Intermediate donor–acceptor electronic coupling leads to a brilliant fluorescence behaviour.
Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor– donor–acceptor or donor–acceptor–donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor–acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti- Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics.
Organic single crystals of charge-transfer complexes: model systems for the study of donor/acceptor interactions
The charge-transfer (CT) state arising as a hybrid electronic state at the interface between charge donor and charge acceptor molecular units is important to a wide variety of physical processes in organic semiconductor devices. The exact nature of this state depends heavily on the nature and co-facial overlap between the donor and acceptor; however, altering this overlap is usually accompanied by extensive confounding variations in properties due to extrinsic factors, such as microstructure. As a consequence, establishing reliable relationships between donor/acceptor molecular structures, their molecular overlap, degree of charge transfer and physical properties, is challenging. Herein, we examine the electronic structure of a polymorphic system based on the donor dibenzotetrathiafulvalene (DBTTF) and the acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) in the form of high-quality single crystals varying in the donor–acceptor overlap. Using angle-resolved photoemission spectroscopy, we resolve the highest occupied molecular orbital states of the CT crystals. Analysis based on field-effect transistors allows us to probe the sub-gap states impacting hole and electron transport. Our results expand the understanding on the impact of donor and acceptor interactions on electronic structure and charge transport.
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- Award ID(s):
- 1810273
- NSF-PAR ID:
- 10344599
- Date Published:
- Journal Name:
- Materials Horizons
- Volume:
- 9
- Issue:
- 1
- ISSN:
- 2051-6347
- Page Range / eLocation ID:
- 271 to 280
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Electron donor–acceptor co-crystals are receiving increasing interest because of their many useful optoelectronic properties. While the steady-state properties of many different co-crystals have been characterized, very few studies have addressed how crystal morphology affects the dynamics of charge transfer (CT) exciton formation, migration, and decay, which are often critical to their performance in device structures. Here we show that co-crystallization of a pyrene (Pyr) electron donor with either N , N ′-bis(2,6-diisopropylphenyl)- or N , N ′-bis(3′-pentyl)-perylene-3,4:9,10-bis(dicarboximide) (diisoPDI or C 5 PDI) electron acceptors, respectively, yields mixed π-stacked Pyr–diisoPDI or Pyr–C 5 PDI donor–acceptor co-crystals. Femtosecond transient absorption microscopy is used to determine the CT exciton dynamics in these single crystals. Fitting the data to a one-dimensional charge transfer CT exciton diffusion model reveals a diffusion constant that is two orders of magnitude higher in the Pyr–diisoPDI co-crystal compared to the Pyr–C 5 PDI co-crystal. By correlating the co-crystal structures to their distinct excited-state dynamics, the effects of each mixed stacked structure on the exciton dynamics and the mechanisms of CT exciton diffusion are elucidated.more » « less
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null (Ed.)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 S n ← S 0 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.more » « less
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Abstract A major breakthrough in the field of organic photovoltaics (OPVs) was the development of the donor/acceptor heterojunction that aids in separating Coulombically bound excitons that are generated upon photoabsorption. Additionally, bound charge transfer (CT) states that result from the exchange of charge carriers across the donor/acceptor interface are believed to play an important role in charge generation. Though organic thin films are often disordered, enhancements to the local structural order at the donor/acceptor interface have recently been shown to greatly influence CT state energetics and the charge generation process. In this progress report, recent efforts to understand the role that donor/acceptor morphology plays in the behavior of CT states and the resulting implications on OPV function are presented. It is aimed to provide a survey of different experimental approaches and to present a balanced examination of current interpretations of key results, and to offer best practices for the fabrication and study of morphologically tunable donor/acceptor CT states.
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Photoinduced charge transfer between neighboring bases plays an important role in DNA. One of its important effects is shown in its ability to affect the photochemical yields of the formation of cyclobutane pyrimidine dimer (CPD) products between adjacent pyrimidine bases. In this work we examine how the energies of charge transfer states depend on the sequences of oligonucleotides using a hybrid quantum and molecular mechanics (QM/MM) methodology combined with the algebraic diagrammatic construction through a second order electronic structure method for excited states. Specifically, we examine 10 sequences with guanine being on the 5′ or 3′ position of two pyrimidine bases. The results show that the energies of charge transfer states are affected by the nature of the donor acceptor pair, by the distance between them, and by other electrostatic effects created by the surrounding environment.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1MH01214B
