Abstract
Organic donor–acceptor (D–A) co-crystals have attracted much interest due to their important optical and electronic properties. Co-crystals having ⋯DADA⋯ π-stacked morphologies are especially interesting because photoexcitation produces a charge-transfer (CT) exciton, D˙+–A˙−, between adjacent D–A molecules. Although several studies have reported on the steady-state optical properties of this type of CT exciton, very few have measured the dynamics of its formation and decay in a single D–A co-crystal. We have co-crystallized a peri-xanthenoxanthene (PXX) donor with a N,N-bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) (Ph4PDI) acceptor to give an orthorhombic PXX–Ph4PDI ⋯DADA⋯ π-stacked co-crystal with a CT transition dipole moment that is perpendicular to the transition moments for Sn ← S0 excitation of PXX and Ph4PDI. Using polarized, broadband, femtosecond pump–probe microscopy, we have determined that selective photoexcitation of Ph4PDI in the single co-crystal results in CT exciton formation within the 300 fs instrument response time. At early times (0.3 ≤ t ≤ 500 ps), the CT excitons decay with a t−1/2 dependence, which is attributed to CT biexciton annihilation within the one-dimensional ⋯DADA⋯ π-stacks producing high-energy, long-lived (>8 ns) electron–hole pairs in the crystal. These energetic charge carriers may prove useful in applications ranging from photovoltaics and opto-electronics to photocatalysis.
Highlights
IntroductionOrganic donor–acceptor (D–A) co-crystals have been shown to possess many interesting properties, among them: semiconducting behavior with appreciable ambipolar charge transport capabilities,[1,2,3,4,5,6,7,8] tunable emission spectra,[5,9,10,11,12,13] and ferroelectricity.[5,9,14] These structures are believed to be good candidates for organic eldeffect transistors,[15,16] organic photovoltaics,[17,18,19] photonic logic,[20,21] and near-IR photothermal imaging.[22]
At early times (0.3 # t # 500 ps), the charge transfer (CT) excitons decay with a tÀ1/2 dependence, which is attributed to CT biexciton annihilation within the one-dimensional /DADA/ p-stacks producing high-energy, long-lived (>8 ns) electron–hole pairs in the crystal
These energetic charge carriers may prove useful in applications ranging from photovoltaics and opto-electronics to photocatalysis
Summary
Organic donor–acceptor (D–A) co-crystals have been shown to possess many interesting properties, among them: semiconducting behavior with appreciable ambipolar charge transport capabilities,[1,2,3,4,5,6,7,8] tunable emission spectra,[5,9,10,11,12,13] and ferroelectricity.[5,9,14] These structures are believed to be good candidates for organic eldeffect transistors,[15,16] organic photovoltaics,[17,18,19] photonic logic,[20,21] and near-IR photothermal imaging.[22]. To probe the ultrafast dynamics of CT excitons in D–A cocrystals, it is most informative to conduct femtosecond timeresolved spectroscopic measurements over a broad spectral range, taking advantage of the anisotropic absorption changes that can be directly related to the orientation of the donor and acceptor molecules relative to the crystal axes. By combining single wavelength pump with broadband probe beams we were able to measure a spectrum spanning $300 nm at different polarizations Using this technique, the results presented here determine the CT exciton hopping rate constant and show that CT biexciton annihilation to form high-energy free charge carriers occurs in the PXX–Ph4PDI co-crystal by kinetically outcompeting charge recombination of the individual CT excitons. For details on the form of the Hamiltonian and the parameters used, see the ESI.†
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