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Retrieving the Rate of Reverse Intersystem Crossing from Ultrafast Spectroscopy Jiahua Hu, Qun Zhang, and Yi Luo J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 21, 2016

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The Journal of Physical Chemistry Letters

Submitted for publication in Journal of Physical Chemistry Letters

Retrieving the Rate of Reverse Intersystem Crossing from Ultrafast Spectroscopy Jiahua Hu, Qun Zhang*, and Yi Luo* Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China

Corresponding Authors *[email protected] (Q. Zhang) *[email protected] (Y. Luo)

ACS Paragon Plus Environment

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ABSTRACT Among various non-radiative photophysical processes related to energy migration in singlet–triplet coupled molecular systems, unlike such processes as internal conversion, intersystem crossing (ISC), and intramolecular vibrational relaxation that have been extensively addressed, the reverse ISC (rISC) is a unique one that apparently lacks sufficient interrogation probably owing to its intrinsically elusive nature. Particularly, it still remains a non-trivial task to quantitatively describe the rISC pathway. Here we introduce a new, simple route to this end, just through explicit modeling and simulations on routinely available, experimental data from ultrafast transient absorption spectroscopy. We demonstrate on a proof-of-concept, rare-earth chelate molecular system that our approach, featuring spectral profile analysis together with wavelength-dependent global kinetics fitting, enables facile retrieval of the rISC rate from experimental data. TOC GRAPHICS

KEYWORDS reverse intersystem crossing, ultrafast transient absorption spectroscopy, spectral profile, global kinetics fitting, rare-earth chelate molecule


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In singlet–triplet coupled molecular systems, effective manipulation between the “bright” (singlet) and “dark” (triplet) spaces1,2 usually holds the key to a variety of important photophysical and photochemical applications such as organic light-emitting diodes (OLEDs),3–7 photovoltaics,8,9 and photocatalysis.10,11 In terms of conventional fluorescent OLEDs, the maximum internal efficiency is limited to 25% unless one manages to convert triplets to emissive singlets12 or to harvest triplets using phosphorescent materials.13 The singlet-to-triplet intersystem crossing (ISC) process has been extensively addressed in the context of Jablonski diagram.14,15 Its reverse process of population transfer from triplets to singlets, i.e., the so-called reverse ISC (rISC), proven vital to improving the OLEDs efficiency,3–7 is however far from being explored and understood. Specifically, it still remains a challenge to quantitatively characterize the rISC rate in singlet–triplet coupled molecular systems. The currently existing approaches to this end mainly include (i) temperature-dependent measurements of thermally activated delayed fluorescence as well as prompt fluorescence and phosphorescence,16,17 and (ii) two-laser induced fluorescence and transient absorption measurements.18–21 Unfortunately, both methods necessitate sophisticated procedures to deduce the rISC rate from experiments in an indirect, nontrivial manner. Alternatively, we here introduce a new, facile approach for retrieving the desirable rISC rate simply from routine, femtosecond time-resolved transient absorption (TA) spectroscopy. We demonstrate on a proof-of-concept molecular system that explicit modeling and simulations on TA spectral profiles, in conjunction with wavelength-


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dependent global kinetics fitting based on experimental data, allow for determining the rISC rate, a feat that has never, to the best of our knowledge, been attained in similar ultrafast spectroscopy studies in the past. The selected model molecular system in this work is a rare-earth chelate molecule (TTA)3–Er3+–(TPPO)2, where three central ligands of thenoyltrifluoroacetonato (TTA) and two synergetic ligands of trioctylphosphine oxide (TPPO) are coordinated with Er3+ (see the inset of Figure 1). Such kinds of lanthanide coordination complexes with organic ligands have been widely used as photoluminescence materials and subjected to decades of investigation.22–25 The photophysical picture describing various pathways of energy migration involved in such molecular systems has also been established.26–28 For the (TTA)3–Er3+–(TPPO)2 system of interest, as schematically illustrated in Figure 1, upon photoexcitation from the ground state (S0) to the vibrational manifold of the first excited singlet state (S1), the excited TTA molecule may undergo a non-radiative, singlet-to-triplet ISC process, followed by resonance energy transfer (RET) from its triplet state(s) to adjacent ion state(s) of Er3+. Subsequently, population relaxation from the upper ion state(s) to the lower ones takes place, finally resulting in certain characteristic line emission(s).26 The main reasons for us to choose such a rare-earth chelate molecule as the model system are twofold. On the one hand, the presence of rare-earth Er3+ ion is known to greatly enhance singlet–triplet coupling due to the socalled heavy-atom effect,29 which would benefit the observation of the elusive rISC pathway. On the other hand, the fast TTA-to-Er3+ RET process (on a time scale of sub-


