Intramolecular Electron Transfer in Frozen Solvents: Charge Transfer

Sep 11, 2017 - In frozen solvents at 77 K, ultrafast (≤250 fs) photoinduced intramolecular electron transfer (ET) in bichromophoric donor–acceptor...
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Intramolecular Electron Transfer in Frozen Solvents: Charge Transfer and Local Triplet States Population Dynamics Revealed by Dual Phosphorescence Jerzy Karpiuk,*,† Alina Majka,‡ Ewelina Karolak,‡,§ and Jacek Nowacki†,⊥ †

Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

‡

S Supporting Information *

ABSTRACT: In frozen solvents at 77 K, ultrafast (≤250 fs) photoinduced intramolecular electron transfer (ET) in bichromophoric donor−acceptor ([D−A]) diarylmethane lactones produces a covalently linked radical ion pair, 1[D•+−A•−]. Steady state and timeresolved luminescence measurements reveal that 1[D•+−A•−] decays to charge-separated (3[D•+−A•−]) and donor-centered ([3D*−A]) triplets, which display dual phosphorescence. 3[D•+−A•−] and [3D*−A] are formed in parallel via two intersystem crossing mechanisms: spin orbit charge transfer (SOCT) and hyperfine coupling (HFC), with solvent dependent branching ratio. The solvent drives the D−A alignment during the freezing process to adapt to increasing solvent polarity, producing inhomogeneous groundstate population distribution with solvent-dependent D−A exchange interaction, which plays a key role in partitioning into SOCT and HFC mechanisms. In polar glasses, a third phosphorescence band appears due to dissociative back ET in 3[D•+−A•−] resulting in excited open ring biradical.

R

state level,13 and, to our best knowledge, never as one coming from 3LE and 3CT states of a single molecule. Triarylmethane lactones (LTAM) are compact D−A−D triads composed of two electron-donating (D) groups weakly coupled via an sp3 carbon bridge to an electron-accepting (A) moiety. Low energy UV photoexcitation of a typical LTAM− malachite green lactone (MGL; D = dimethylaniline (DMA), A = phthalide (Pd), Chart 1) is basically localized on one of the D subunits (the S1 state of the Pd moiety is much higher in energy),14 and at room temperature (RT) does not lead to any observable fluorescence from the photoexcited chromophore15

adical ion pairs (RP) offer a stage for spin-selective chemistry, where spin degrees of freedom can significantly affect the reaction outcome in terms of product yields and reaction kinetics.1,2 Excited state dynamics of singlet and triplet RP are of fundamental importance for a number of chemical or biologically relevant processes, including long-lived charge separation,3 photosynthesis,4 or magnetoreception,5,6 where spin dynamics can provide a basis for transformation of physical signals into chemically or biologically relevant information. The spin dynamics has recently been exploited in organic light emitting devices (OLED), where charge transfer triplet states (3CT) can be used as reservoir of energy from electron−hole recombination for upconversion of a triplet exciton into singlet excited state via reverse intersystem crossing (RISC).7 The mechanism of the RISC-based thermally activated delayed fluorescence (TADF) is under debate, with experimental8 and theoretical work9 emphasizing that vibronic coupling between the lowest locally excited (LE) triplet and the lowest 3CT state is important for TADF to be operative. One of the reasons hindering progress in this area is the incomplete understanding of structure- and environment-related mechanisms behind spin correlation and dephasing in 1CT↔3CT conversions,3 likely because intramolecular 3CT states have been exceedingly rarely studied by phosphorescence.10 Herein we report on donor− acceptor (D−A) molecules displaying phosphorescence from 3 CT state and enabling to visualize the spin dynamics due to coexistence of luminescent 3CT and 3LE states. Unlike the wellknown dual fluorescence, dual phosphorescence was reported rarely,11,12 exceptionally as evidence for reactivity on the triplet © XXXX American Chemical Society

Chart 1. Malachite Green Lactone (MGL), Its Structural Analogue with Single Electron-Donating Unit (MGLA), and Their Electron Donating (D) and Accepting (A) Subunits, Dimethylaniline (DMA) and Phthalide (Pd)a

a

θ indicates DMA−Pd torsion angle in MGLA.

