Bichromophoric Photosensitizers: How and Where to Attach Pyrene Moieties to Phenanthroline to Generate Copper(I) Complexes

Pyrene is a polycyclic aromatic hydrocarbon and organic dye that can form superior bichromophoric systems when combined with a transition metal-based chromophore. However, little is known about the effect of the type of attachment (i.e., 1- vs 2-pyrenyl) and the individual position of the pyrenyl substituents at the ligand. Therefore, a systematic series of three novel diimine ligands and their respective heteroleptic diimine-diphosphine copper(I) complexes has been designed and extensively studied. Special attention was given to two different substitution strategies: (i) attaching pyrene via its 1-position, which occurs most frequently in the literature, or via its 2-position and (ii) targeting two contrasting substitution patterns at the 1,10-phenanthroline ligand, i.e., the 5,6- and the 4,7-position. In the applied spectroscopic, electrochemical, and theoretical methods (UV/vis, emission, time-resolved luminescence and transient absorption, cyclic voltammetry, density functional theory), it has been shown that the precise choice of the derivatization sites is crucial. Substituting the pyridine rings of phenanthroline in the 4,7-position with the 1-pyrenyl moiety has the strongest impact on the bichromophore. This approach results in the most anodically shifted reduction potential and a drastic increase in the excited state lifetime by more than two orders of magnitude. In addition, it enables the highest singlet oxygen quantum yield of 96% and the most beneficial activity in the photocatalytic oxidation of 1,5-dihydroxy-naphthalene.


■ INTRODUCTION
The efficient capture and usage of solar energy provides a sustainable alternative to the burning of fossil fuels. 1−5 In this context, photosensitizers (PS) offer a viable tool for harvesting sunlight and converting solar energy directly into chemical energy. This has been successfully demonstrated for the conversion of small molecules such as the photocatalytic production of hydrogen from water, 6−11 the photoreduction of CO 2 to CO, 12−17 or the generation of reactive singlet oxygen. 18−20 Furthermore, PS also enable a wide range of organic transformations like the E → Z isomerization of different alkenes, 20−23 cyanation 24,25 or even the challenging alkylation 26,27 and arylation 28,29 of various organic substrates. 30−33 The main goals of current photochemical research are to further improve the catalytic activities, to expand the range of applications, and to better understand structure-activity relations. This includes the design of novel molecular PS with appropriate photophysical properties, such as long-lived excited states, high absorptivity in the visible region, and sufficient (photo)stability. 11,34−38 Among the different types of PS, especially transition-metal complexes are attracting a great and long-lasting interest due to their largely tunable redox and excited state properties. 11,37−39 In particular, complexes based on noble and expensive 4d/5d metals such as Pt, 40,41 Ru, 9,30,42 and Re 6,43,44 have been intensively studied. Replacing the metal center in such systems with non-precious and more earth-abundant 3d metals, e.g., Cr, 45−48 Fe, 49−52 and Cu 53−59 has emerged as an attractive step toward large-scale applications.
In this context, especially heteroleptic copper(I) complexes of the type [Cu(N^N)(P^P)] + bearing a diimine and diphosphine ligand enable long-lived excited states, sufficient quantum yields, and high excited state redox potentials. 35,39,55,60−64 Moreover, they have already successfully demonstrated their competitiveness with noble metal-based Ir(III) and Ru(II) complexes in photocatalysis. 7,10,11,65,66 As the absorptivity of the heteroleptic Cu(I) complexes in the visible region is rather limited, and with the aim of further improving the excited state properties, several studies have targeted this challenge by modifying the diimine ligand. 33,35,65,67 One promising strategy is the introduction of extended π-systems in the backbone of the diimine moiety by directly fusing an additional organic chromophore to form large and highly conjugated ligand scaffolds. 39,68−70 It was proven that the implementation of conjugated groups in this way can strongly increase the excited state lifetimes through the introduction of long-lived ligand-centered triplet ( 3 LC) states. 20,69,71,72 However, this sometimes comes at the cost of significantly shifting reduction potentials toward less negative values and therefore a loss in possible driving force in catalytic transformations. 