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100 ps, as verified experimentally) would facilitate the data acquisition of TA relaxation kinetics within the nanosecond time window of our pump–probe spectrometer.

Figure 1. Schematic illustration of the relevant state energetics and energy-migration pathways involved in the model system of rare-earth chelate molecule (TTA)3–Er3+–(TPPO)2, whose chemical structure is displayed as an inset.

In our femtosecond time-resolved TA measurements, a routine scheme that features an ultraviolet pump and a white-light continuum probe was adopted. The center wavelength of the pump was set at 340 nm (~29400 cm-1), corresponding to the S0→S1 (vibrationally excited) transition in TTA (Figure 1), roughly 4400 cm-1 higher than the bottom of S1 state (~25000 cm-1) [refer to the steady-state absorption spectrum in Figure S1, Supporting Information, in which one can also see that the synergetic ligand TPPO molecule shows nearly no absorption in the UV region around 340 nm]. Displayed in Figure 2a are the TA spectra registered in a broad wavelength range (385– 760 nm, or ~13000–26000 cm-1) at several representative probe delays of 1, 5, 15, 100, and 300 ps, from which one can find two distinct spectral profiles labeled “Band-A” (~13000–21000 cm-1) and “Band-B” (~21000–26000 cm-1). Figure 2b exhibits the typical


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kinetic traces taken at the band maxima, i.e., 16400 and 25000 cm-1, for Band-A and Band-B, respectively. As for Band-B, the TA signal rapidly builds up (within the instrument response function, ~100 fs) and then decays in a bi-exponential fashion: τ1 = 3.00 ± 0.05 ps and τ2 = 73 ± 2 ps. As for Band-A, however, the TA signal builds up much slower (τ1 = 3.7 ± 0.1 ps), followed by a single-exponential recovery (τ2 = 77 ± 1 ps). In light of the consistency in time constants, one can safely infer that τ1 describes the singlet-to-triplet ISC process that features depletion of singlet population and concomitant growth of triplet population, while τ2 accounts for the triplet-to-Er3+ RET process in common for both bands. In other words, Band-B (A) arises most likely as a result of excited-state absorption occurring in the singlet (triplet) space. Similar observation and assignments can be found in the literature.28 The above assignments basically capture the essence of photophysical picture; nevertheless, care must be taken in another undeniable fact pertaining to the evolution of spectral profiles. That is, no matter whether at early or late delay times, Band-A and Band-B always emerge in a concomitant manner (see Figure 2a). As for Band-A, one may argue that the occurrence of a discernable spectral red-shift at early times, as compared to the late-time triplet profiles therein, could hint a blending with some other singlet excited-state absorption features. But, it would be farfetched to make a similar argument for the late-time persistence of pure singlet feature in Band-B, as nearly no discernable red- or blue-shift occurs there. Presumably, this sort of spectral evolution could have also been observed in other singlet–triplet coupled molecular systems but


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unfortunately overlooked simply because attention was paid only to extracting certain kinetic constants of interest (e.g., kISC ).

