Nature of Excited States of Ruthenium-Based Solar Cell Dyes in

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Nature of Excited States of Ruthenium-Based Solar Cell Dyes in Solution: A Comprehensive Spectroscopic Study Raphael Horvath,† Michael G. Fraser,‡ Charlotte A. Clark,† Xue-Zhong Sun,† Michael W. George,*,†,§ and Keith C. Gordon*,‡ †

School of Chemistry, University of Nottingham, Nottingham NG7 2RD, United Kingdom Department of Chemistry, University of Otago, Dunedin 9001, New Zealand § Department of Chemical and Environmental Engineering, University of Nottingham Ningbo China, 199 Talking East Road, Ningbo 315100, China ‡

S Supporting Information *

ABSTRACT: The photophysical properties of a number of ruthenium complexes of the general structure [Ru(L1)(L2)(NCS)2], related to the prominent solar cell dye [Ru(dcb)2(NCS)2] (dcb = 4,4′-dicarboxylato-2,2′-bipyridine) are investigated. For L1 = dcb and dmb (dmb = 4,4′-dimethyl2,2′-bipyridine), several variations of L2 show very little difference in the lowest energy absorption peak. Resonance Raman and density functional theory calculations have been used to assign the corresponding transitions as {Ru(NCS)2} → dcb with significant contributions of the NCS ligands. Transient absorption, time-resolved infrared, and transient resonance Raman spectroscopic techniques were used to probe the photophysics of the complexes and relatively shortlived {Ru(NCS)2} → dcb/dpb (dpb = 4,4′-diphenylethenyl-2,2′-bipyridine) excited states were observed with the exception of [Ru(dcb)(dab)(NCS)2] (dab = 4,4′-dianthracenethenyl-2,2′-bipyridine), which showed a long-lived excited state assigned as ligand centered charge separated.

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from the metal is excited to the ligand π* orbital, forming a radical anion in a similar fashion to [Ru(bpy)3]2+ (bpy = 2,2′bipyridine).32−34 There has been debate in the literature as to the origin of this transition, with some reports describing it as containing significant NCS character,35−37 tending toward interligand charge transfer (ILCT). Shoute and Loppnow24 have investigated the resonance Raman profile of [Ru(dcb)2(NCS)2], both in DMSO solution and bound to TiO2. Upon binding to TiO2 a slight blue shift of the absorption band was found with a similar absorption coefficient and almost a 2-fold increase in Raman activity compared to the free dye. For both free and bound [Ru(dcb)2(NCS)2], a number of resonantly enhanced modes in the range 700−1700 cm−1 were assigned as bpy-based, with the exception of a mode at 812 cm−1, assigned as CS stretch, modes at 1261 and 1310 cm−1, which contain some C−O stretching character, and a symmetric COOH stretch at 1388 cm−1 for the complex bound to TiO2. An NCS-based CN stretch at 2115 cm−1 with a shoulder at 2140 cm−1 and a weak CO stretch at 1713 cm−1 were also observed, the latter only in the free complex. An analysis of the change in normal

INTRODUCTION Dye sensitized solar cells (DSSCs) are an intensely studied field. One of the most prominent solar cell dyes is [Ru(dcb)2(NCS)2] (dcb = 4,4′-dicarboxylato-2,2′-bipyridine, see Figure 1), also known as N3, which was first synthesized by Nazeeruddin et al.1,2 two decades ago. The efficiency initially recorded was ≈10%, which was not significantly bested until more than a decade later.3,4 There have been a large number of studies investigating the photophysical properties of [Ru(dcb)2(NCS)2] and closely related derivatives thereof, in many cases bound to TiO2 or other semiconductors.2,5−31 Absorption and emission properties of [Ru(dcb)2(NCS)2] in a number of environments1,2,24,28,29 have been reported. Free [Ru(dcb)2(NCS)2] shows a weak emission peak in the range 720−890 nm, depending on the solvent, with a transient decay time (τ) of ca. 50 ns (at 298 K), which is quenched if the dye is bound to TiO2.24 Three main electronic transitions are found in the visible absorption spectrum; the one of lowest energy is a weak shoulder at ca. 600 nm and has been assigned as a spinforbidden triplet metal-to-ligand charge transfer (3MLCT) excitation. The lowest-energy absorption band has been reported ranging from 500 nm in water to 544 nm in dimethyl sulfoxide (DMSO) and is assigned as a singlet metal-to-ligand charge transfer (1MLCT) transition, in which a dπ electron © XXXX American Chemical Society

Received: July 26, 2015

A

DOI: 10.1021/acs.inorgchem.5b01690 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Structures of the ruthenium complexes investigated.

cm−1 and a grow-in of new bands to lower wavenumbers. This has been interpreted as evidence for a 3MLCT state. While bound to TiO2, this process occurs in ca. 50 ± 25 fs, in agreement with transient absorption studies. Consistent with luminescence studies, the excited state signals from solutionphase infrared studies persist for nanoseconds. Here we investigate [Ru(dcb)2(NCS)2] and a number of derivatives, using a combination of the experiments discussed above, particularly comparing resonance Raman spectroscopic results with fast TRIR measurements which has previously shown insight into the photophysics and the nature of excited states of metal diimine complexes.39−42 Derivatization was carried out at the 4- and 4′-positions of bpy, which is an established strategy for modifying [Ru(dcb)2(NCS)2]. It was possible that including groups with large absorption cross sections would significantly alter, and preferably enhance, the light-absorbing abilities of these complexes; however, this was not the case, as the lowest-energy peak is nearly identical for all investigated complexes (vide infra). We believe this unusual behavior warrants further investigation as it is important to have a full understanding of the nature of the excited states in order to produce more efficient photosensitizers for DSSCs. The complexes investigated are [Ru(dcb)2(NCS)2], [Ru(dcb)(dpb)(NCS)2] (dpb = 4,4′-diphenylethenyl-2,2′-bipyridine), [Ru(dcb)(dnb)(NCS)2] (dnb = 4,4′-dinaphthylethenyl2,2′-bipyridine), [Ru(dcb)(dab)(NCS)2] (dab = 4,4′-dianthracenethenyl-2,2′-bipyridine), and [Ru(dmb)(dpb)(NCS)2] (dmb = 4,4′-dimethyl-2,2′-bipyridine), as shown in Figure 1. These complexes have been investigated using a number of techniques, including FT-Raman, resonance Raman, transient absorption and emission, and time-resolved infrared spectros-