55,69 As a result, the proper selection of the coordinating ligands and substituents plays a key role in the design of novel PS. Pyrene and its derivatives, having four fused benzene rings, are promising alternatives, due to their appealing advantages: extensive π-electron delocalization, high attenuation coefficients, long fluorescence lifetimes (τ = 354 ns in toluene), and good ability to transport holes. 73 Hence, they already attracted a great interest for applications in organic electronics, solar cells, and light-emitting electrochemical materials. 74−77 Moreover, there are several studies applying pyrenes as substituents for ligand modifications to increase the catalytic activity of the resulting Ir, 71,[78][79][80][81][82]7,83,84 Cr, 47 Fe, 85 or Cu 86 complexes. The incorporation of pyrene substituents into metal-organic chromophores successfully improves the photophysical properties of the complexes by introducing pyrene-based 3 LC states, which (i) have a quasi-isoenergetic behavior compared to triplet metal-to-ligand charge transfer ( 3 MLCT) states and can serve as an energy reservoir for excited states; 83,84,87,88 (ii) or act as the lowest-lying excited state with long excited state lifetime. 71,72 There are two different options to attach a pyrene substituent to a potential ligand, i.e., in the 1-position and in the 2-position (Scheme 1). Studies indicated that the 1position of pyrene is markedly more active than other positions. 89 In strong contrast, the 2-position is much more difficult to directly be substituted because the nodal planes of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) pass through it. 90,91 The direct comparison between a 1-and 2-pyrene substituted deoxyuridine nucleoside illustrated that the differ-ent positions of pyrene can alter the optical properties. This is due to the fact that 1-pyrenyl substitution causes a stronger electronic coupling between the two aromatic parts relative to that at the 2-position. 87 Nevertheless, as a rigid and linear linker, the particular long axis along 2-and 7-positions makes pyrene attractive for the synthesis of metal−organic frameworks (MOFs) and covalent organic frameworks (COFs). 92,93 To address this issue in more depth, this study has examined two different effects: (i) the introduction of two pyrene substituents at two different positions (i.e., 1-vs 2-substitution) and (ii) of two different positions at the 1,10-phenanthroline itself (i.e., 4,7-vs 5,6-position) (Scheme 1). By comparing the resulting three novel ligands and their corresponding copper(I) complexes, the impact of the substitution positions of pyrene and phenanthroline on the photophysical and photocatalytic properties was explored in detail. Moreover, these properties were compared with those of the unsubstituted reference complex CuNeo. Cyclic and differential pulse voltammetry, various steady-state and time-resolved spectroscopic techniques, as well as time-dependent density functional theory (TD-DFT) were used to investigate this novel class of heteroleptic copper(I) PS. Finally, all three complexes were successfully applied in the photocatalytic generation of reactive singlet oxygen ( 1 O 2 ) over several cycles. In addition, the formed 1 O 2 was also used for the catalytic photooxidation of 1,5dihydroxynaphthalene (DHN) to Juglone 94−96 to examine their catalytic potential and to study structure-activity relationships.
■ RESULTS AND DISCUSSION Synthesis and Structural Characterization. Ligand Synthesis (Pyr1−3). The novel diimine ligands Pyr1, Pyr2, and Pyr3 were synthesized via Suzuki−Miyaura cross-coupling reactions from their respective pyreneboronic acid (and its pinacol esters) utilizing a XPhos-Pd-G2 catalyst. 97 The required cross-coupling substrates 4,7-dichloro-2,9-dimethyl-1,10-phenanthroline (Neo-4,7-Cl 2 ) 98 and 5,6-dibromo-2,9dimethyl-1,10-phenanthroline (Neo-5,6-Br 2 , see Scheme 1) 36 were synthesized on gram-scale following literature-known procedures. Subsequent cross-coupling reactions toward the desired ligands Pyr1−3 varied strongly due to the different reactivity of the coupling substrates and reagents. Pyr1 was synthesized from commercially purchased pyrene-1-boronic acid (Pyr1BA) and Neo-4,7-Cl 2 in a biphasic mixture of THF and an aqueous 0.5 M K 3 PO 4 solution, similar to our previously described approach 36 (see SI Chapter 2 for details). Adjustments had to be made to the work-up procedure due to the polarity and solubility of Pyr1, Pyr1BA, and the byproduct pyrene: column chromatography was conducted first on basic aluminum oxide to remove the remains of the catalyst and boronic acid. Further chromatography on silica was necessary to remove pyrene as the remaining impurity. Due to the extended work-up procedure, the isolated yield was 55%, although quantitative conversion can be assumed as only the 4,7-disubstituted ligand was isolated from the reaction.
