to the torsional angle. This leads to a strong dependence of the vibronic band shape of the fluorescence spectra on the excitation wavelength. The marked temperature dependence of the band shape may be attributed to the viscosity‐dependent deceleration of the torsional relaxation of BA in the highly viscous polymer which prevents the excited state ensemble from reaching thermal equilibrium. The predominant broadening mechanism affecting the band shape is given by a combination of the S1 torsional distribution and the torsional angle‐dependent transition frequency. These effects are interpreted quantitatively within a torsional relaxation model based on the description of the torsional dynamics according to a Smoluchowski equation which is adapted to the steady state case. The band shape analysis of the fluorescence spectra enables the determination of the effective S1 torsional potential as well as the friction coefficient related to the internal torsional motion of BA in the polymer matrix. A value of ΔV = 325 cm−1 is deduced for the local barrier of the S1 double minimum potential at ϕ = 90° while the minimum torsional angle is found to be ϕmin = 61°. Additionally, model calculations on time‐resolved fluorescence spectra of BA in apolar solvents are carried out as a function of excitation wavelength and temperature showing the importance of conformational relaxation for the interpretation of transient spectra of flexible molecules.
CV-' m2. Es zeigte sich, da13 die elektrooptischen Absorptions-und Emissionsuntersuchungen an DMANS in unpolaren Losungen zu einem konsistenten Satz von Dipolmomenten fiihren, daB also die Aquilibrierung mit der Umgebung nach Anregung im unpolaren Cyclohexan keinen EinfluB auf das permanente Dipolmonient hat.From electrooptical emission and absorption measurements in liquid and glassy frozen solutions and with dielectric investigations the dipole moment pf and the component & of the polarizability parallel to p: of trans-p-dimethylamino-p'-nitrostilbene (DMANS) in its first excited singlet state has been determined to be p: = (63 f 5 ) -10-30 Cm and a:z = (442 f 6 5 ) . 10-' O CV-' m2, respectively. It could be shown that both electrooptical absorption and emission measurements give the same dipole moments, indicating that there is no change of the dipole moment with equilibration of DMANS and its surrounding after excitation in nonpolar cyclohexane solution. Voraussetzungen Elektrooptische Emissionsuntersuchungen an trans-p-dimethylamino-p'-nitrostilben (DMANS) wurden in nicht entI" = ( m , A " p ( ' .( 1 5)Temperatur. Weiter ist in unpolaren Liisungsmitteln. wo in guter Niiherung die Dielektrizitiitszahl c gleich dem Quadrat der Brechzahl 12 ist: Uni den Beitrag der Anderung der Polarisierbarkeit mit der Anregung A"a abzuschiitzen, wurden bei 121 K in gla,ig erstarrten 1 : 1 n-Butanol-iso-Propanolgemischen elektrooptische Absorptionsmessungen an DMANS durchgefiihrt und analog dem oben Gesagten nach Liptay [5] ausgewertet. Seibold et al. [9] Auswertung a) AllgemeinesSchiitzt man mit D" aus Tab. 2 den Beitrag der von mi abhiingigen Glieder zu E", F" und G" ab, so zeigt sich, daD diese Beitriige weit unterhalb der Fehler von E". Fa und G" liegen und selbst bei den Tieftemperaturmessungen, Tab Die Meljwerte der dielektrischen Untersuchungen aus [ 101 sind zusainmen mit den Ergebnissen der Auswertung nach den Gleichungen (36) und (37) DiskussionDie aus elektrooptischen Absorptions-und Emissionsuntersuchungen gefundenen, noch nicht feldkorrigierten Dipolmomente von DMANS in Cyclohexan sowie die Dipolmomente des freien DMANS nach Berucksichtigung des inneren Feldes in einer Losung sind in Tab. 9 einander gegeniibergestell t. Mit der Beriicksichtigung des inneren Feldes nach den Gleichungen (17) und (23)
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