coordinates upon excitation showed an elongation of bpy-based bonds, consistent with promoting an electron into the antibonding π*L orbital; however, the Ru−N coordinate bonds were found to be largely unaffected by the transition. Early ultrafast transient absorption measurements16,25 were carried out on [Ru(dcb)2(NCS)2] bound to TiO2, and injection times (≈25 fs) of an electron into TiO2 from the excited state were reported. However, no assignment was made for the injecting state of the dye and this can be difficult since electronic absorption bands are broad and featureless and can show interference with ground state depletions. Later studies by Kallioinen et al.19 reported two contributing components for the excited state decay, with time constants of 50 and 75 fs, attributed to electron injection from the vibrationally hot 1 MLCT state and intersystem crossing (ISC), respectively. Electron injection from the 3MLCT state was assigned to be on a slower time scale. More recently, Juozapavicius et al.31 have carried out studies on complete cells. Evidence for relatively slow injection was observed, suggesting that longer-lived excited states following Franck−Condon excitation are significant in the efficient operation of DSSCs. Fast time-resolved IR has proved a powerful tool for examining excited state structure and reactivity of coordination compounds.38 An advantage of probing the excited states of [Ru(dcb)2(NCS)2] using mid-IR spectroscopy is that NCS and COOH both possess stretching modes that can be used as reporter groups, particularly as they are sensitive to electron density on various parts of the complex. As such, several groups have examined [Ru(dcb)2(NCS)2] in solution and bound to TiO2 using time-resolved infrared (TRIR) spectroscopy.5,8−11,14,17,26 Photoexcitation of [Ru(dcb)2(NCS)2] results in bleaching of a peak at ca. 2115 cm−1 and shoulder at ca. 2140 B


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Inorganic Chemistry

and LUMO, respectively) can be found in Table S1 (see Supporting Information). The assignment of the lowest energy peaks to a {Ru(NCS)2} → bpy transition is justified when considering that the donor MOs (HOMO − 2 to HOMO, for most complexes) show significant amplitude on the NCS units. This is also the case for the dab complex, where increased mixing occurs and a larger number of MOs are involved. It is also interesting to note that a recent study by Jeon et al.30 found that the location of the hole on the NCS π-orbitals facilitates electron transfer from the electrolyte of the oxidized dye. It is suggested that replacement of the NCS with a better electron donor could enhance the interaction with the electrolyte. We have modeled the shift of electron density, and Table 1 includes the change in wave function amplitude over selected portions of the complexes during excitation, as indicated in Figure 3. All but the dmb complex show greater contributions from (NCS)2 than the metal, for every transition in the visible range. In the case of [Ru(dcb)2(NCS)2], the difference is approximately a factor of 2. The energies of the first two lowest unoccupied orbitals of dcb, dpb, dnb, and dab are affected very little by the change in substituent, since the orbitals show only minimal extension beyond bpy, as shown in Table S1 (see Supporting Information). As the acceptor orbitals associated with the {Ru(NCS)2} → bpy transitions at ca. 545 nm are LUMO and LUMO + 1, this is consistent with the invariance of these transitions toward changing substituents. In contrast, the shift in electron density in the higher-energy transitions terminates on MOs that show overlap with the arylethylene substituents and are thus significantly affected by the nature of the substituent. Fourier Transform Raman (FT-Raman) Spectroscopy. Solid-state FT-Raman spectra are displayed in Figure 4. All spectra show bpy-related vibrations, notably ring vibrations at ca. 1600 cm−1, breathing mode peaks at ca. 1020 cm−1, as well peaks at 1264 cm−1 for [Ru(dcb)2(NCS)2] and 1271 cm−1 for the other compounds. The feature at 1609 cm−1 in [Ru(dcb)2(NCS)2] splits in spectra of the other compounds due to an increased number of ring systems. Peaks specific to the ligands dpb (1599 cm−1), dnb (1573 cm−1), and dab (1556 cm−1) have been identified, while all arylethenyl complexes contain CC stretching peaks at 1632 cm−1; these are useful in determining electronic activity in the resonance Raman spectra (vide infra). Calculation of the mean absolute deviation (MAD) between experimental and calculated spectra can be useful in determining the efficacy of a calculation (see Experimental Section for methodology).47,48 MADs were determined as 5−11 cm−1, which indicates a satisfactory agreement. A more detailed summary of the MADs and the scale factors used can be found in the Table S2 (see Supporting Information). While the spectra show very few features below 900 cm−1, all compounds except [Ru(dcb)2(NCS)2] display a peak between 555 and 580 cm−1 (not shown in Figure 4), which is attributed to a Ru−bpy stretching vibration. [Ru(dcb)(dnb)(NCS)2] and [Ru(dcb)(dab)(NCS)2] also show weak peaks between 400 and 500 cm−1, assigned to flexing of the naphthyl and anthracenyl groups, respectively. Assignment of these peaks is difficult and somewhat ambiguous due to their low intensities. Resonance Raman Spectroscopy. Resonance Raman spectroscopy is a useful tool to probe the shift of electron density upon photoexcitation (i.e., the Franck−Condon state). Resonance enhancement of a vibrational mode occurs if this

copies as well as ground and triplet state density functional theory (DFT) calculations.

