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Singlet Fission Mediated Photophysics of BODIPY Dimers Raul Montero, Virginia Martinez-Martinez, Asier Longarte, Nerea EpeldeElezcano, Eduardo Palao, Iker Lamas Frejo, Hegoi Manzano, Antonia R Agarrabeitia, Iñigo Lopez-Arbeloa, Maria José Ortiz, and Inmaculada García-Moreno J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03074 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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The Journal of Physical Chemistry Letters

Singlet Fission Mediated Photophysics of BODIPY Dimers Raúl Montero1*, Virginia Martínez-Martínez2*, Asier Longarte3, Nerea Epelde-Elezcano2, Eduardo Palao4, Iker Lamas3, Hegoi Manzano5, Antonia R. Agarrabeitia4, Iñigo López Arbeloa2, Maria J. Ortiz4 and Inmaculada Garcia-Moreno6 1

SGIKER Laser, Universidad del País Vasco, UPV/EHU, Apartado 644, 48080 Bilbao, Spain

2

Molecular Spectroscopy Laboratory and 3Spectroscopy Laboratory, Departamento Química

Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco, UPV/EHU, Apartado 644, 48080 Bilbao, Spain 4

Departamento de Química Orgánica I, Facultad de CC. Químicas, Universidad Complutense de

Madrid, Ciudad Universitaria s/n, 28040, Madrid, Spain 5

Departamento de Física de la Materia Condensada, Universidad del País Vasco, UPV/EHU,

Apartado 644, 48080, Bilbao, Spain 6

Departamento de Sistemas de Baja Dimensionalidad Superficies y Materia Condensada

Instituto Química Física “Rocasolano” C.S.I.C., Serrano 119, 28006, Madrid, Spain Corresponding Authors [email protected] and [email protected]

ACS Paragon Plus Environment

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ABSTRACT. The photodynamics of an orthogonal BODIPY dimer, particularly the formation of triplet states, has been explored by femtosecond and nanosecond transient absorption measurements. The short time-scale data show the appearance of transient features of triplet character that according to a quantitative analysis of their intensities, account for more than 100% percent of the initially excited molecules, which reveals the occurrence of a singlet fission process in the isolated dimers. The formation rate of the triplet correlated state 1(TT) is found to depend on the solvent polarity, pointing to the mediation of a charge transfer character state. The dissociation of the 1

(TT) state into pairs of individual triplets determines the triplet yield measured in the long time-

scales. The kinetic model derived from the results provides a comprehensive view on the photodynamics of BODIPY dimers and permits to rationalize the photophysical parameters of these systems.

TOC GRAPHICS

KEYWORDS. Singlet fission, BODIPY dimer, femtosecond transient absorption, triplet photosensitizer, solvent dependence, charge transfer.


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During the last years the development of new triplet photosensitizers has attracted a great deal of attention due to their potential application in a wide variety of fields such as solar energy harvesting,1–3 photoinduced H2 production4, organic chemistry and catalisys,5–7 and photodynamic therapy,8–11 among others. For this purpose, Boron dipyrromethene (BODIPY) multi-chromophores appear as a very promising candidates, since these species, while retaining the required photostability of the monomers, are able to provide high triplet quantum yields12,13 without the toxicity associated to the introduction of heavy atoms, which is key for future bioaplications, and particularly for therapies.14–17 The experimental characterization and theoretical modeling of their photophysical properties, specially intended to control triplet state formation, has accordingly gained a great interest in recent times.18–21 Aimed to further explore the relaxation mechanisms behind the ultimate photophysical behavior of these systems, we have investigated the solvent dependent photodynamics of a recently synthesized orthogonal BODIPY dimer,21 by femtosecond (fs) and nanoseond (ns) transient absorption (TA) measurements. The short time-scale measurements reveal that, as a function of the medium polarity, triplet states are formed in a few picoseconds to reach transitory efficiencies >100%. This observation points to singlet fission (SF)22–25 as the mechanism controlling the system relaxation. SF is known from long ago,26–29 but due to its potential ability to multiply the number of excitons, a quite recent interest in modeling and exploiting its properties has developed in the field of photoactivated processes, especially in photovoltaics.1,3 For SF tficiently compete with ordinary radiationless relaxation routes a number of requirements have to be fulfilled: i) an energy level structure where the T1 state energy is less than or equal to half that of the S1←S0 excitation and ii) a suitable interaction between the chromophores, among others.


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The phenomenon has been observed to occur in crystals26–30, polymers31,32 aggregates33,34 and, as in the present case, in isolated molecules containing more than a covalently linked chromophore.35–41 Although the latter are promising materials in terms of applicability, the number of these systems presenting SF is still quite limited. In this context, the characterization of intramolecular SF in BODIPY dimers not only permits us to rationalize previous findings on the photophysics of these systems,21 it also allows us to build a general relaxation scheme that can help in developing BODIPY based photomaterials with targeted properties.