Received: August 2, 2017 Accepted: September 11, 2017 Published: September 11, 2017 4659

DOI: 10.1021/acs.jpclett.7b02020 J. Phys. Chem. Lett. 2017, 8, 4659−4667

Letter

The Journal of Physical Chemistry Letters because of 50−150 fs16 electron transfer (ET) resulting in quantitative population of a highly polar intramolecular 1CT state (ICT). Replacement of one D moiety in MGL with −CH3 group transforms MGL into diarylmethane lactone (LDAM), resulting in modification of the spatial alignment of the D and A moieties, but essentially retaining the sp3-carbon based D−A structure.17 At RT, such MGL analogue, 3-methyl-3-(4dimethylaminophenyl)phthalide (MGLA, Chart 1), displays fluorescence (Figure 1a), in a given solvent closely matching

than any other intramolecular deactivation channel of the primarily excited [1D*−A] state. In particular, intersystem crossing (ISC) from [1D*−A] to a donor-centered triplet state, [3D*−A], is much slower (for DMA in ACN at RT, kISC ≤ 2.6 × 108 s−1, as estimated from the S1 decay time of 3.8 ns19) and hence unlikely to compete with so fast CS. ISC from [1D*−A] to the acceptor-centered triplet, [D−3A*], can be likewise ruled out, as in addition to a spin flip it would require a slower energy transfer step between D and A moieties. Monophotonic photoionization known to occur from nonrelaxed singlet state in aromatic amines and leading to formation of solute−solvent ion pairs20 can be excluded as well as an alternative ultrafast deactivation path of [1D*−A] because of high luminescence quantum yields (see below), strongly suggesting purely intramolecular character of the primary photophysical process. It can be hence concluded that in a 77 K glass, the primary CS transforms quantitatively photoexcited MGL or MGLA to a nonrelaxed 1[D•+−A•−] population, which is the starting point for subsequent relaxation steps. The absence of any LE fluorescence proves that the transformation is irreversible. In contrast to nearly identical fluorescence spectra at RT17 (Figure 1a), the total luminescence spectra of MGL and MGLA in a given solvent glass at 77 K differ in shape and maximum position (Figure 1b). Based on mechanical separation with choppers (4 kHz), we previously concluded that in butyronitrile (BTN) both MGL and MGLA displayed shortlived fluorescence and long-lived phosphorescence from 1 [D•+−A•−] and 3[D•+−A•−] states, respectively, differently overlapping for each molecule. For MGL, the long-lived band maximum was by 550 cm−1 red-shifted versus the short-lived one,15 while for MGLA in the same solvent, a blue-shift of the long-lived band maximum by 250 cm−1 was found,17 indicating that MGLA phosphorescence could be higher in energy than fluorescence. In the present paper we show that this finding is a consequence of the co-occurrence and spectral overlap of CT and LE phosphorescence bands emitted from the 3[D•+−A•−] and the [3D*−A] states, respectively. Compared with RT, the electronic absorption spectra of MGLA at 77 K show a distinct long-wave absorption tail, extending from 30500 down to 27500 cm−1, and a noticeable intensity increase of the lowest energy Lb band relative to the second La band (Figure S3). The absorption intensity increase is not due to the solvent contraction only, but results also in part from changes in the molar absorption coefficient of the DMA moiety (Figure S2) and in part possibly from changes in CT absorption intensity17 due to frozen medium-induced changes in D−A orientation and, consequently, modified electronic coupling between the MGLA subunits. More importantly, the long-wave absorption tail, barely noticeable at RT, also appears clearly in the luminescence excitation spectra recorded at 77 K (Figure S4−S6), pointing to a low temperature enhanced charge transfer absorption transition directly populating the 1[D•+−A•−] state. For all solvents used, the luminescence excitation spectra monitored at various emission energies generally match the low temperature absorption spectra (Figure S3), with some low-temperature induced difference in the La to Lb band ratio in nonpolar methylcyclohexane (MCH) (Figure S4). The dependence of the luminescence intensity upon the excitation intensity is approximately first order across the entire spectrum (Figure S7), and the shape of the spectrum, except for MCH, is independent of sample concentration (in a 10−6 − 10−4 M range), indicating one-photon one-molecule origin of the