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RESULTS AND DISCUSSION Electronic Absorption Spectroscopy. Figure 2 presents the absorption spectra of the complexes. The lowest energy

Figure 2. Electronic absorption spectra, acquired in DMSO.

peaks at ca. 545 nm are essentially unaffected by the 4,4′substituents on the bpy moieties. While some variation in peak intensities is observed, the majority of this can be attributed to changes in intensities of higher-energy peaks. The 545 nm bands will be referred to as {Ru(NCS)2} → bpy or mixed MLCT/ILCT transitions, involving oxidation of the NCS units and the metal, and reduction of the bpy ligand. Higher energy peaks are significantly perturbed upon substitution of bpy. These appear at 403 and 437 nm for [Ru(dcb)2(NCS)2] and [Ru(dmb)(dpb)(NCS)2], respectively. A peak at 413 nm in the spectrum of [Ru(dcb)(dpb)(NCS)2] is likely to be a combination of contributions from transitions to the dcb and dpb ligands. Compounds [Ru(dcb)(dnb)(NCS)2] and [Ru(dcb)(dab)(NCS)2] possess similar transitions that are visible as shoulders at ca. 400 nm but are partially masked by strong arylethenyl-based (π → π*) peaks at 368 and 421 nm, respectively. Analogous phenylethylene-based transitions are observed as shoulders at ca. 345 nm for compounds containing the dpb ligands. At shorter wavelengths (ca. 320 nm) bpycentered (π → π*) transitions are observed. Time-dependent DFT (TD-DFT) calculations were carried out in DMSO solvent fields, using the B3LYP and CAMB3LYP functionals. CAM-B3LYP appears to yield satisfactory results for arylethenyl transitions, which are modeled to red shift from 322 nm to 342 nm to 415 nm for the dpb, dnb, and dab substituents, respectively. However, as the lowest-energy peak is absent for all complexes, the CAM-B3LYP data were considered unreliable and are not included. [The absence of these transitions in the CAM-B3LYP calculations is intriguing. CAM-B3LYP has a well-known propensity to overestimate transition energies,43−46 and therefore it is possible that the “missing” transitions are calculated to overlap with higher energy ones. We have previously reported that CAM-B3LYP predicts a reduced delocalization of MOs compared to B3LYP,45 which is qualitatively consistent with this observation.] In contrast, B3LYP appeared to produce satisfactory results, which are summarized in Table 1. A number of groups of transitions can be identified; the corresponding highest occupied and lowest unoccupied molecular orbitals (HOMO C


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Inorganic Chemistry

Table 1. Electronic Absorption Data Acquired in DMSO with Corresponding Calculated Electronic Transitions and the Change in Wave Function Amplitudea % change in wave function amplitude λabs/nm (ε/10 × M 3

−1

−1

cm )

[Ru(dcb)2(NCS)2] 642f,g (2.6) 542 (12.0) 403 (11.6)

λB3LYP/nm ( f) 639 550 445 376 299

(0.052) (0.133) (0.253) (0.217) (1.083)

318 (37.4) [Ru(dcb)(dpb)(NCS)2] 632f,g (3.2) 676 (0.027) 546 (13.0) 543 (0.332) 413 (14.6) 447 (0.332) 411 (0.261) 344f (31.5) 388 (0.839) 320 (45.3) 327 (1.180) [Ru(dcb)(dnb)(NCS)2] 632f,g (3.2) 683 (0.029) 548 (15.0) 547 (0.393) 446 (0.760) 422 (0.411) 368 (34.6) 366 (0.651) 319 (38.3) [Ru(dcb)(dab)(NCS)2] 638f,g (4.2) 675 (0.030) 546 (17.6) 544 (0.655) 532 (0.647) 421 (32.8) 456 (0.671) 318 (46.7) 311 (2.007) [Ru(dmb)(dpb)(NCS)2] 638f,g (2.6) 620 539 (10.0) 535 437 (10.1) 463 422 347f (26.4) 383 334 (29.8) 328

(0.060) (0.209) (0.270) (0.269) (0.817) (1.163)

donor MOs

b,c

acceptor MOs

b,c

(NCS)2

Ru

dxbd

bpy

ArCHCHe

H H−2, H−1 H−3 to H H−6 to H−2 H−8 to H−4

L, L+1 L, L+1 L to L+4 L, L+1, L+4 L to L+5

−51 −58 −67 −60

−36 −28 −29 −34

87 87 97 94

H H−2 to H H−3 to H H−5, H−3 to H−1 H−6 to H−4, H H−6 to H−4

L L, L+1 L, L+2 to L+4 L to L+3 L, L+1, L+5 L+1 to L+3

−41 −45 −53 −46 −45

−38 −36 −42 −14 −14

86 25 71 16 32

−2 41 10 29 61

−4 17 14 14 −33

H H−2 to H H−5 to H H−4, H−1 H−8, H−6 to H−3 H−1

L L, L+1 L to L+4 L+1, L+3 L to L+3, L+5

−41 −44 −38 −28 −37

−38 −37 −26 −15 −11

86 25 57 1 46

−2 42 15 49 24

−4 16 −7 −7 −21

H−2 to H H−4 to H H−4 to H H−5 to H H−16, H−13, H−11 H−8 to H−6

L L, L+1 L+1 L, L+2, L+3 L to L+4, L+15

−22 −28 −33 −35 −35

−18 −19 −25 −13 −15

87 65 5 43 45

−1 16 58 11 8

−45 −32 −5 −6 −4

H H−2, H H−2 to H H−3 to H H−5, H−4, H−1, H H−8, H−6 to H−4 H−2, H−1

L L, L+1 L+1, L+2 L, L+2, L+3 L, L+3, L+4 L to L+2, L+4 L+5

−36 −36 −39 −44 −43

−46 −45 −47 −36 −23

−3 33 35 −1 11

60 35 20 42 81

25 14 30 39 −24

a All calculations were carried out in a DMSO solvent field. bMOs are included if the transitions they contribute to have f ≥ 10% of the most intense transition in their group. c“H” and “L” refer to HOMO and LUMO, respectively. ddmb for [Ru(dmb)(dpb)(NCS)2] and dcb for all other complexes. eArylethylene substituent. fObserved as shoulder. gApproximate value due to broad peak.