Figure 2. (a) Representative TA spectra recorded in 385–760 nm (i.e., ~13000–26000 cm-1) at probe delays of 1, 5, 15, 100, and 300 ps, from which one can detect distinctly different evolution of spectral profiles for the two bands labeled “Band-A” and “Band-B” (refer to the dashed, grey arrows that guide the eye). (b) Typical kinetic traces taken at the band maxima for Band-A (~16400 cm-1) and Band-B (~25000 cm-1). The matching between the kinetics fitting results ( 1 and  2 ) is highlighted by dashed, grey ovals. The TA signal (i.e., absorbance change, A ) is given in the unit of mOD (OD: optical density).


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Intuitively, the subtle observation that Band-A (triplet feature dominated) and Band-B (pure singlet feature) coexist at both early and late times would most likely correlate with the usually ignored, triplet-to-singlet rISC process. It is worth noting that in our current case study the energy position (within S1 state) reached by the 340-nm pump is ~2000 cm-1 higher than the bottom of a nearby excited triplet state (denoted Tn in Figure 1b; refer to Figure S2 in Supporting Information for Tn and T1 state energetics). As is known, such sort of photoexcitation configuration usually favors the rISC process7 thanks to enhanced singlet–triplet coupling in relatively high, singlet–triplet overlapping energy region. To examine the above prediction, we took a further step to conduct populationevolution modeling with the rISC pathway being involved. The time-dependent singlet and triplet population, denoted S (t ) and T (t ) , respectively, can be described by S (t ) / t  kISC S (t )  krISCT (t )

(1.1)

T (t ) / t  kISC S (t )  krISCT (t )  kRETT (t )

(1.2)

where kISC , krISC , and kRET denote the ISC, rISC, and RET rates, respectively. Jointly solving eqs 1.1 and 1.2 gives the following simplified, analytical expressions S (t )  (1  κ )1 exp[ (1  κ )kISCt ]  κ (1  κ )1 exp[ (1  κ )1 kRETt ]

(2.1)

T (t )  (1  κ )1 exp[ (1  κ )kISCt ]  (1  κ )1 exp[(1  κ )1 kRETt ]

(2.2)


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where κ  krISC / kISC

(2.3)

The details of formulation are given in Supplementary Note (Supporting Information), where we adopted certain proper approximations kISC  krISC and kISC  kRET that are known as usually being the case for rare-earth chelate systems (see, e.g., ref 28) and also well justified in the current case study. The TA signal ( A ) detected in the pump–probe configuration should comprise both the singlet and triplet contributions in the form of A  ASS (t )  ATT (t )

(3)

where AS(T) represents a wavelength-dependent amplitude of the singlet (triplet) spectral profile. Substitution of eqs 2.1 and 2.2 into eq 3 yields A  P exp[ (1  κ )kISCt ]  Q exp[ (1  κ )1 kRETt ]

(4.1)

where the two pre-exponential coefficients are

P  (1  κ )1 AS  (1  κ )1 AT

(4.2)

Q  κ (1  κ )1 AS  (1  κ )1 AT

(4.3)

Interestingly, summation of the two coefficients produces the pure singlet profile, i.e.,


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P  Q  AS

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(4.4)

We realized that the above modeling framework actually opens up the possibility for retrieving all of the key kinetic constants, i.e., the ISC, rISC, and RET rates. By noting that the wavelength-dependent, overall A takes an explicit, bi-exponential form (see eq 4.1), we performed a global kinetics fitting based on the experimental TA data in the entire probing wavelength range 385–760 nm (i.e., ~13000–26000 cm-1) with a 5-nm step, yielding two fitting parameters:  1  3.3 ps and  2  77 ps, which correspond to (1  κ )1 / kISC and (1  κ ) / kRET , respectively, according to eq 4.1. Certainly, the three

quantities of interest ( kISC , krISC , and kRET ) cannot be deduced only from the two relations with  1 and  2 . In an attempt to find one more relation, we resorted to numerical simulations of the