Figure 1. a) fs-TAS of the BODIPY 546 dimer in cyclohexane, recorded after exciting at 509 nm in the S0→S1 transition. b) Absorption spectra corresponding to four characteristic time-delays.


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fs-TAS experiments were conducted on BODIPY 546 dimer solved in cyclohexane (c-hex), chloroform (CHCl3), and acetonitrile (ACN) after excitation at 509 nm (501 nm in ACN), in the S1 absorption band (Figure S1). Figure 1 shows the data collected in c-hex. The spectra, characterized by a slow decay compatible with the ∼5 ns lifetime and the high fluorescence quantum yield reported in Ref. 21 (Φfl = 0.92, Table S1), do not change along the explored delay window. Consequently, we can assign all the spectral features observed to the ground and the initially populated S1 states, that is: i) the main contribution, the negative peak around 500 nm, corresponds to the ground state bleach (GSB) and reflects the population promoted from S0 to S1 due to laser absorption. ii) The stimulated emission (SE) from S1 appears overlapped with the GSB and extends to the red up to 650 nm. iii) A positive contribution, attributed to S1→Sn excited state absorption, is also observed at 360 nm. Figure 2 shows the results obtained in CHCl3, which exhibit substantial differences with respect to those recorded in c-hex. In this case, four contributions are found in the 350-800 nm observation window. Three of them have already been assigned in c-hex, which are: the negative GSB peak around 500 nm, the negative SE band in the 540 y 650 nm range and the positive excited state absorption at the UV edge of the spectrum that exhibits the same dynamics than the SE. The fourth band at ~413 nm, which was absent in the non-polar medium, can be attributed, on the base of the ns-TAS measurements included in the accompanying supplemental information (SI), to the absorption of triplet states (Table 1, Figure S2 and Figure S3). Indeed, a very long triplet lifetime (τT = 386 µs) is recorded in N2 saturated CHCl3 (Table 1, Figure S3), the longest obtained for a BODIPY derivative up to now,42–44 which makes it an excellent photosentiziser even in the presence of low oxygen concentration. The oxygen quenching experiments give rate constant values in agreement with those expected for a triplet-triplet


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energy transfer (Kq = 7 x108 M-1 cm-1 for CHCl3, Table 1, Figure S5), confirming the triplet nature of the band at 413 nm. Accordingly, a high triplet state yield was determined by singlet oxygen measurements (Φ∆=0.75).21 We can anticipate that, as the temporal evolution of the system described below demonstrates, these triplet states result from a SF process. It is worth to mention here that according to molecular simulations, the condition for the relative energies between the singlet and triplet states ES1 ≥ 2ET1, is fulfilled (Figure S6).

Figure 2.a) fs-TAS of the BODIPY 546 dimer recorded in CHCl3 after excitation by 509 nm radiation. b) Absorption spectra corresponding to the indicated time-delays. c) Temporal evolution of the main bands in the fs-TAS (dots) together with the best fit obtained (solid lines). d) The DAS corresponding to the lifetimes extracted from the modeling of the fs-TAS


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Table 1. Triplet state lifetimes in nitrogen (τN2), air (τair) and oxygen (τO2) purged solutions determined by ns-TAS, rate constant of oxygen quenching (Kq) and the comparison of the ΦTT values extracted from the fs-TAS data (ΦTT*) and those calculated by expression 1 (ΦTT**) ΦTT*

ΦTT**

1.71

1.60

1.99

125

0.70

1.32

1.40

0

0

0

-

τN2 (µs)

τair (ns)

τox (ns)

ACN

158

158

65

CHCl3

386

468

Cyclohexane

0

0

Kq (109 M-1 s-1)

Once the spectral features have been identified, we will focus on their time dependence. Figure 2c show the fs-transients corresponding to some characteristic wavelengths, which were modeled by a global fit that yielded a triexponential decay function with the following time constants τ1=16.2±0.2 ps, τ2=391±10ps y τ3 >>1ns, the latter value cannot be precisely determined with our femtosecond set-up. In Fig 2d, the decay associated spectra (DAS), which characterize the weight (ai) of each time constant (τi) along the absorption spectra, are exhibited. The τ1 DAS is dominated by two negative bands at 413 and 540 nm. The first feature at 413 nm corresponds to triplet states and, as the absorption is positive in this region, the negative value of a1 indicates that the population builds up during the first 16 ps. We can interpret it as the formation of a triplet correlated state (TT) that occurs during the SF process (see below for further proof). It has been shown that the absorption spectrum of the TT state contains, particularly in its visible portion, characteristic features of individual triplet states36,37,40,45,46 The second band in the 520620 nm range is the decay of the SE and reflects the lifetime of the initially excited S1 state. This temporal correlation between the S1 and TT states, also evidenced by the isobestic point at 390


