Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 5929-5934
pubs.acs.org/JPCL
Femtosecond Raman Microscopy Reveals Structural Dynamics Leading to Triplet Separation in Rubrene Singlet Fission Kajari Bera, Christopher J. Douglas, and Renee R. Frontiera* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Singlet fission generates multiple excitons from a single photon, which in theory can result in solar cell efficiencies with values above the Shockley−Queisser limit. Understanding the molecular structural dynamics during singlet fission will help to fabricate efficient organic photovoltaic devices. Here we use femtosecond stimulated Raman spectroscopy to reveal the structural evolution during the triplet separation in rubrene. We observe vibrational signatures of the correlated triplet pair, as well as shifting of the vibrational frequencies of the 1430 and 1542 cm−1 excited state modes, which increase by more than 25 cm−1 in 5 ps. Our results indicate that the correlated pair separation into two individual triplets occurs concurrently with the loss of electron density from the tetracene backbone in rubrene. This study provides new insights into the triplet separation process and proves the utility of structurally sensitive ultrafast vibrational techniques to understand the mechanism of singlet fission.
T
properties needed to generate new materials for highly efficient organic photovoltaics. Rubrene is an organic semiconductor that undergoes SF in the orthorhombic crystalline form.20−22 The structure of rubrene is shown in the inset of Figure 1a, consisting of four phenyl rings, two on each side of a tetracene backbone. In rubrene, the energy of the S1 state is approximately equal to twice the energy of the T state,21,23,24 which is one of the required properties for any molecule to undergo SF.3,6 Many studies have shown that rubrene undergoes efficient SF, thereby generating triplets25,21,26 that have long lifetimes (ns−μs) and long exciton diffusion lengths.25,27 The quantum efficiency of triplet generation through SF in rubrene crystals is ∼100%,28 making it a model system to investigate the structural dynamics associated with SF and multiexciton migration. Even though the fluorescence lifetime in a rubrene crystal lasts for picoseconds,21,22,25,26 a recent 2D electronic spectroscopy (ES) study found that the formation of the correlated triplet pair is extremely fast, on the order of 20 fs, and is in equilibrium with the bright S1 state, and the correlated triplet pair has a lifetime of 2 ps before separating into two separate triplets.18 However, to fabricate devices with optimal charge separation and transport properties, it is important to understand the underlying structural dynamics occurring during SF in order to optimize molecular design for efficient solar energy conversion. Thus, structural studies to examine the molecular motions involved with all of the steps in the SF process are urgently needed.
he efficiency of single-junction photovoltaic devices is limited by the Shockley−Queisser limit to values of ∼34%.1 Fortunately, this number can be increased to 45% through a process known as singlet fission (SF) in which two electron−hole pairs are generated from a single photon.2,3 This phenomenon has been used in photovoltaics and photodetectors, with external quantum efficiencies of 1094 and ∼100%,5 respectively, due to the high efficiency of singlet to triplet conversion.6,7 Despite these promising advances, photodriven devices that make use of SF are not yet viable for widespread commercial adoption, and further gains in overall efficiency will likely require a greater understanding of the mechanisms of energy conversion and charge transport in order to minimize losses. The process of SF is depicted in a simplified form as8 S0 → S1 ↔ 1(TT)⥂1(T ··· T)⥂T + T
(1)
After photoexcitation, a singlet exciton (S1) interacts with a neighboring ground-state molecule, forming a correlated triplet pair, 1(TT), followed by the separation of the triplet pair, 1(T··· T), and eventual separation into two individual triplets. Previous studies have largely examined the triplet generation process,3,6,9−12 which is typically extremely efficient in most SF materials and, thus, challenging to optimize further. However, only a few have identified structural signatures of the correlated triplet pair13−16 and its subsequent separation dynamics.15,17,18 Triplets formed after the separation of the correlated pair typically have a very long lifetime, which helps in exciton diffusion to interfaces for energy harvesting.19 However, the correlated triplet pair must first separate to generate the uncorrelated triplets for further solar harvesting. This necessitates understanding the underlying dynamics of the triplet separation process in SF to identify the specific structural © XXXX American Chemical Society
Received: October 18, 2017 Accepted: November 19, 2017
5929
DOI: 10.1021/acs.jpclett.7b02769 J. Phys. Chem. Lett. 2017, 8, 5929−5934
Letter
The Journal of Physical Chemistry Letters
instrumental response function of 250 ± 2 fs with transmission geometry in an optical microscope. A raw FSR spectrum contains stimulated Raman signals from both the ground and excited states. To obtain only the excited-state features, we subtracted the FSR spectra of the ground state taken in the absence of the photoexcitation pump from identical spectra collected in the presence of the photoexcitation pump. We show the FSR spectra after this one-to-one subtraction in Supporting Information Figure S4. The large depletion of the ground-state peaks in the excited-state spectra demanded the addition of a scaled ground-state spectrum, as is customary in FSRS.36−38 Hence, we added the ground state back to the excited-state difference spectra until no depletion of any of the ground-state peaks was observed (kinetics shown in Figure S5, in accordance with previous work18), resulting in Figure 2. We
Figure 1. (a) Steady-state absorption spectrum of the rubrene crystal used for FSRS measurements along with the molecular structure. (b) Spontaneous Raman spectrum of this rubrene crystal with the incident p-polarized 785 nm excitation normal to the ab plane with prominent peaks indicated by the dashed lines. The inset shows a representative rubrene crystal image under the microscope used for our FSRS studies. The scale bar is 0.4 mm in length.
To provide new insights into the molecular conformational changes occurring during SF, we use femtosecond stimulated Raman spectroscopy (FSRS) in a microscope configuration29 to follow the structural evolution of rubrene during SF and subsequent triplet pair separation. FSRS is an ultrafast vibrational technique that allows for the determination of structural changes in a chemical reaction by monitoring the vibrational modes of the system with high temporal (as short as 50 fs) and spectral resolution (