P  Q and Q profiles (refer to eqs 4.4 and 4.3, respectively) based on the above wavelength-dependent global kinetics fitting. The simulated results in the experimental wavelength range (with a 5-nm step) are presented in Figure 3. As compared with the TA spectra in Figure 2a, the spectral profiles of P  Q (Figure 3, upper panel) and Q (Figure 3, lower panel) turn out to nicely reproduce the observed ones at early (e.g., 1 ps, black line in Figure 2a) and late (e.g., 15 ps, green line in Figure 2a) delay times, respectively. Not surprisingly, the achieved spectral reproduction validates again the proposed modeling framework in which the rISC pathway has been taken into account. Notably, the presence of singlet contribution in the Q profile [i.e., the κ (1  κ )1 AS


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term, see eq 4.3] is commensurate with the late-time persistence of pure singlet feature observed in Band-B (Figure 2a). The extent of this late-time singlet contribution relies on the rISC pathway through the κ coefficient (i.e., the ratio of krISC and kISC , see eq 2.3) in the form of κ (1  κ )1 , offering a quantitative description of the role played by rISC in impacting on the spectral evolution. Band-B at around 25000 cm-1 can be undoubtedly identified as arising from excitedstate absorption in the singlet space, no matter in which simulation circumstance ( P  Q or Q ). By virtue of this important fact, we performed a single-profile Gaussian fitting specifically for this pure singlet band in both the P  Q and Q profiles (see the solid, red lines in Figure 3) and used the resulting area ratio,  A , as a crude yet rational measure to evaluate the pure singlet contribution in this specific energy region. According to eqs 4.3 and 4.4,

A  [κ (1  κ )1 AS ] / AS  κ (1  κ )1

(5)

In the current case study, the value of  A is estimated to be ~0.226, and thus the coefficient κ is determined to be ~0.292. Remarkably, for the investigated rare-earth chelate system, krISC is as high as nearly one-third kISC , revealing a rather efficient triplet-to-singlet reverse energy migration therein. Taking together the three relations at hand: (1  κ )1 / kISC  3.3 ps, (1  κ ) / kRET  77 ps, and κ  krISC / kISC  0.292, we can finally retrieve the rates of interest: kISC  0.235 ps-1, krISC  0.068 ps-1, and kRET  0.017 ps-1,


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which can be translated to the more readable, characteristic time constants: ~4.3 ps for ISC, ~14.6 ps for rISC, and ~58.8 ps for RET.

Figure 3. The simulated spectral profiles for (a) P  Q and (b) Q , based on the explicit modeling framework (eqs 1–4) in conjunction with wavelength-dependent global kinetics fitting based on the experimental TA data. In each case, the pure singlet contribution (Band-B, at around 25000 cm-1) singled out with a Gaussian fitting profile is highlighted in solid, red line. The simulated TA signal (i.e., absorbance change, A ) is given in the unit of mOD (OD: optical density).

Last but not least, it would be helpful to address the question why not to directly extract the component of rISC from the routine kinetics fitting. On the one hand, the


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kinetic traces for both Band-A and Band-B turned out to be fitted only by using a biexponential function, as exemplified in Figure 2b. Such a fitting yielded two characteristic time constants of roughly 3 and 70 ps, which are nominally responsible for ISC and RET, respectively, when rISC is not taken into account (similar treatment and results can be found in the literature; see, e.g., ref 28). As a matter of fact, it is just the annoying failure to directly extract the component of rISC from the routine kinetics fitting that has motivated us to introduce the new approach reported in this work. On the other hand, it seems reasonable that the rISC process may be observable in the TA spectra given that krISC  kRET . Nevertheless, it is worth noting that the ISC and rISC processes rapidly establish a dynamic equilibrium between the singlet and triplet populations in a few picoseconds with the triplet state being activated, keeping both the singlet and triplet populations (no matter how many) in a sort of steady state. The subsequent slow RET process serves as a rate-limiting pathway to deplete the triplet population on the molecular side, which can be viewed as a persistent perturbation to the singlet–triplet equilibrium. In such a dynamic equilibrium, rISC turns out to exert less influence on the equilibrated populations than ISC does, thereby making it difficult, if not impossible, to directly extract the component of rISC from the routine kinetics fitting. In summary, this work introduces a new, simple method to determine the elusive rISC rate in singlet–triplet coupled molecular systems. Through explicit and physically meaningful modeling of singlet/triplet spectral profiles in combination with