mode mimics the structural changes of the molecule upon photoexcitation at a particular excitation wavelength (λex).49 Figure 5 presents resonance Raman spectra at representative λex values of [Ru(dcb)(dnb)(NCS)2], [Ru(dcb)(dab)(NCS)2], and [Ru(dmb)(dpb)(NCS)2]; spectra of the remaining complexes can be found in Figure S1 (see Supporting Information). Only bands discussed in the text are marked in the spectra; a list of Raman bands at selected λex values is shown in Tables S3 and S4 (see Supporting Information). Spectra were also acquired at λex = 406.7, 444.3, 457.9, 488.0, and 514.5 nm. As no additional peaks are observed, they are represented by the spectra shown. Figure 6 displays a number of vibrational modes of [Ru(dcb)(dnb)(NCS)2]; their analysis is also relevant to other complexes, as many of the observed modes are very similar (vide infra). At λex = 350.7 nm the spectrum of [Ru(dcb)2(NCS)2] (Figure S1, see Supporting Information) shows dcb vibrations, as assigned by comparison to the FT-Raman spectrum, and is

Figure 3. Partitioning scheme employed for the calculation of the change in wave function amplitude, as presented in Table 1.

D


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disappearing. This is indicative of the same state being probed in all cases, and thus excitation of similar portions of the bpy ligands. Analysis of the vibrational modes reveals that peaks at 1470, 1540, and 1609 cm−1 show little involvement of the carboxylate and linker units at the 4,4′-positions of bpy (Figure 6). Importantly, the molecular activity relating to these modes correlates with the regions that the MLCT-accepting MOs occupy on the substituted ligands (LUMO + 1 in Table S1), and is located predominantly on the bpy unit; this is true for all complexes, regardless of the size of the arylethenyl substituent. This indicates that the substituents at these positions show little structural distortions during excitation with longer wavelengths and can thus be said to exhibit only minimal effects on the natures of the transitions. Resonance enhancements of NCS stretching peaks are observed at ca. 2010 cm−1 for all complexes, indicating electronic activity in these units. The greatest relative intensity of NCS peaks is observed at λex = 532 nm. Both of the visible absorption peaks can therefore be assigned as transitions originating from the {Ru(NCS)2} moiety, as predicted by TD-DFT calculations. The transition at ca. 445 nm appears to terminate on a portion of the bpy ligand that is largely unaffected by substituents, while the higher energy transition extends over the substituents, as evidenced by the presence of arylethylene peaks in the resonance Raman spectra at the relevant wavelengths. Not shown in Figure 5 and Figure S1 are metal−ligand bond vibrations, which are expected at 318 and 364 cm−1.24 Possible candidates for these are found (for λex = 488.0) at 351 and 365 cm−1 for [Ru(dcb)2(NCS)2], and approximately 351 and 372 cm−1 for the other complexes; their observation is made difficult by nearby DMSO peaks at 308, 335, and 384 cm−1. No resonance enhancement of these modes is observed at shorter wavelengths. Since both MLCT and ILCT transitions are expected to involve a change in the metal to ligand bond length, it is difficult to draw a conclusion as to the exact nature of the transition from this information. Excited State Electronic Spectroscopy. The excited state electronic spectra are summarized in Table S5 (Supporting Information). Weak transient emission signals were observed between 760 and 790 nm for all complexes except [Ru(dcb)(dab)(NCS)2]. Due to low signal intensities, fitting of emission lifetimes proved to be difficult for most complexes and no satisfactory fit could be obtained for [Ru(dcb)(dpb)(NCS)2].

Figure 4. FT-Raman spectra of the complexes, acquired in KBr disks. Vertical lines indicate common peaks throughout the series of complexes.

thus consistent with excitation of a (π → π*) state on this ligand. The arylethylene complexes also show enhancement on the arylethylene substituents. These include naphthyl peaks50 at 1371 and 1574 cm−1 (see Figure 5) and anthracenyl peaks51 at 1259, 1409, and 1559 cm−1. The spectra of [Ru(dcb)(dpb)(NCS)2] and [Ru(dmb)(dpb)(NCS)2] are very similar at λex = 350.7 nm and show enhancement of stilbene-like peaks,52,53 indicating activity of the dpb ligands. Strong enhancement of the ethenyl CC stretches at 1634 cm−1 can also be seen, the intensity of which decreases for the visible wavelengths. No NCS stretching contributions are observed at λex = 350.7 nm. These data are consistent with (π → π*) transitions at short wavelengths. At λex = 413.1 nm, corresponding to the highest energy visible absorption peak, a number of substituent-specific peaks are still observed, as well as some peaks localized on the bpy unit (ca. 1540 and 1609 cm−1). This is consistent with a transition that terminates on the bpy as well as the substituent, as also indicated by MO pictures (vide supra). At longer excitation wavelengths, especially at λex = 532 nm, the resonance Raman spectra of all complexes appear strikingly similar, with most peaks due to substituents essentially

Figure 5. Resonance Raman spectra of (a) [Ru(dcb)(dnb)(NCS)2], (b) [Ru(dcb)(dab)(NCS)2], and (c) [Ru(dmb)(dpb)(NCS)2]. All spectra were acquired in DMSO. Solvent peaks are marked with asterisks. Vertical lines indicate common peaks throughout the range of wavelengths probed. E


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Inorganic Chemistry

Figure 6. Illustration of vibrational modes observed in the FT-Raman and resonance Raman spectra of [Ru(dcb)(dnb)(NCS)2].

