Excited-State Dynamics of Perylene-Based Chromophore Assemblies on Nanoporous Anodic Aluminum Oxide Membranes

Abstract

Exploring excited-state decay pathways in organic chromophore assemblies often requires significant synthetic effort to systematically change interchromophore distances. Moreover, the solubility of these assemblies is often limited. Herein, we examine how nanoporous anodic aluminum oxide (AAO) membranes can address both of these issues by covalently attaching 3-(phenylethynyl)perylene (PEP) and 1,6,7,12-tetrakis-(4-tert-butylphenoxy)perylene(3,4:9,10)-bis(dicarboximide) (tpPDI) chromophores with different loadings to AAO membranes, where the estimated average distance between chromophores on the AAO membranes for PEP-AAO-1, -2, and -3 is 5, 8, and 16 Å, respectively, while that between tpPDI cores in tpPDI-AAO-1, -2, -3, and -4 is 4, 6, 10, and 17 Å, respectively. When PEP-AAO-1, -2, and -3 are photoexcited, we observe a distribution of excimer-like states with varying degrees of charge-transfer (CT) character depending on the interchromophore spacing and solvent polarity. Increasing solvent polarity or decreasing the interchromophore distance increases the CT character in the excimer state. Notably, a PEP excimer state having significant CT character still forms in air, that is, in the absence of solvent. In contrast, when tpPDI-AAO-1 is photoexcited in solvents of varying polarity, as well as in air, we observe full symmetry-breaking charge separation (SB-CS) between tpPDIs attached to the AAO membrane. The SB-CS and charge recombination rates increase with increasing solvent polarity as the energy of the CT state is lowered. As the average distance between the tpPDIs increases in the series tpPDI-AAO-1, -2, -3, and -4, the SB-CS rate decreases with no SB-CS observed in tpPDI-AAO-4. This demonstrates the utility of AAO membranes to control the fate of electronic excited states by tuning the various contributions from Coulombic and charge transfer coupling.

π-Stacking-Dependent Vibronic Couplings Drive Excited-State Dynamics in Perylenediimide Assemblies

“…S2 displays solvent-dependent TA spectra of the EX state, evidenced for example by increased prominence of CT excited-state absorption (ESA) signals in polar solvents due to the enhanced CT state contribution in the EX state. Thus, S2 in THF exhibits stronger CT character compared to that in toluene (Figure S10), which agrees with previously reported TA measurements of a PDI cofacial dimer that exhibited a charge-resonance enhanced EX state in polar solvents. , In addition, the fluorescence quantum yield of S2 (<0.005) is significantly lower than that of the cofacial dimer (0.05) in dichloromethane, which is likely due to the stronger interchromophoric interactions (the shorter interchromophoric distance) and optimal rotational angle (1° vs 20° for S2 and the cofacial dimer, respectively) for efficient EX formation in S2 . ,, …”

“…Thus, S2 in THF exhibits stronger CT character compared to that in toluene (Figure S10), which agrees with previously reported TA measurements of a PDI cofacial dimer that exhibited a charge-resonance enhanced EX state in polar solvents. 38,59 In addition, the fluorescence quantum yield of S2 (<0.005) is significantly lower than that of the cofacial dimer 38 (0.05) in dichloromethane, which is likely due to the stronger interchromophoric interactions (the shorter interchromophoric distance) and optimal rotational angle (1°vs 20°for S2 and the cofacial dimer, respectively) for efficient EX formation in S2. 43,60,61 Vibronic Coherences of the Excimer State by Broadband Transient Absorption Measurements.…”

Excimer Diffusivity in 9,10-Bis(phenylethynyl)anthracene Assemblies on Anodic Aluminum Oxide Membranes

“…Such effects have been inferred previously in perylene-based AAO studies where alteration in excimer formation and symmetry-breaking charge transfer times showed a solvent polarity dependence. 46,47 The strong dependence of free carrier mobilities on packing morphology has also been demonstrated in OPV materials 48 and is expected in other systems where disorder can influence the excited-state dynamics.…”

Rigid Bay-Conjugated Perylene Bisimide Rotors: Solvent-Induced Excited-State Symmetry Breaking and Resonance-Enhanced Two-Photon Absorption

“…PCBP-EPE and PCBP-EE resembled the optical characteristics of monomeric PBIT except for the emission shoulders around 600 nm, indicating main origination from the conjugated PBI skeletons. We know that PBIs tend to aggregate strongly influenced by the solvents, thanks to the π–π stacking and the intensity changes of vibronic features that are indicative of their strong exciton coupling. , As reported by previous works, the spectral changes upon aggregation of PBIs were commonly described in terms of exciton-vibrational coupling, where the electronic transition is coupled to the vibrational modes of the chromophores. − Würthner et al also pointed out that the exciton coupling was mainly assigned to long-range Coulomb coupling between the transition dipole moments of the chromophores leading to H- or J-type coupling with positive or negative exciton coupling energy, respectively . Intensity reversal of the A 0‑0 and A 0‑1 absorption bands was observed for PCBP-EPE and PCBP-EE with respect to PBIT (Figure b), which should arise from the interplay of exciton and vibrational coupling.…”

“…We know that PBIs tend to aggregate strongly influenced by the solvents, thanks to the π−π stacking and the intensity changes of vibronic features that are indicative of their strong exciton coupling. 15,18 As reported by previous works, the spectral changes upon aggregation of PBIs were commonly described in terms of exciton-vibrational coupling, where the electronic transition is coupled to the vibrational modes of the chromophores. 18−20 Wurthner et al also pointed out that the exciton coupling was mainly assigned to long-range Coulomb coupling between the transition dipole moments of the chromophores leading to H-or J-type coupling with positive or negative exciton coupling energy, respectively.…”