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wavelength-dependent global kinetics fitting based on routinely available, experimental data from ultrafast transient absorption spectroscopy, we demonstrate a first example of quantitatively retrieving the seldom addressed rISC rate in a proof-of-concept model system of rare-earth chelate molecule (TTA)3–Er3+–(TPPO)2. We expect that this facile yet robust approach can be readily extended to similar studies on other singlet–triplet coupled molecular systems.

 EXPERIMENTAL METHODS The ultrafast TA measurements were performed, under ambient conditions, on a modified ExciPro pump–probe system (CDP) in combination with an amplified femtosecond laser system (Coherent). The 340-nm pump pulses were delivered by an optical parametric amplifier (TOPAS-800-fs), which was excited by a Ti:sapphire regenerative amplifier (Legend Elite-1K-HE; center wavelength 800 nm, pulse duration 25 fs, pulse energy 3 mJ) seeded with a mode-locked Ti:sapphire laser system (Micra 5) and pumped with a 1-kHz Nd:YLF laser (Evolution 30). The stable white-light continuum (WLC) probe pulses were generated by focusing the 800-nm beam (split from the regenerative amplifier with a portion of 10%) onto a rotating CaF2 crystal plate (385–760 nm, i.e., ~13000–26000 cm-1 for the TA measurements presented in the main text) or onto a fixed sapphire crystal plate (1020–1450 nm, i.e., ~6900–9800 cm-1 for the TA measurements presented in Supporting Information). The linear chirp of the WLC


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spectrum was compensated with the aid of the cross-phase modulation signals recorded on a liquid hexane sample using an ExiPro 2.6 software. The WLC pulses were free of pre-pulsing and after-pulsing, as verified by our routine pulse characterizations. The instrument response function was determined to be ~100 fs by cross-correlating the pump and probe pulses at the sample cell. Precise spatial overlap of the pump and probe beams at the 1.2-mm-thick sample cell (quartz) was attained by optimizing the transient signals with the aid of a laser beam analyzer (BG-USB-SP620, Ophir-Spiricon). The delay times between the pump and probe pulses were varied by a motorized optical delay line (minimum step ~1.6 fs; maximum delay ~3 ns). The delay time zero was determined by cross-correlating the pump and probe pulses at the sample cell in situ and also carefully cross-checked with other chemical samples such as the DCM and LDS698 dyes. The sample cell containing the (TTA)3–Er3+–(TPPO)2 samples (well dissolved in pure DMSO) was mounted on a rapidly rotating stage (5000 rpm) to ensure that the photoexcited volume of the sample was kept fresh during the course of the TA measurements. The temporal and spectral profiles of the pump-induced differential transmission of the WLC probe light were visualized by a 1024-pixel imaging spectrometer (CDP2022i) and further processed by the ExiPro 2.6 software equipped with the CDP pump–probe spectrometer.


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 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.xxxxxxx. Steady-state absorption spectrum (Figures S1), energetics identification for T1 and Tn states (Figure S2), and details of modeling formulation (Supplementary Note) (PDF)

 AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] Notes The authors declare no competing financial interests.

 ACKNOWLEDGMENTS This work was supported by the MOST (nos. 2016YFA0200602, HH2060030013), the NSFC (nos. 21573211, 21421063), the CAS (XDB01020000), and the Fundamental Research Funds for the Central Universities (WK2340000063). We are grateful to Dr. Z.-J. Hu and Prof. Q.-J. Zhang at the University of Science and Technology of China for providing us with the investigated samples.


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TOC Graphics 69x68mm (150 x 150 DPI)


Retrieving the Rate of Reverse Intersystem ... - ACS Publications

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