Considering this, the value of τ = 33 ns obtained for [Ru(dcb)2(NCS)2] compares well to the literature value of 50 ns (in methanol), as does the emission wavelength reported at 755 nm.1 As shown in Figure 7, depletions can be seen in the transient absorption spectra of all complexes, matching the electronic

involved in the transition. An interpretation of the 680 nm peak consistent with the data presented here is a dcb → {Ru(NCS)2} transition, equivalent to a conversion of a {Ru(NCS)2} → bpy to a 3(π → π*) state. The complexes containing dpb or dnb show broad bands in this region, possibly corresponding to two or more peaks. These are not inconsistent with transitions from each of the available polypyridyl ligands to {Ru(NCS)2}. [Ru(dcb)(dpb)(NCS)2] and [Ru(dcb)(dnb)(NCS)2] show similar transient absorption spectra, with excited state lifetimes of 24 and 34 ns, respectively. They differ from the spectrum of [Ru(dmb)(dpb)(NCS)2], in that this shows a relatively strong peak at 610 nm and a transient decay time of 57 ns. The spectrum of [Ru(dcb)(dab)(NCS)2] appears unlike those of the dpb and dnb analogues. A high energy band at 340 nm as well as a strong band at 500 nm indicate that a different type of excited state is probed, which is supported by the prolonged excited state lifetime of 210 ns. No emission or excited state absorption due to anthracene is observed.55,56 Considering this, these data are consistent with a ligandcentered charge transfer (LCCT) state. Two potential electron configurations involve oxidation of the arylethylene and reduction of either the bpy or the second arylethylene. These configurations are discussed below in reference to TRIR data. Time-Resolved Infrared Spectroscopy. TRIR has the advantage over electronic spectroscopies that the narrower line widths of IR bands can be more diagnostic toward the electronic changes in a molecule. It has therefore been used extensively to examine the photophysical properties of transition metal complexes.38,39,57 The TRIR peak positions and lifetimes are summarized in Table 2. Figure 8 shows the picosecond TRIR (ps-TRIR) spectra of [Ru(dcb)2(NCS)2], [Ru(dcb)(dpb)(NCS)2], [Ru(dcb)(dnb)(NCS)2], and [Ru(dmb)(dpb)(NCS)2]. The ps-TRIR spectra of [Ru(dcb)(dab)(NCS)2] are shown in Figure 9. For reference, a number of kinetic traces pertaining to all complexes are shown in Figure S2 (see Supporting Information). The ground state NCS mode of [Ru(dcb)2(NCS)2] at 2106 cm−1 is bleached at the earliest time delay, concurrent with the appearance of excited state peaks at 2075 and 2041 cm−1. These peaks are in accordance with previously published results5,8−11,14,17 and persist for the duration of the experiment. Few effects of vibrational cooling are observed, with the 0.7 ps spectrum appearing very similar to the 1200 ps spectrum. The observed shifts have previously been attributed to an MLCT state of this complex; however, they are also consistent with an ILCT or {Ru(NCS)2} → dcb transition, as indicated by TD-

Figure 7. Transient absorption spectra, acquired in DMSO, at 298 K with λex = 532 nm.

absorption peaks. In the case of dab-containing complex, the depletion corresponding to the peak at 546 nm is masked by a strong peak in the excited state at 500 nm. The transient absorption spectra of [Ru(dcb)2(NCS)2] show peaks at 360 and 680 nm, which decay with a lifetime of 22 ns. The lower energy band has previously been observed25,54 (in ethanol and acetonitrile) and attributed to NCS as it was not observed for an analogous complex where the NCS units were replaced by acetylacetone. It has been assigned to a {NCS} → Ru transition, that is, a conversion of MLCT to ILCT states. This assignment is inconsistent with the data shown in Figure 7, as similar peaks are shifted for [Ru(dcb)(dpb)(NCS)2] or [Ru(dcb)(dnb)(NCS)2], indicating that the ligands are F


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Inorganic Chemistry Table 2. Ground and Excited State Infrared Absorption Peaks, Acquired in DMSO at 298 K peak position/cm−1

complex [Ru(dcb)2(NCS)2] [Ru(dcb)(dpb)(NCS)2] [Ru(dcb)(dnb)(NCS)2] [Ru(dcb)(dab)(NCS)2]

[Ru(dmb)(dpb)(NCS)2] a

2075, 2109, 2074, 2103, 2076, 2103, 2072, 2099, 2109, 2085, 2105,

2041, 1713, 2041, 1717, 2040, 1719, 2036, 1578, 1717, 1625, 1612,

1696, 1609, 1680, 1629 1681, 1628 1686, 1558, 1620, 1595, 1482

τ/ns

1686, 1550, 1496, 1490, 1455 1465, 1438 1610, 1597, 1559, 1542, 1489, 1455

16

1610, 1559, 1544, 1491, 1450

13

1605, 1558, 1543, 1499, 1457 1507, 1499, 1458 1523 1573, 1562, 1545, 1526, 1511, 1493, 1475

14

0.15 150 39

assignment {Ru(NCS)2} GSa {Ru(NCS)2} GSa {Ru(NCS)2} GSa {Ru(NCS)2} LCCTAr→Ar GSa {Ru(NCS)2} GSa

→ dcb → dcb → dcb → dcb

→ dpb

Bands observed as depletions.

Figure 8. TRIR spectra, acquired 50 ps after photoexcitation. (a) NCS spectral region of [Ru(dcb)2(NCS)2], [Ru(dcb)(dpb)(NCS)2], [Ru(dcb)(dnb)(NCS)2], and [Ru(dmb)(dpb)(NCS)2] in DMSO. (b) The bpy spectral region of [Ru(dcb)2(NCS)2], [Ru(dcb)(dpb)(NCS)2], and [Ru(dcb)(dnb)(NCS)2] in DMSO-d6 and (c) [Ru(dmb)(dpb)(NCS)2] in DMSO-d6.

Figure 9. ps-TRIR spectra of [Ru(dcb)(dab)(NCS)2]. (a) NCS spectral region, acquired in DMSO. (b) The bpy spectral region, acquired in DMSO-d6. (c) Short-time dynamics of (i) the parent at 2109 cm−1 and peaks at (ii) 2072 and (iii) 2099 cm−1, each showing time constants of 15 and 150 ps.

orbitals, respectively. The COOH region shows a bleach at 1721 cm−1 and excited state peaks at 1696 and 1686 cm−1 that persist throughout the duration of the experiment, which has also been observed previously.17 This is consistent with a weakening of the CO bonds of the COOH units and is good evidence for the localization of the excited state electron on dcb, although it does not inform as to its origin. In the aromatic region, ground state depletions are observed at 1609, 1465, and 1438 cm−1, which show counterparts in the ground state spectra at 1610, 1468, and 1438 cm−1 respectively. The expected depletion from a ground state peak at 1550 cm−1 is likely masked by overlap of an excited state peak that possesses