Pyr2 was synthesized from pyren-2-boronic acid pinacol ester (Pyr2Pin), which was prepared directly from pyrene in an Ir-catalyzed C−H activation reaction with bis(pinacolato)diboron according to a known procedure (see SI Chapter 2 for further details). 99 More importantly, under basic conditions, the reacting species Pyr2BA is slowly released from Pyr2Pin, 100 significantly suppressing an assumed deboronation side reaction. Neo-4,7-Cl 2 was therefore reacted with Pyr2Pin under similar conditions as Pyr1. To ensure a constant release of the reactive boronic acid from its pinacol ester, an aqueous 1 M Cs 2 CO 3 solution was chosen, as an increased solubility in the organic solvent was expected. The reaction was prolonged by a factor of three (48 h vs 16 h). After column chromatography, the product could be recrystallized from toluene in a final isolated yield of 49%.
Pyr3 was synthesized from Pyr2Pin and Neo-5,6-Br 2 in a biphasic mixture of water with Cs 2 CO 3 as the base. When using THF at reflux temperature as described for Pyr1 and Pyr2, the reaction also yields the mono-substituted phenanthroline derivative as a side product, which could not be separated from the crude mixture. Applying the more classical [Ph(PPh 3 ) 4 ] catalyst expectedly yielded unreacted substrates as the main component after the reaction, highlighting the need for the more reactive XPhos-Pd-G2 catalyst 36 and more harsh conditions. Therefore, THF was replaced by toluene, which allowed higher temperatures, and the reaction time was increased to 60 h to achieve full conversion without producing the mono substituted byproduct. Owing to its low solubility and unfavorable crystallization behavior, Pyr3 was purified by preparing its crude homoleptic complex as an intermediate (SI Chapter 2). The ligand Pyr3 was finally obtained after liberation from the complex using potassium cyanide (please see SI Chapter 2 for details concerning safety) in an isolated yield of 54%.
Complex Synthesis. The syntheses of the heteroleptic copper(I) PS CuPyr1, CuPyr2, and CuPyr3 were conducted following a well-known one-pot two-step procedure (Scheme 1) starting from [Cu(MeCN) 4 ]PF 6 (MeCN = acetonitrile). 7,20,34,36,69,70 To ensure a slow and precise addition of the diimine ligand to the reaction mixture, an automatic syringe pump was utilized for the preparation of CuPyr1 and CuPyr3. In contrast, due to its low solubility in CH 2 Cl 2 , Pyr2 was added directly as a solid under inert conditions at −20°C for the synthesis of CuPyr2.
Contrary to earlier studies, 7,20,34,36,69,70 the novel copper(I) complexes could not be precipitated as solids by adding nhexane. Instead, the compounds form a viscous deep red oil even at −20°C, possibly due to competing intermolecular π−π interactions induced by the pyrenyl substituents. 101 After decanting the remaining solution and treating the oil with an excess of n-hexane in an ultrasonic bath, the compounds were carefully dried to obtain a solid. Isolated yields for CuPyr1, CuPyr2, and CuPyr3 were 71, 41, and 61%, respectively.
The compounds were then fully characterized by nuclear magnetic resonance (NMR) spectroscopy ( 1 H, 13 C, 31 P) and high-resolution mass spectrometry (HRMS) (SI Chapters 2, 3, and 4). Attempts to grow single crystals for X-ray crystallography with various methods did not result in any crystal formation so far.
Molecular Structure Analysis. As the growth of single crystals was not successful, the structural analysis is based on theoretically predicted structures obtained by TD-DFT calculations (see Figure 1). Selected parameters of CuPyr1− 3 and the reference CuNeo are gathered in Table 1, along with experimental results for CuNeo for comparison.