Figure 1. (a) Solvent effect on fluorescence spectra of MGLA (solid lines) and MGL (dashed lines) at 295 K. (b) Total luminescence spectra of MGLA in various solvents at 77 K. The inset compares total luminescence spectra of MGLA (thick lines) and MGL (thin lines) in different solvents at 77 K. Colors mark the same solvents in panels a and b. Excitation at 32260 cm−1 (310 nm). The MGLA and MGL concentrations were between 3 × 10−6 M (MCH) and 4 × 10−5 M (BTN). See text for solvent abbreviations.

that of MGL, proving that photoinduced charge separation (CS) in MGL involves a single D group, and indicating that ET in MGLA (estimated kET ≥ 4 × 1012 s−1 at RT) occurs on the same time scale as in MGL. The dipole moments of the emitting states in MGL and MGLA, 25.0 and 24.8 D, respectively, indicate ET occurring on a D−A center-to-center distance (5.2 Å) and generating a highly polar charge separated singlet state with a structure of covalently linked radical ion pair, 1[D•+−A•−]. With decreasing temperature, the MGLA fluorescence spectrum is first red-shifted, while its shape is retained, similarly as for MGL.15 The red shift continues approximately down to the melting point of the solvent, and on further temperature decrease of a supercooled solvent glass, the spectrum is blueshifted with a large gain in intensity.18 Nevertheless, even at 77 K both MGLA and MGL do not display fluorescence characteristic of insulated photoexcited chromophores (DMA or Pd), indicating very high CS rates also in rigid solution. Based on the low temperature-dependent measurements, fluorescence of MGLA and MGL observed at 77 K was ascribed to come from the 1[D•+−A•−] state,15,17 which is likely formed as fast as at RT, indicating that intramolecular CS occurs in the Franck−Condon region also at 77 K, and is faster 4660

DOI: 10.1021/acs.jpclett.7b02020 J. Phys. Chem. Lett. 2017, 8, 4659−4667

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The Journal of Physical Chemistry Letters observed emissions. The concentration dependence of the 77 K luminescence spectrum of both MGL and MGLA observed in MCH will be reported and discussed in detail elsewhere. For clarity of presentation we confine reporting the data for this solvent to low (2−5 × 10−6 M) concentrations of MGLA. The steady-state luminescence spectrum of MGLA at 77 K shows unusual variability and dependence on glass medium (Figure 1b). In MCH, the luminescence spectrum consists of a lower intensity short-wave (26500−31000 cm−1) band (F) and a dominating intense, slightly structured main (M) band peaking at 24050 cm−1. With increasing solvent polarity, in 1propanol (1-PrOH) and 2-methyltetrahydrofuran (2-MTHF) glasses, the F band is red-shifted and visible as a short-wave tail only, essentially becoming a part of the M band. In polar solvent glasses,21 BTN and propionitrile:butyronitrile mixture, 4:5, v/v (PRN:BTN), the M band overlaps with the F band, and is strongly red-shifted (in PRN:BTN the maximum of the M band shifts to 21400 cm−1, Table S1). The shift is accompanied by an increase of the bandwidth (the full width at half-maximum (fwhm) increases from 3900 to 5200 cm−1 between MCH and PRN:BTN) and a noticeable intensity change of a distinct shoulder on the blue edge (25400 cm−1) of the M band with solvent independent position and solvent dependent intensity (relative to the M band maximum), observed in all frozen solvents used in this study (Figures S8− S11). In addition, yet another red luminescence band (R), not observed in MCH nor in 2-MTHF, appears in polar solvents between 13000 and 16000 cm−1.22 Unlike the F and M bands, the position of the R band does not show significant dependence on solvent glass polarity, the R band is, however, not observed with excitation energies within the abovementioned long-wave absorption tail. A similar red shift of the M band is observed for MGL (2750 cm−1 between MCH and PRN:BTN), with much weaker R band and less distinct blue shoulder (Figure 1b, inset). The MGLA luminescence decays recorded in various regions of the 77 K spectrum extend over multiple time scales from nanoseconds to seconds as a result of a complex overlap of short and long-lived luminescence components. The short-lived (ns, comprising prompt fluorescence) and the long-lived (μs and longer) components to the total steady-state spectrum (T, Figure 2) are extracted with time-resolved measurements with the following procedure. Upon excitation with a 3-μs flash lamp, the long-lived component is obtained by gating the total luminescence, typically with 50 ms gating time (tg), after a 50 μs initial delay time (td). The initial delay time of 50 μs is used to avoid any interference from the prompt fluorescence (e.g., due to a 45 μs tail of the flash lamp; see Experimental Procedures in the Supporting Information). In view of the longlived decays extending to seconds, we compared the luminescence spectra recorded with tg 50 and 500 ms to find whether the spectrum obtained with 50 ms is representative for the entire long-lived luminescence and found satisfactory agreement. To quantitatively assess the prompt fluorescence contribution in polar solvent glasses to the total luminescence spectrum, we make use of the fact that the R band, well separated from the M band in the steady-state luminescence spectrum, is absent in the prompt luminescence spectrum measured by gating the first 50 μs of the luminescence decay signal (td = 0 μs) and decays on the time scale of tens of milliseconds, which shows that the R band comprises exclusively the long-lived luminescence. This observation allows for extracting the