DFT calculations (vide supra). A similar assignment was also made for the lowest energy triplet state of the complex [Re(CO)3(NCS)(bpy)],58 although it should be noted that the NCS vibrational shifts of this complex are not directly comparable due to mixing with carbonyl vibrations in the excited state. Pictures of the highest occupied, singly occupied (SO), and lowest unoccupied MOs (Figure 10) are also consistent with this assignment. The triplet state frontier orbitals have been calculated to possess very similar spatial distributions to the ground state MOs for all complexes, where the HOMO − 1, HOMO, LUMO, and LUMO + 1 orbitals are equivalent to the HOMO, SOMO1, SOMO2, and LUMO G


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Figure 10. Illustration of the triplet state frontier MOs of [Ru(dcb)2(NCS)2] (top) and [Ru(dcb)(dab)(NCS)2] (bottom), calculated using B3LYP, in a DMSO solvent field.

removed from the CC bonds. Additionally, a new peak can also be seen at ca. 1459 cm−1. As with [Ru(dcb)2(NCS)2], the COOH region shows a depletion of the carboxylate peaks and a grow-in of a red-shifted excited state feature, indicating that the lowest excited triplet state involves reduction of the dcb ligand. Calculations of the lowest energy triplet states of these complexes (albeit carried out in vacuo) are consistent with this assignment. The bpy stretching region of [Ru(dmb)(dpb)(NCS)2] (Figure 8c) on the other hand appears very different. In the ligand skeletal mode region (1000−1700 cm−1) ground state depletion signals are observed at 1482 and 1612 cm−1, and excited state features appear as a number of peaks, including at 1526, 1545, and 1562 cm−1. The fact that the transient features are dramatically different compared to TRIR spectra of the dcbcontaining complexes indicates the presence of a different type of excited state. The excited state peaks are observed to diminish slightly on the time scale of the picosecond experiments. Triplet state calculations suggest that the excited state electron in the lowest energy triplet state of this complex is delocalized over the dpb ligand. Geometry optimization of the radical anion indicates that the dpb ligand possesses a lower reduction potential than the dmb ligand, which is also consistent with this assignment. As for [Ru(dcb)2(NCS)2], no further spectral shifts are observed, with excited states decaying with lifetimes of tens of nanoseconds (see Table 2). The observed excited state lifetimes match well with those observed in transient absorption spectroscopy (vide supra). A slight discrepancy can be attributed to a difference in concentrations; the TRIR experiments are carried out in solutions that are approximately an order of magnitude more concentrated, leading to an increased likelihood of annihilation. [Ru(dmb)(dpb)(NCS)2] shows a slightly longer excited state lifetime. This is consistent with an assignment of the excited state as {Ru(NCS)2} → dpb, where the increase can be attributed to an increased energy gap between the ground and excited states compared to complexes where a dcb-based

a larger oscillator strength. Excited state features observed at 1455, 1490, 1496, and 1550 cm−1 appear similar to peaks of the excited state of [Ru(bpy)3]2+, previously observed in TRIR59 spectra at 1449, 1490, 1500, and 1548 cm−1 and in transient resonance Raman32,60 spectra at 1547 and 1504 cm−1. However, as the remaining peaks are unlike the radical anion of bpy, it appears that the COOH units significantly perturb the structure of the excited state compared to unsubstituted bpy. Pictures of the higher energy singly occupied MO (SOMO2) as well as the ground state LUMO (Table S1, Supporting Information) also support this, showing some overlap with the carboxylate groups and thus electronic communication. Triplet state DFT frequency calculations have been carried out, showing major peaks at 1539, 1500, 1488, 1464, and 1453 cm−1. Vibrations corresponding to these bands are located on the reduced dcb unit. A number of peaks corresponding to vibrations of the remaining dcb are also calculated; for the most part these are considerably weaker. The most significant of these is calculated at 1416 cm−1, which is at a similar position to the ground state peak calculated at the same frequency. The NCS stretching regions in the ps-TRIR spectra of complexes [Ru(dcb)(dpb)(NCS)2], [Ru(dcb)(dnb)(NCS)2], and [Ru(dmb)(dpb)(NCS)2] appear very similar to that of [Ru(dcb)2(NCS)2]. Depletion of the ground state peaks and a growth of excited state bands at lower energies is observed. The decay of excited state features is negligible on the picosecond scale. The excited state bands shift and narrow slightly at early times, and this is due to vibrational cooling over the first 10 ps. The phenylethylene and naphthylethylene complexes (shown in red and green in Figure 8a, respectively) display similar features in TRIR spectra in the bpy spectral region (Figure 8b). IR bands are observed at 1610 cm−1, as well as possible depletions at ca. 1629 cm−1, that show no counterparts in the spectra of [Ru(dcb)2(NCS)2]. The band at 1634 cm−1 in the ground state IR and Raman spectra is assigned to vibrations of the ethenyl linkers, and this could be the origin of this feature. In other words, this is consistent with a red shift of these peaks in the excited state, which would indicate electron density being H


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NCS units; this pathway appears to be hindered for the excited state observed here. The bpy region of the nanosecond TRIR (ns-TRIR) spectra shows peaks at 1458, 1499, 1507, 1558, and 1578 cm−1. Potential correlation with experimentally observed peaks at 1570 and 1540 cm−1 in the radical anion spectrum of anthracene exists;73 however, this is only speculative. No direct correlation to cation74 or triplet state75 frequencies was found. Calculation of the molecular orbitals of the lowest energy triplet state predicts an LCCT excited state, whereby one ArCC unit is oxidized and one is reduced, with a small contribution from the bpy (see Figure 10). This interpretation is consistent with a lack of COOH peaks in the ns-TRIR spectra and is not unlike 9,9-bianthryl, for which partial charge transfer states with nanosecond lifetimes have been observed following photoexcitation.76 The system examined here has a significantly greater distance between anthracene moieties, which could explain the increased excited-state lifetime observed. Triplet state frequencies are inconclusive in this regard, as calculated frequencies are invariant with respect to the ground state, which implies no change in electron density in the NCS units. The calculations show that the acceptor ArCC unit is distorted, being bent into a “saddle-like” structure, which appears to be driven by the carbon atom in the 9-position becoming more sp2-like. Such a distortion has not been reported for radical anion77 or radical cation78 species of anthracene, nor is it predicted by triplet state DFT calculations of anthracene. However, it is well represented by triplet state calculations of propenylanthracene, which indicates significant involvement of the −CHCH unit in this excited state.