The Cu−P and Cu−N bonds lengths as well as the N−Cu− N and P−Cu−P angles are well described by theory and are mostly uniform within the computed structures. C−C bond lengths of the bond between the pyrenyl substituent and the phenanthroline moiety do also not differ significantly. They are predicted to be slightly shorter when compared to a related phenyl substituted complex (149.(5) pm in [Cu(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)(xantphos)] + . 102 It is noteworthy that the average distances between the two pyrene substituents vary between 636 pm (CuPyr3) and 1231 pm (for CuPyr2 measured between the centers of each substituent), indicating a possible stronger pyrene−pyrene interaction for CuPyr3 with the substituents in the 5,6- position. This can include, for example, interactions via intramolecular π−π interactions in the ground state or in an excited state. Significant differences can also be found when comparing the torsion angles around the new C−C bond τ Sub GS in the singlet ground state. Here, CuPyr2 tends to exhibit a more coplanar structure (−50°) compared to CuPyr1 and CuPyr3 (85°and 73°, respectively), most likely due to less steric hindrance. Considering the same parameter but in the lowest triplet state τ Sub ES, the difference Δτ Sub between these angles (i.e., Δτ Sub = τ Sub GS − τ Sub ES) is the largest for CuPyr1 and the smallest for CuPyr3. This suggests that especially in CuPyr1, a possible triplet state may be partially delocalized over the phenanthroline and the pyrenyl moiety, which could be due to the increased orbital overlap. In contrast, CuPyr3 is therefore suggested to have the weakest orbital overlap. However, the small difference could also be due to greater repulsion between the two pyrenyl-substituents and/or possible intramolecular π−π interactions preventing stronger rotation.
Electrochemical Studies. The electrochemical properties of CuPyr1−3 and the corresponding ligands Pyr1−3, as well as of pyrene as a reference, were determined by cyclic and differential pulse voltammetry (CV and DPV, Figures 2 and S30−S34,  6 ] as the electrolyte. Based on scan-ratedependent CV measurements ( Figure S34), CuPyr1−3 show three reversible cathodic events. Compared to the only singly reducible CuNeo, the first reversible reduction waves of CuPyr1, CuPyr2, and CuPyr3 (−1.97, −2.02, and −2.10 V, respectively) can be assigned to the reduction of the phenanthroline moiety. Interestingly, the first reduction potential of CuPyr3 is identical with that of the reference CuNeo (both −2.10 V), which lacks pyrene substituents. Contrarily, the reduction waves of CuPyr1 and CuPyr2 are shifted anodically by 130 and 80 mV, respectively. This indicates that the pyrenyl substitutions at the 4,7-position of the phenanthroline have a greater impact on the electrochemical behavior than those at the 5,6-position.
A comparison with uncoordinated pyrene (−2.58 V) reveals that the second and third reversible reductions in CuPyr1−3 can be attributed to the reduction of both pyrenyl substituents (−2.32/−2.42, −2.46/−2.54, and −2.54/−2.67 V, see Table  2). Only CuPyr1 has a fourth reduction, which might indicate an even doubly reduced pyrenyl substituent. Furthermore, a maximum difference of ∼200 mV was observed between these sets of reductions for CuPyr1−3, with CuPyr1 being the easiest (−2.32 V) and CuPyr3 being the most difficult (−2.54 V) to reduce. These findings suggest that the electronic communication between the substituent and the phenanthroline is most pronounced in the pyrene-1-yl-substituted CuPyr1. In contrast, pyrene-2-yl substitution in the 5,6position (CuPyr3) is expected to induce barely any electronic interaction as the phenanthroline-and pyrene-based reductions are almost identical to those of the two unsubstituted building blocks (i.e., phenanthroline and pyrene).
Comparing the ligands with their complexes, potential differences of 123, 90, and 40 mV were found for the reduction of the pyrenyl substituents for CuPyr1, CuPyr2, and CuPyr3, respectively. The fact that the largest potential difference is observed for CuPyr1 is again in line with the argument that the strongest electronic interaction between phenanthroline and the pyrene moieties is present in the 1-pyrenyl system.
The three complexes CuPyr1−3 show an irreversible oxidation event, which can be attributed to the literatureknown oxidation of the Cu−P bond at about +0.9 V ( Figures  S31 and 32) 39,69 and to the irreversible oxidation of pyrenes at similar potentials. 85 Photophysical Properties. The UV/vis absorption spectra of Pyr1−3 (Figure 3, top) show strong features between 250 and 370 nm in acetonitrile solution, with Pyr1 having the most red-shifted absorption profile. Based on the spectra of pure pyrene and unsubstituted phenanthroline, the bands of Pyr1−3 between 250 and 300 nm can be assigned to a mixture of (i) ligand-centered (LC) π−π* transitions at the phenanthroline core as well as (ii) LC transition from S 0 to S 3 at the pyrenyl substituent. 101 In the range from 300 to 360 nm, the spectra of the pyrenyl substituted ligands are dominated by   Inorganic Chemistry pubs.acs.org/IC Article intense and clearly structured vibrational bands of the S 0 to S 2 LC transition at the pyrenyl moiety. 103,104 The redshift of Pyr2 compared to pure pyrene is 4 nm, while 9 nm is observed for Pyr1 and Pyr3. Overall, the impact of pyrene substituents on the absorption of the phenanthroline moiety is not significant, in contrast to 5,5′-bis(pyren-1-yl)-2,2′-bipyridine, where a strong and broad absorption band between 320 and 420 nm was observed. 79 The absorption features of the bichromophoric CuPyr1−3 complexes possess similar features compared to those of their corresponding pyrenyl-substituted ligands Pyr1−3 ( Figure  S42). Especially between CuPyr2 and Pyr2, the difference in the attenuation coefficients is rather small (i.e., Δ 340 nm = 4 × 10 3 Lmol −1 cm −1 , cf. Figure S42 left). In strong contrast, CuPyr1 and CuPyr3 show more pronounced absorptivity compared to their respective ligands (Δ 343 nm = 13 × 10 3 Lmol −1 cm −1 for CuPyr1 vs Pyr1 and Δ 327 nm = 31 × 10 3 Lmol −1 cm −1 for CuPyr3 vs Pyr3; Figure S42 right).