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nm (Fig. 2b), indicates that the triplet exciton is created directly from S1 or through a short living intermediate state possibly with charge transfer character (see below). The τ2=391±10 ps DAS reproduces the TT and GSB absorption bands (Figure 2d). The former positive sign points out to a decay process, while the latter means that the ground state is recovered at the same rate. Consequently, we can assign the τ2 component to direct conversion of the TT to the ground state. However, a positive feature with the form of the triplet states absorption is also present in the DAS of the τ3 long time constant. This means that in addition to give rise to ground state dimers, an important part of the population survives excited in individual triplet states (T+T) being responsible of the high measured Φ ∆=0.75. In essence, we can conclude that the population of the initially prepared S1 state gives raise during the first 16 ps to TT correlated excitons that subsequently relax in 391 ps, to yield ground state dimers and the localized T+T states. From these dynamical data, and taking the Φ∆=0.75 derived from the singlet oxygen formation measurements21 as reference, we can go further and extract quantitative data on the efficiency of the SF process considered as the unique process to populate the triplet state, neglecting the intersystem crossing. Since the formation of the singlet oxygen takes place in a long µs time scale, we can assume that Φ∆ reflects the portion of the population that ends in long living T+T states. In the fs-TAS experiments, this number would correspond to the intensity that the triplet states band (413 nm) reaches at long delay times (>1ns), and can be extracted directly from the a3 DAS at 413 nm. Thus, by comparison with the maximum value reached by the 413 nm band after the excitation, which can also be derived from the DAS as a2+a3 at this wavelength (more details of this calculations in the SI), the TT state


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yield can be estimated to reach a value of 132% (ΦTT=1.32±0.1). This number above 100%, together with the fast formation of the triplets, is the fingerprint of the SF process. Additionally, it is also possible to demonstrate that the high triplet yield measured in the singlet oxygen experiments is due to the presence of doubly excited chromophores (T+T) in isolated dimers. At long delays, the contribution of the GSB band (measured at 480 nm to avoid scattering) is provided by the a3 coefficient, representing the fraction of molecules that have not returned to the ground state. This value is 0.59 of the maximum reached at shorter delays, being the latter given by the sum of a2 and a3. Knowing from the photoluminescence experiments that the fluorescence quantum yield is 0.22 and the fluorescence lifetime 5 ns (Figure S1, Table S1),21 we can estimate that the fraction left in the long living T+T states is ~0.37 (see SI for further detail on this calculation). Since these molecules yield the Φ∆=0.75 measured in the singlet oxygen experiments, it means that each dimer contributes doubly. From this observation, we can conclude that the formed triplets correspond exclusively to localized T+T states that result from the SF process. In order to gain more insights in the SF mechanism, fs-TAS experiments were conducted also on the dimer solved in ACN, see Figure 3. The spectra show essentially the same features already observed in CHCl3, plus an additional positive contribution that overlaps with the SE in the 540-650 nm region. The feature follows a dynamical behavior similar to the TT band at ~413 nm, suggesting an analogous origin. The assignment is confirmed by previous ns-TAS measurements on BODIPY dimers and iodinated derivatives having large ΦICS,19,47–49 and by our own experiments (Table 1, Figure S4). It is worth to mention that this band is also present in the


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CHCl3 experiments (Figures 2a and b); however, in the time scale of the fs-TAS measurements, it is buried under a higher negative contribution of the SE.

Figure 3. a) fs-TAS of BODIPY 546 dimer excited at 509 nm in ACN. b) Absorption spectra corresponding to the indicated time-delays. c) The DAS for the lifetimes derived from the modeling of the fs-TAS.