excited state is present. According to the energy gap law, this leads to an increase in the excited state lifetime.61 The ps-TRIR spectra of [Ru(dcb)(dab)(NCS)2] (see Figure 9) allude to significantly perturbed photophysics compared to the remaining complexes. At short time delays, the spectra appear similar to those of the other dcb-containing complexes, with a depletion of the ground state peak accompanied by the appearance of excited state peaks at 2072 and 2036 cm−1. Some vibrational cooling is observed, on a time scale on the order of tens of picoseconds. As these peak shifts are comparable to other complexes, this state is assigned to a {Ru(NCS)2} → dcb transition. The location of the excited state electron on the dcb ligand is supported by the presence of a COOH depletion in the bpy spectral region. After approximately 150 ps the initial excited state peaks can be seen to give way to a new peak at 2099 cm−1 that partially masks the parent depletion. In addition to the transients indicated in Figure 9c, a long decay transient (≈150 ns) was also included, the time constant of which was determined from the lifetime of the excited state. The decreased shift in this state is indicative of decreased oxidation of the NCS units. An interpretation that is consistent with this observation is a shift of the excited state hole away from the NCS units to the ruthenium atom or the attached bpy or dcb ligands. This would leave the {Ru(NCS)2} moiety slightly more electron rich than in the initial state, giving rise to a relatively small shift in NCS stretching frequency. The spectral features in the COOH and bpy regions undergo a change on a similar time scale. Initially peaks are observed at ca. 1686, 1605, 1558, 1543, 1499, and 1457 cm−1, of which the former two disappear after ca. 150 ps, while a new peak appears to grow in at ca. 1578 cm−1. The depletions at 1620 and 1523 cm−1 persist, while the 1717 cm−1 depletion appears to return toward the baseline. The almost complete disappearance of COOH contributions is consistent with a shift of the excited state electron away from dcb to the bpy or ArCC units; it also makes a shift of the excited state hole to dcb unlikely, as one would expect to observe a change in the COOH stretching frequencies in such a case. Migration of excited states between ligands and from MLCT to ligand-centered states of similar energies has previously been reported.62−69 A particularly relevant example is that of [(pyrvbpy)2RuCl2] (pyrv-bpy = 4-(pyrenylvinyl)-4′-methyl-2,2′-bipyridine) and related complexes.63 Similar to the present case, substitution of the bpy of the parent complex was found to lead to the formation of a long-lived pyrenylvinyl-based excited state. In this example only a single pyrenylvinyl unit is present, which may result in the formation of an excited state that is not analogous the one observed here. Another example is that of a heteroleptic Ru complex based on dppn (dppn = benzo[i]dipyrido[3,2-a;2′,3′-c]phenazine), where initial excitation of a 1 MLCT state resulted in the formation both of metal-centered and ligand-centered states. These were found to effect ligand loss and 1O2 sensitization, respectively.69 [Ru(dcb)(dab)(NCS)2] shows a lifetime that is prolonged significantly compared to the other ruthenium complexes investigated. This is also consistent with previous results of ligand-centered excited states.62,63,70 Previous studies have reported excited state lifetimes of [Ru(bpy-X2)3]2+ and [Ru(bpy)2(bpy-X2)], (X = COOH, COO−, CH3, CH2CO2) to be on the order of several hundred nanoseconds.2,71,72 This implies that nonradiative deactivation of [Ru(dcb)2(NCS)2] and related complexes proceeds, at least in part, through the

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CONCLUSIONS We have presented a number of ruthenium complexes related to and including the N3 dye and examined them using a range of spectroscopic and computational techniques. Minor variations in the lowest energy absorption peaks were observed regardless of bpy substituent, which was attributed to minor orbital overlap of the substituent with the absorbing MO. The lowest energy absorption peak has been assigned as a {Ru(NCS)2} → dcb transition, with significant contributions of the NCS units. In comparison, the higher-energy absorption bands are modified by changing the substituent, reflecting the conjugative and inductive effects of the substituents. Resonance Raman spectra at λex = 532 and 568.2 nm appear almost identical for all complexes, indicating the transitions at these energies are unaffected by the 4,4′-substituents, as also indicated by the electronic absorption spectra. The observation of NCS peaks at low energies supports the assignment of the transitions. Relatively short-lived {Ru(NCS)2} → dcb/dpb excited states were found, with the exception of [Ru(dcb)(dab)(NCS)2], which has longer lived excited state peaks that differ significantly. Using time-resolved infrared spectroscopy and triplet state calculations, this was found to be consistent with an LCCT state where the excited state electron and hole are both located on the dab ligand. This state is formed from an initial short-lived {Ru(NCS)2} → dcb state that is similar in nature to those found in the remaining complexes.

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EXPERIMENTAL SECTION

Spectroscopic Measurements. Data interpretation, solvent subtraction, baseline correction, and splicing of acquisition windows were carried out using Grams v8.0 (Thermo, Inc.). Peak identification, fitting, analysis of exponential decays, and visualization of spectra were I