As known from the literature, the absorption of CuNeo features a weak broad band between 360 and 450 nm attributed to singlet metal-to-ligand charge transfer ( 1 MLCT) transitions from d(Cu) to π*(phen). 35,39 In this spectral region, the absorption profile of CuPyr2 is very similar to that of CuNeo, with almost identical attenuation coefficients. Together with the fact that CuPyr2 has the lowest redshift compared to CuPyr1 and CuPyr3, it is suggested that there is only a small orbital overlap between the pyrenyl substituent and the phenanthroline core. It can be concluded that CuPyr2 has the lowest electronic communication among the complexes CuPyr1−3.
In contrast to the pyren-2-yl substitution in 4-and 7positions (as in CuPyr2), such a substitution in 5-and 6positions in CuPyr3 almost doubles the attenuation coefficient of the 1 MLCT band and simultaneously shifts the S 2 ← S 0 transition (Figure 3). A stronger electronic interaction between phenanthroline and pyrene is therefore likely in CuPyr3 compared to CuPyr2.
The absorption of CuPyr1 in the 1 MLCT region (i.e., around 350−450 nm) exhibits a greater ability to absorb visible light compared to pyren-2-yl substituted systems, as already seen for the Pyr1 ligand, where the broadest low energy band stretches to longer wavelengths. DFT calculations also indicate that the LUMO+1, which is a dominant part of the 1 MLCT transition of CuPyr1, consists of a mixture of π* phen and π* pyr localized orbitals (Table S1 transition 2). This is quite different    3, S41, and  Tables S1−S3). Taken together, the observed improvement in the light-harvesting properties of CuPyr1 is attributed to the supposed strong electronic interaction between the two moieties, which is in agreement with the literature. 89 Emission measurements were then conducted to gain a better understanding of the excited states. Pyrene and Neo exhibit structured emissions in the range of 350 to 450 nm (Figure 4 top), 104,107 whereas the novel ligands Pyr1−3 show a much broader emission spectrum, extending from 370 to 600 nm. Pyr1 and Pyr2 have broad, barley-structured emission bands in acetonitrile, with maxima that are red-shifted relative to the emission of pyrene (59 and 45 nm, respectively) and Neo (67 and 53 nm, respectively; Figure 4 top and Table S4). Pyr3 exhibits a broad emission, consisting of two bands at 400 and 480 nm.
The emission profiles of Pyr1−3 show considerable differences when compared in dichloromethane and acetonitrile because two bands appear in dichloromethane ( Figure  S45). This emission behavior has been confirmed by TD-DFT calculations, which show that intraligand charge transfer transitions from π pyr to π* phen ( 1 ILCT pyr,phen ) and transitions from one pyrenyl substituent to the other ( 1 ILCT pyr,pyr , Figure  5) are possible. Furthermore, the intensity of the bands depends on the excitation wavelength (SI for further details) in the experiment.
