Triplet exciton migration in a conjugated polyfluorene

Abstract

The diffusion and triplet energy relaxation in amino endcapped poly ͓9,9-bis͑2-ethylhexy1͒fluorene-2,7-diyl͔ has been studied using photoluminescence and photoinduced absorption, both time-resolved covering ten decades of time, dependent on temperature, excitation dose, and concentration. The results are analyzed employing the concept of dispersive hopping in a Gaussian distribution of states ͑DOS͒, characterized by width ␦ of 40 meV. It is found that the initial photogenerated triplet population in the polymer films decays fairly equally by triplet-triplet annihilation and intrinsic monomolecular decay, respectively. In the latter case, the triplet excitons survive the initial 100 ms either in trap states outside the DOS at ambient temperature or within the DOS at low temperature. Further, it is clearly found that triplet migration in this conjugated polymer is both an intrachain and interchain process. Interchain hopping of triplet excitons must occur, which subsequently leads to the observation of delayed fluorescence. The nature of interchain hopping of the triplets is discussed.

Violation of the Exponential-Decay Law at Long Times

“…In consequence, the algebraic decaying fluorescence contribution cannot be assigned to spectral diffusion to low-lying energetic states with a potential distribution of long lifetimes or other external impurity or defect sites. As with the exponentially decaying contribution, the long-time fluorescence exhibits a linear dependence on concentration (tested between 10 ÿ5 and 10 ÿ3 g=g solvent) and on laser excitation dose (tested between 0.1 and 1000 J=cm 2 ) [10]. Both of these observations rule out any potential bimolecular origin of the long-time fluorescence, such as diffusion limited triplet exciton annihilation, which obeys clearly different decay kinetics [10].…”

“…As with the exponentially decaying contribution, the long-time fluorescence exhibits a linear dependence on concentration (tested between 10 ÿ5 and 10 ÿ3 g=g solvent) and on laser excitation dose (tested between 0.1 and 1000 J=cm 2 ) [10]. Both of these observations rule out any potential bimolecular origin of the long-time fluorescence, such as diffusion limited triplet exciton annihilation, which obeys clearly different decay kinetics [10]. Also, intersystem crossing of the (relatively long-lived) triplet excitons back to the singlet manifold (E-type delayed fluorescence) can safely be excluded as for these materials the singlet-to-triplet exciton energy splitting is large compared to the thermal energy (typically ST 0:7 eV).…”

“…Albeit not exactly identical, certainly the spectra are isoenergetic. Also, the relative intensities of both emission contributions are invariant of excitation energy [10]. In consequence, the algebraic decaying fluorescence contribution cannot be assigned to spectral diffusion to low-lying energetic states with a potential distribution of long lifetimes or other external impurity or defect sites.…”

“…Using this solvent interaction, one arrives at a significantly reduced ratio E=ÿ, which is the essential condition for nonexponential decay [1,3,9]. The decay curves of a range of organic molecules and polymers dissolved in common solvents where thus measured over 20 -30 lifetimes, using time-resolved spectroscopy as previously described in detail [10]. Briefly, in our spectrometer the sample is excited by a short laser pulse (170 ps, 7 mJ at 3.5 eV, repetition rate 10 Hz).…”

Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

“…Emission was focused on a spectrograph and detected in a sensitive gated iCCD camera (Stanford Computer Optics) with sub-nanosecond temporal resolution. Luminescence, PF and DF time-resolved measurements were performed by exponentially increasing gate and delay times; details can be found elsewhere [17]. Figure 1 represents the energy diagram describing the kinetics of TADF in the long-time regime, i.e.…”