Figure 2. Steady state total luminescence spectrum, T, of MGLA in MCH (a) and BTN (b) at 77 K, decomposed into single fluorescence, F CT , and single (MCH) or three (BTN) phosphorescence components: PCT (triplet charge transfer state), PLE (donor-localized triplet state), and R (excited open-ring photoproduct). PDMA is phosphorescence spectrum of DMA in BTN red-shifted by 400 cm−1. P is the long-lived luminescence spectrum observed in experiment: P = PCT + PLE + R; T = FCT + P. The inset in diagram (b) compares normalized FCT and PCT bands of MGLA in BTN at 77 K.

prompt fluorescence contribution to the steady-state luminescence spectrum, FCT, in PRN/BTN, BTN, and 1-PrOH, by subtracting the long-lived spectrum (P), normalized to the maximum of the R band, from the T spectrum. Extremely large Stokes shift (∼11000 cm−1 in BTN), the absence of any band structure, red-shift of the fluorescence maximum by 5150 cm−1 between MCH and PRN:BTN, and a large half width of the fluorescence band (e.g., 5100 cm−1 in BTN, Table S1) confirm that MGLA fluorescence at 77 K is emitted from a highly polar 1 [D•+−A•−] state, similar to that at RT.17 Another spectral feature useful in the analysis of the MGLA luminescence spectra recorded at 77 K is the shoulder at 25400 cm−1. The presence of a very similar feature in the phosphorescence spectrum of DMA and virtual match of the M band of MGLA in MCH with a red-shifted (400 cm−1) DMA phosphorescence spectrum (Figure 2a), combined with the absence of any observable shift in the shoulder’s position with changing medium polarity, strongly point to a contribution from the DMA-centered triplet state, [3D*−A], to the P spectrum in the blue region. Assuming that only the donor-centered triplet state emits phosphorescence in that spectral region, we use the 25400 cm−1 shoulder intensity as a measure of the [3D*−A] contribution, PLE, to the total phosphorescence spectrum, and by subtracting PLE from P, we obtain another long-lived luminescence band (PCT). The PCT band is distinctly different from FCT (inset, Figure 2b) and cannot be considered delayed fluorescence, as the latter should be spectrally indistinguishable from prompt fluorescence. In addition, by comparing the luminescence quantum yields and decay times of the deoxygenated and nondeoxygenated samples and following the procedure described by Dias et al.,8 we found no symptoms of delayed fluorescence of MGLA, both at RT and 77 K. 4661