To prove the involvement of the phenanthroline moiety in the assumed 1 ILCT pyr,phen transition, protonation experiments with trifluoroacetic acid (TFA, 100 equiv.) were conducted on Pyr1−3 ( Figures S46, 47 and Table S4). Upon protonation in acetonitrile, the emission of Pyr2 displayed a characteristic pyrene-based ligand-centered emission at 375 nm and a redshifted (Δ = 269 nm) emission band originating from the protonated phenanthroline-1-ium to pyrene at 685 nm. The emission band of Pyr1 exhibits a redshift of 217 nm, while the second emission band of Pyr3 has a redshift of 121 nm. Similar results with TFA were also reported by Constable et al. 79 Interestingly, in dichloromethane, the second emission from Pyr1 and Pyr3 ( Figure S46) does not change significantly as a result of protonation, indicating the 1 ILCT pyr,pyr emission. A similar interaction between two pyrenyl substituents is referred to in the literature as excimer emission. 74,107 The complexes CuPyr1 and CuPyr2 show identical emission bands in acetonitrile to their corresponding ligands (Figure 4 middle), when excited at 334 nm. These bands are invariant to the excitation wavelength (in the range between 334−355 nm, Figure S48) and can be assigned to a decay of an ILCT phen,pyr state. The emission quantum yield of CuPyr1 is 32%, much higher than that of CuPyr2 (9%), which again indicates a significantly stronger electronic interaction between the two chromophores of CuPyr1. In contrast to CuPyr1 and CuPyr2 in acetonitrile, CuPyr3 displays a pyrene-structured emission centered at 400 nm and a very broad but weak emission attributed to an ILCT phen,pyr . This latter emission is analogous to Pyr3.
CuPyr1 is the only emissive compound in acetonitrile when excited at 407 nm, with an emission peak at 539 nm, attributed to 3 MLCT from d(Cu) to π*(phen). In contrast, the emission upon excitation at 387 nm appears to originate from a mixture of ILCT pyr,pyr and MLCT Cu,phen ( Figure S49). This assignment is supported by the absorption analysis and TD-DFT calculations.
The emission of CuPyr1 in dichloromethane, excited at 334 nm, is very weak and red-shifted compared to its emission in acetonitrile. CuPyr2 and CuPyr3 do not really emit and the two emission bands, one based on LC pyr (at about 375 nm) and the other centered at 505 and 515 nm, respectively ( Figure  4 bottom), are only in the order of Raman scattering.
The excited-state characteristics of Pyr1−3 and CuPyr1−3 were further studied by time-resolved spectroscopy excited at 355 nm in deaerated acetonitrile and dichloromethane solutions (Figures S50−S53). The emission lifetimes of Pyr1 and protonated Pyr1H were below 10 ns, while Pyr2 and Pyr3 also possess rather short-lived emissive states (28.6 and 29.7   (Table S5). Based on similarities with other reports of pyrene-based systems, 71,72 in which the pyrene-localized triplet states also act as the lowestlying excited states with extended excited state lifetimes, the observed lifetimes of Pyr1 can be assigned to 3 LC pyr (16.03 and 26.43 μs in acetonitrile and dichloromethane, respectively). Compared to the triplet lifetime of pyrene (1.23 μs in dichloromethane), 105 the excited state lifetime of Pyr1 in the same solvent is increased 22-fold due to the attached phenanthroline moiety.
In line with the results obtained for Pyr1, CuPyr1 also has a short-lived emission (<10 ns) and a long-lived dark excited state (22.42 and 35.95 μs in acetonitrile and dichloromethane, respectively). Therefore, the lowest excited state in CuPyr1 can be assigned to a non-emissive pyrene-localized 3 LC state. However, similar results were obtained for Pyr2 and the respective copper(I) complex (Table S5). Although the excited state lifetimes of CuPyr2 are somewhat shorter (e.g., 17.70 (acetonitrile) and 25.02 μs (dichloromethane)), the similar magnitude also suggests a similar pyrene-localized 3 LC state as the final excited state.
For Pyr1 and Pyr2 and their respective complexes CuPyr1 and CuPyr2, the excited state lifetimes generally show an increase by a factor of about 1.5−2.5 when the solvent is changed from acetonitrile to dichloromethane. This is consistent with the expected behavior when a coordinating solvent is replaced by a non-coordinating solvent. 108,109 This solvent effect in our pyrene-based systems differs from other results regarding the solvent-tuning effect of MLCT triplet energies. 47,110 In these studies, both MLCT emission and solvatochromic behavior of the MLCT states were observed. 47,110 Even a switch between both the excited state could be achieved when changing the solvent environment. 110 In contrast, the small impact on the emission wavelength in CuNeo (only 4 nm, Table 2) and the slight increase in the excited state lifetime for CuPry1−3 in dichloromethane renders a change in the nature of the final excited state unlikely.