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Two fundamental differences can be found between the CHCl3 and ACN measurements. First, regarding the spectra, the amplitude of the GSB and TT contributions at long delays (a3 trace in Figure 3c) is clearly lower in ACN, meaning that the fraction of molecules that survive in triplets is smaller. This observation agrees with the lower singlet oxygen quantum yield measured for this solvent (Φ∆ = 0.25).21 Second, with respect to the dynamics, the formation of the TT state occurs much faster in ACN, as indicated by the value of τ1=0.75±0.2 ps derived from the temporal evolution of the spectra. Conversely, not much variation is observed for the rate of the TT decay constant, τ2=453±35 ps. Applying the analysis already described above, we can estimate the TT quantum yield and fraction of molecules that reach the T+T states along the relaxation process in this solvent. The obtained values are 1.60±0.35 and 0.09±0.03 (more details in the SI), respectively. The latter corresponds approximately to one half of Φ∆, (0.25)21, meaning that the both chromophores in each excited molecule remain in triplets state at long time delays (T+T state). Consequently, the results in ACN, also unambiguously identify SF as the process that determines the final triplet yield. Considering the data derived from the three used solvents, it is possible to put together a relaxation scheme that gives a physical meaning to the temporal constants extracted from the time-resolved data. Note here that SF is a phenomenon of coherent nature25 and any kinetic approach will not able to reproduce some fundamental aspects of the problem. The proposed model, shown in Scheme 1, can sustain a comprehensive interpretation of the results. The locally excited S1 state couples to the TT state, very likely, through a charge transfer (CT) state. The mediation of the SF by CT states has been pointed out theoretically35,49 and demonstrated


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experimentally in polymers32 and covalent dimers, in which the dependence of the SF rate on the solvent, has been associated, as it has been observed here, with the mediation of the process by CT states.36–40

Scheme 1. Schematic representation of the relaxation route followed by the BODIPY dimer after excitation in polar solvents. The formation of the TT state (described by the τ1 constant) competes with vibrational relaxation and intramolecular vibrational redistribution; τ2 is related with the direct conversion of TT to the ground state and the formation of localized T+T states; τ3 accounts for the decay of the longer living species: T+T and relaxed S1 states, depending on the considered wavelength. The formation of the correlated TT state, characterized by τ1, competes with the electronic coherence loss (τR) introduced by processes as vibrational relaxation and intramolecular vibrational redistribution, which produce a relaxed S1 state. Therefore, the τ1/τR ratio determines the efficiency of the SF process. This fact can be deduced from the presented data when comparing the TT state formation quantum yields in CHCl3 and ACN. The faster rate of τ1 in the latter results in a higher ΦTT, 1.60±0.35 vs 1.32±0.1. The same argument can be invoked to explain the absence of SF in the non-polar CH solvent.


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The TT correlated pair simultaneously gives rise to two independent triplets (T+T), by the action of decoherence mechanisms, and to the ground state. A single temporal constant, with similar values in CHCl3 and ACN, τ2~400 ps, is found to account for both processes. The branching ratio controls the yield of localized triplet states formed in the long term, Φ∆=0.75 in CHCl3 vs Φ∆=0.25 in ACN. In the light of the proposed model, it is possible to rationalize most of the photophysical properties found for BODIPY dimers. For example, in ACN the sum of the dimer ΦFl+Φ∆ has been measured to be as low as 0.25,21 which was difficult to conciliate with the low rate of direct S1→S0 internal conversion (IC) found for the monomer (Table S1). However, an interpretation in terms of the TT correlated pair formation, followed by conversion to the ground state, permits to fully understand these facts. Another interesting result derived from the proposed model is that we can estimate the triplet quantum yield of BODIPY dimers in different solvents, knowing the monomer and dimer fluorescence quantum yields. If we assume that the fluorescence and IC rate measured for the monomer are conserved in the relaxed S1 state of the dimer, the ΦTT can be calculated by:

Φ =2∙(1-

Φ

Φ

)

(1)

where ΦflD and ΦflM are the fluorescence quantum yield of the dimer and monomer, respectively. The agreement with the values obtained from fs-TA data in c-hex, CHCl3 and ACN sustains the validity of the proposed model, which might be applied in many other solvents, i.e. acetone, THF and toluene with a predicted ΦTT of 1.98, 1.61 and 0.22, respectively, according to the data in Table S1.


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In conclusion, the presented experimental evidences reveal that the relaxation routes of an orthogonal BODIPY dimer, after excitation along the S0→S1, can be understood in terms of a SF mechanism. The model proposed to explain the observed dynamical opens the door to the rational design of new multi-chromophore BODIPYs for application as triplet photosensitizers. The work also identifies key aspects of the problem that might guide the future research intended to gain a deeper knowledge of the SF process in isolated systems, such as the role of the CT states and the exact nature of the other competing relaxation pathways.

ASSOCIATED CONTENT Supporting information. Experimental details, absorption and emission spectra and fluorescence decay curves of BODIPY 546 dimer, fluorescence quantum yields of BODIPY 546 monomer and dimer in different solvents, results of ns-transient absorption spectra (ns-TAS) experiments, computational simulations of the dimer, and details of the calculation of ΦTT are included. AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was funded by the Spanish MINECO through the CTQ2015-68148-C2-1-P, MAT2017-83856-C3-1-P, MAT2017-83856-C3-2-P, MAT2017-83856-C3-3-P and MAT2016-


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Singlet Fission Mediated Photophysics of BODIPY ... - ACS Publications

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