DOI: 10.1021/acs.jpclett.7b02020 J. Phys. Chem. Lett. 2017, 8, 4659−4667

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The Journal of Physical Chemistry Letters Therefore, the PCT band is interpreted as phosphorescence from a charge transfer triplet state, 3[D•+−A•−], and the P band - as total phosphorescence emitted by MGLA. For PRN/BTN and BTN glasses, PCT and PLE are of comparable intensity, whereas in 1-PrOH and 2-MTHF the PLE band strongly dominates and the PCT contribution is negligibly small (Figure S8−S10). The shoulder wavenumber (25400 cm−1) is identified with the energy of the [3D*−A] state in MGLA. Separation of Fluorescence f rom Phosphorescence. The spectral decompositions (Figures 2 and S8−S10) clearly show that, at 77 K, MGLA emits single fluorescence and single (P band in MCH), dual (PCT and PLE bands in medium polar solvents) or triple phosphorescence (PCT, PLE and R bands in more polar glasses). The PCT and PLE bands come from 3[D•+−A•−] and [3D*−A] states, respectively, and the R band is assigned to an excited open-ring photoproduct in the triplet state, likely triplet biradical, 3[D−A••]*, formed by severe stretching23 or, more likely, cleavage of the C−O bond in the lactone ring. The PCT band is assigned to the 3[D•+−A•−] state based on its spectral position, decay time, red shift with increasing glass polarity (1200 cm−1 between 2-MTHF and PRN/BTN, exactly like the FCT band), and large spectral width (Table S1). The [3D*−A] state is identified by close similarity of the PLE band to the DMA phosphorescence spectrum red-shifted by 400 (MCH) or 750 cm−1 (BTN), or even identity with that nonshifted of tertbutyldimethylaniline (Figure S11). It is to be noted that the acceptor-centered triplet state (localized on the Pd moiety), [D−3A*], is too high in energy (27800 cm−1)15 to be a source of any emission observable in our experiments. Low transition energy in the R band is indicative for an extended π-conjugated electronic system that could emerge in MGLA upon dissociation of the C−O bond, and the R band is tentatively interpreted as the emission from the excited open-ring triplet biradical, 3[D−A••]*, formed from the 3[D•+−A•−] state. The assignment is strongly suggested by the propensity of the Pd radical-anion to open the lactonic ring on the time scale of seconds. 24 In MGLA, the presence of DMA and CH 3 substituents in position 3 and the spin-selective bond cleavage (dissociative back electron transfer) in the parent 3[D•+−A•−] triplet state could likely contribute to acceleration of the dissociation to a low ms region. This is also supported by the fact that, in strained ring systems, the recombination of triplet radical pairs generates biradicals that may undergo rearrangements in mechanisms involving formation of biradicals via dissociative return ET.25 Similar strongly red-shifted phosphorescence was recently observed in diphenylcyclopropanes and assigned to a triplet biradical.23,26 Yet another argument in favor of the interpretation of the R band origin is provided by close similarity of the R band shape and position to the intense red phosphorescence of structurally related rhodamine lactones (LR) containing an MGLA-type subunit incorporated in their structures.27,28 Further work to ultimately verify and confirm the assignment is underway and may shed new light on triplet state dynamics in spirocyclic D−A systems. The spectra obtained from spectral decompositions (Figures 2 and S8−S10) are used to determine quantum yields of spectral components based on the total luminescence quantum yield at 77 K (Table 1; see Supporting Information for details). High and comparable overall yields, Φ(T), of the radiative processes in MGLA at 77 K in all solvents used indicate low efficiency of nonradiative deactivation modes leading directly to the ground state, including possible lactonic C−O bond dissociation in the 1[D•+−A•−] state. Direct radiationless

Table 1. Fluorescence and Phosphorescence Quantum Yields, and Quantum Yield of Donor-Centered Triplet State Formation for MGLA in Solvent Glasses at 77 K quantum yield

PRN: BTN

BTN

2-MTHF

1-PrOH

MCH

Φ(T)a Φ(FCT)b Φ(P)c Φ(PCT)d Φ(PLE)e Φ(R)f ΦDg ε, Tgh

0.66 0.39 0.27 0.15 0.09 0.03 0.10

0.59 0.31 0.28 0.15 0.10 0.03 0.11 37.2

0.79 0.28 0.51 0.02 0.49 0.00 0.53 19.0

0.49 0.12 0.37