Interestingly, CuPyr3 differs significantly from CuPyr1 and CuPyr2, as a second, slightly shorter decay component with a lifetime of ∼1 μs is observed in both solvents. Within this time, an excited state is populated and subsequently depopulated. Since the lifetime of pure triplet pyrene in dichloromethane is 1.23 μs, 105 it is possible that a triplet state of pyrene forms initially and interacts with the neighboring pyrene, which is still in the ground state. As the distance between the two pyrenes in CuPyr3 (see Figure 1) is much shorter than in CuPyr1 and CuPyr2, such an interaction could be present. This could be a reason for the much shorter excited state lifetime of CuPyr3 compared to CuPyr1 and CuPyr2.
Singlet Oxygen Measurements. The time-resolved measurements indicate that the lowest triplet excited state changes from a 3 MLCT state in CuNeo to a pyrene-localized 3 LC state in CuPyr1−3, associated with an increase in the excited state lifetime of more than one order of magnitude compared to CuNeo (vide supra). Therefore, the next step was to test whether these promising photophysical properties can be used to generate reactive singlet oxygen ( 1 O 2 ) and to determine the individual catalytic activities. 1 O 2 is the energetically higher form of triplet oxygen ( 3 O 2 ) and, due to its singlet electronic configuration, commonly used as a reagent for oxidation reactions. Furthermore, 1 O 2 is also used in photodynamic therapy (PDT) in the treatment of cancer, for example. Mechanistically, 1 O 2 is generated through energy transfer from 3 O 2 . The efficiency of this process can generally be evaluated from the characteristic emission band of 1 O 2 at about 1276 nm. As a universal standard to quantify this emission, phenalenone (PN) can be used. 111,112 In aerated acetonitrile, all four copper(I) photosensitizers display a clear 1 O 2 emission band when excited at 407 nm ( Figure S54). As expected, CuPyr1, which has the longest excited state lifetime, provides the highest 1 O 2 quantum yield of 0.96 with respect to PN (ϕ 1O2 = 1). 111−114 Thus, CuPyr1 outperforms CuPyr2 (0.83) and CuPyr3 (0.66) and is also clearly superior to the reference complex CuNeo (0.20). This To evaluate the ability of the photosensitizers to produce 1 O 2 continuously and to be reusable without significant loss of activity, successive 1 O 2 emission spectra of the same sample were recorded in conjunction with subsequent photostability measurements. In addition, the photostability of CuPyr1−3 was investigated over a period of 3 h in inert acetonitrile and at least 1 h under aerated conditions (Figures S57 and S58). Figure 6 successfully demonstrates the constant activity in 1 O 2 generation of CuPyr1 over the course of 8 measurements, with only a slight decrease of the MLCT band (Δ = 9%). This small depletion is possibly due to photobleaching by in situ generated 1 O 2 or photo-induced ligand exchange processes, as known from the literature. 115−117 Similarly, for CuPyr2 and CuPyr3, the continuity of 1 O 2 generation and the subsequent photostability are not affected by the substitution characteristics (compare Figures 6 and S55, S56). Nevertheless, all three bichromophoric complexes exhibit increased photoactivity, highlighting the success of this design strategy. Photocatalytic Oxidation of 1,5-Dihydroxy-Naphthalene. The pyrene-substituted copper(I) complexes exhibit a promising activity to generate 1 O 2 and an excellent photostability, making them attractive for other visible-light photocatalytic applications. 5-Hydroxy-1,4-naphthoquinone, commonly known as juglone, has been demonstrated to have interesting anticancer properties. 118 Juglone can be derived from 1,5-dihydroxynaphthalene (DHN) by an oxidation reaction utilizing 1 O 2 (Figure 7 bottom). 78,96 The oxidative conversion of DHN to juglone represents a significant increase in value (∼$1/g vs ∼$300/g). Therefore, we aimed to investigate the novel photosensitizers CuPyr1−3 for the photooxidation of DHN in acetonitrile solution. Main attention was given to the kinetics of the photocatalytic reaction and the estimation of the final yields of juglone. The progress of the photooxidation of DHN was monitored by in situ UV/vis spectroscopy by measuring the changes in the absorption (Figures S59−S62) of DHN at 301 nm and the formation of juglone at 427 nm, as described in previous studies. 94 −96 The measurements (Figure 7) indicate that the catalytic process proceeds via a pseudo first-order reaction, which is in agreement with earlier studies. 78,95,96 All pyrenebased complexes display larger rate constants k obs compared to both the reference system (CuNeo alone) and a physical mixture of CuNeo and pyrene (Table 3). Among the complexes, CuPyr1 has a rate constant that is up to ten times larger than CuNeo. The yield of juglone obtained with the most efficient copper(I) photosensitizer, CuPyr1 (85%) is comparable to those achieved with noble metal-based iridium(III) complexes (80−99%). 78 The observed rate constant for CuPyr1 (12.9 × 10 −5 s −1 ) is also in a similar range to those of the precious metal complexes (25−58 × 10 −5 s −1 ). 78 Although an exact comparison is difficult due to many parameters affecting the kinetics, the more earth-abundant copper(I) derivatives represent a viable alternative to noble metal-based photosensitizers.