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be adequate solvents for these measurements. Probing below ca. 1420 cm−1 is prevented by intense solvent bands for both DMSO-h6 and DMSO-d6. Computations. All calculations were carried out using the B3LYP or CAM-B3LYP methods using the Gaussian (Gaussian, Inc.) suite of computational chemistry programs. The 6-31G(d) basis set was used for all atoms except metals, for which the effective core potential (ECP) basis set LANL2DZ was used. This method has previously been shown to be effective for a wide range of transition metal complexes.82−85 All calculations were carried out in DMSO solvent fields. The method used in this work is the integral equation formalism polarizable continuum (IEF-PCM) model.86 Frequency calculations were performed, to ensure the presence of local minima. Electronic excitations were calculated by TD-DFT using B3LYP and CAMB3LYP functionals. For cases where multiple electronic transitions contributed to an experimental peak, the effective calculated peak center was determined as a weighted average with respect to the transition oscillator strength. By carrying out a population analysis, the contributions of each atom to electronic transitions were calculated. The total electron shift for each peak is calculated as a weighted average with respect to the oscillator strength of each transition and then normalized. Frequencies, orbital partitioning factors, and electronic information were extracted from the output files using GaussSum.87 The mean absolute deviations (MADs) of experimental to calculated infrared and Raman frequencies were calculated as described previously47,48 by considering all peaks between 700 and 1700 cm−1 with greater than 20% of the maximum intensity. We consider a MAD of ≈10 cm−1 to be a satisfactory agreement. The frequencies were scaled to minimize the MAD for each compound; the scale factors are listed in Table S2 (Supporting Information). Molecular orbital visualization was carried out using GaussView 5.0 (Gaussian, Inc.), and visualization of vibrational modes was done using the Molden package.88 Compounds. All ruthenium complexes were kindly donated by Prof. Paul L. Burn from the University of Queensland.89,90

carried out using OriginPro 7.5 (OriginLab, Inc.). All solvents were of spectroscopic grade and were used without further purification. FT-Raman spectra were obtained in KBr disks. A Bruker Equinox 55 interferometer coupled with a FRA-106 Raman module and a D418T liquid nitrogen cooled germanium detector was used, controlled by the Bruker OPUS v6.0 software package. A Nd:YAG laser operating at 1064 nm and 250 mW was used. The spectra were acquired with a resolution setting of 4 cm−1, using 2048 coadded scans. Electronic absorption spectra were acquired using a Jasco V-550 spectrophotometer controlled by Spectrum Measurement v1.53. Resonance Raman spectroscopy was carried out on an instrument described previously.70 Briefly, the excitation beam was focused on a spinning sample tube, arranged in an obtuse-angle backscattering geometry with collection lens. The scattered photons were focused on the 30 μm entrance slit of an Acton Research SpectraPro500i. RazorEdge (Semrock, Inc.) long-pass filters or narrow band-line rejection filters (Kaiser Optical, Inc.) were used to reject laser radiation prior to the spectrograph slit. A 1200 grooves mm−1 grating was used to horizontally disperse the beam on a liquid nitrogen cooled Spec10:100B CCD, controlled by WinSpec/32 software (Roper Scientific). A continuous-wave Innova I-302 krypton ion laser (Coherent, Inc.) was used for 350.7, 406.7, 413.1, and 568.2 nm excitation, and diodepumped lasers (CrystaLaser, Inc.) provided 444.3 and 532 nm. Excited state emission and absorption transients were acquired using an LP920 K transient absorption system (Edinburgh Instruments), which has been described previously.43 All samples were degassed using argon prior to measurements and fluorescence correction was applied for the transient absorption spectra as part of the optical density (ΔOD) calculation. The TRIR apparatus is based upon the PIRATE facility at the Rutherford Appleton Laboratory, which has been described previously.79−81 Briefly, the output from a commercial Ti:sapphire oscillator (MaiTai)/regenerative amplifier system (Spitfire Pro, Spectra Physics) was split and used to generate 400 nm pump pulses and a tunable mid-IR pulse with a spectral bandwidth of 180 cm−1 and a pulse energy of ca. 2 μJ at 2000 cm−1. Approximately half of the IR pulse was reflected onto a single-element mercury cadmium telluride (MCT) detector (Kolmar Technology) to serve as a reference, while the remainder serves as the probe beam, which is focused and overlaps with the pump beam at the sample position. The 400 nm pump was optically delayed (up to 3 ns) by a translation stage (LMA Actuator, Aerotech) and focused onto the sample with a quartz lens. The polarization of the pump pulse is set at the magic angle (54.7°) relative to the probe pulse to recover the isotropic absorption spectrum. The focus spot of the probe beam was adjusted to be slightly smaller than that of the pump beam in order to ensure that the area probed corresponds to excited molecules. The broad-band-transmitted probe pulse was detected with an MCT array detector (Infrared Associates), which consists of 128 elements (1 mm high and 0.25 mm wide). The array detector was mounted in the focal plane of a 250 mm IR spectrograph (DK240, Spectra Product) with a 150 grooves mm−1 grating, resulting in a spectral resolution of ca. 4 cm−1 at 2000 cm−1. Signals from the array detector elements and the single-element detector were amplified with a 144-channel amplifier and digitized by a 16-bit analog-to-digital converter (IR-0144, Infrared Systems Development Corp.). For the ns-TRIR a Q-switched Nd:YVO laser (1064 nm, 600 ps, Advanced Optical Technology) was frequency doubled to produce a 532 nm pump pulse. This was synchronized to the probe pulse from the regenerative amplifier and the delay is controlled by a pulse generator (DG535, Stanford Research Systems) allowing delays of 0.5 ns to 100 μs to be used. A Harrick flowing solution cell with 2 mm thick CaF2 windows and a path length of 500 μm was mounted on a motorized cell mount that moves the cell rapidly in x and y dimensions throughout the experiment. Consequently, each laser pulse illuminated a different volume of the sample, reducing heating and degradation of the sample solution. Spectrograph settings were 4.85, 6.00, and 6.60 μm for the probe regions 2160−1950, 1770−1570, and 1590−1420 cm−1 respectively. DMSO-h6 (for the 2160−1950 cm−1 region) and DMSO-d6 (for the other spectral regions) were found to

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01690. Molecular orbital pictures, MAD values and scale factors, Raman band positions, summarized transient absorption data, TRIR kinetic traces, and transient Raman spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.C.G.). *E-mail: [email protected] (M.W.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.H. would like to thank the University of Otago for a postgraduate scholarship. M.W.G. gratefully acknowledges receipt of a Royal Society Wolfson Merit Award. The support of the MacDiarmid Institute for Advanced Materials and Nanotechnology is gratefully acknowledged.

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