■ CONCLUSIONS
A trio of different pyrene-substituted phenanthroline-based ligands Pyr1−3 and their respective heteroleptic copper(I) photosensitizers CuPyr1−3 were analyzed concerning two strategies: (i) substitution at different positions on pyrene (i.e., 1-yl vs 2-yl) and (ii) alternation of the position on phenanthroline (i.e., 4,7 vs 5,6). TD-DFT calculations revealed that there is a remarkable difference between the singlet and triplet state of the torsion angles around the newly formed C− C bond Δτ Sub (GS-ES), where the substituents in CuPyr1 are more twisted compared to CuPyr2 and CuPyr3. This already indicates the presence of significant electronic coupling, especially in CuPyr1. This was also demonstrated by electrochemistry, as CuPyr1 showed the most anodically shifted reduction potential among the complexes. In general, the phenanthroline-based reduction potential is determined by the substitution position at the phenanthroline (i.e., 4,7 vs 5,6), irrespective of the substitution pattern at pyrene (1-yl vs 2-yl).  The reduction potential of the pyrene moiety, however, is strongly dictated by the substitution pattern at pyrene. Importantly, all novel complexes gained the ability to reversibly store a total of at least three electrons, while the reference complex CuNeo can only be reduced once. The general observation that for Pyr1 and CuPyr1 the electronic interaction between the two building blocks is most pronounced (i.e., between phenanthroline and pyrene) is reflected in all the spectroscopic techniques applied (UV/vis, emission, time-resolved luminescence, and transient absorption). For instance, the absorption is more intense and most redshifted, the emission intensities are highest, and the final excited 3 LC state is the most long-lived (e.g., Pyr1 16.03 μs and CuPyr1 22.42 μs). Compared to CuNeo (0.20 μs), the excited state lifetime is increased by more than two orders of magnitude. This directly correlates with the highest singlet oxygen quantum yield (96%) and the most beneficial catalytic activity in the photocatalytic oxidation of 1,5-dihydroxynaphthalene to the anti-cancer drug juglone. In this reaction, the observed rate constant is twelve times greater, and the yield four times higher compared to the unsubstituted reference complex CuNeo.
In conclusion, all three bichromophoric copper(I) complexes are highly efficient photosensitizers that can rival with noble metal-based competitors. Especially, the fact that a total of three electrons can be reversibly stored on the ligand scaffold in combination with the high driving force of the underlying reduction potentials (e.g., −2.67 V vs Fc/Fc + ) calls for more demanding catalytic reactions like the dehalogenation of aryl chlorides in the future. Author Contributions † F.D. and Y.Y. have contributed equally to this work and share first authorship. F.D. prepared the ligands and complexes, including their purification and structural characterization. Y.Y. performed and evaluated the electrochemical, photophysical, and photocatalytic experiments. F.D. conducted the (TD)-DFT calculations. F.D., Y.Y., M.K., and S.T. analyzed and discussed the data. F.D. and Y.Y. drafted the manuscript. M.K. and S.T. guided the project and revised the manuscript. All authors approved the final version of the publication.

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
This work was supported by the Deutsche Forschungsgemeinschaft (DFG) within the Priority Program SPP 2102 "Light-controlled reactivity of metal complexes" (TS 330/4-1 and KA 4671/2-1). We thank the RWTH Aachen Compute Cluster (Project No. 4706) for the provision of computing time. We also thank the group of Prof. Christoph Jacob (TU Braunschweig) for access to their compute cluster. Y.Y. is grateful to the TU Braunschweig Equal Opportunity Office (Promotionsabschlussförderung der TU Braunschweig) for funding her PhD.