Polycyclic aromatic hydrocarbon chromophores tune photo-physical properties in bichromophoric Cu(I) photosensitizers

Abstract

A modular design approach for bichromophoric Cu(I) complexes is reported, combining a [Cu(N^N)(P^P)]⁺ scaffold that supports metal-to-ligand charge transfer with two polycyclic aromatic hydrocarbon chromophores covalently attached to the diimine ligand. Three new systems incorporating 9,9-dimethyl-9H-fluorene, phenanthrene, and anthracene units at the 4,7-positions of neocuproine were synthesized and fully characterized. Structural analysis reveals twisted geometries of the attached chromophores, which reduce conjugation with the diimine core. Electrochemical measurements indicate only moderate shifts in reduction potentials, indicating preservation of the Cu(I)-centered redox behavior. Across the series, the visible MLCT band is retained without pronounced red-shifts, while the molar attenuation coefficients increase markedly (×1.7–3.4 vs. the reference), in line with calculated oscillator strengths. Fragment comparisons and computations attribute the additional UV intensity to intraligand charge transfer (9,9-dimethyl-9H-fluorene, phenanthrene) and to vibronically structured π_A* ← π_A transitions (anthracene), consistent with the non-coplanar geometries. Complexes bearing 9,9-dimethyl-9H-fluorene and phenanthrene substituents exhibit yellow emission involving the Cu(I)-based metal-to-ligand charge transfer state (λ_em ≈ 565 nm, τ up to 20.8 μs), whereas the anthracene-substituted system populates a non-emissive ³π–π* triplet state localized on an anthracene moiety (τ > 35 μs), as evidenced by transient absorption spectroscopy and time-dependent density functional theory. These results establish clear structure–property relationships in bichromophoric Cu(I) systems and illustrate how polycyclic aromatic hydrocarbon chromophores influence excited-state character and dynamics.

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Introduction

The class of heteroleptic Cu(I) complexes has experienced a rise in popularity due to their diverse and successful applications in the field of photochemistry, including photoredox catalysis,^1–3 dye-sensitized solar cells (DSSCs),^4–7 light-emitting diodes^8,9 or solar fuels production.^10–16 In comparison to established and commercially available standards such as Ru(II) or Ir(III) complexes,^17–19 the attractiveness of Cu(I)-based photosensitizers (CuPSs) can be attributed to different factors: (i) the significantly lower cost of copper due to its much higher abundance in the Earth's crust (Cu: 27 ppm vs. Ru: 37 × 10⁻⁶ ppm),^20,21 (ii) the d¹⁰ shell, which prevents low-energy metal-centered (MC) states that can cause undesirably rapid excited-state deactivation, and (iii) the potential to outperform their noble-metal competitors.^1,10,22,23 Numerous studies have already demonstrated that noble-metal-free systems for the reduction of water or carbon dioxide can be achieved using a heteroleptic CuPS along with a suitable earth-abundant-metal catalyst (e.g. based on Fe or Mn).^{11,14,24–27} Thus, further research and development of improved CuPSs can contribute to a more sustainable conversion of solar energy.

In general, heteroleptic CuPSs of the type [Cu(N^N)(P^P)]⁺ combine a diimine (N^N) and diphosphine (P^P) ligand that can both be tuned separately. The diimine ligand, e.g. neocuproine (2,9-dimethyl-1,10-phenanthroline, neo), has strong π-accepting properties that allow for the formation of low-lying excited metal-to-ligand charge transfer (MLCT) states.²⁸ Hence, the diimine ligand is typically modified to alter the photophysical properties of the CuPS.^3,29,30 One strategy to improve the relevant MLCT states, i.e. absorptivity and lifetime, is to attach an additional organic chromophore to the diimine to create a bichromophoric system.^31–40 The attachment of the chromophore can be achieved via a single covalent bond with or without a linking group. The variety of linker groups is large and some common examples include alkyl, phenyl, triazole, thiophenyl, or alkynyl moieties.^33,41–44 Moreover, the use of linking groups adds another dimension to the ligand design, because each bridge has its own properties e.g. rigidity and size.^41,44,45 The advantages of combining chromophores can be numerous: (i) increased absorptivity (antenna effect), (ii) potential red-shifted absorption through π-conjugation, (iii) exploitation of high spin–orbit coupling on the sensitizer for efficient triplet generation and low spin–orbit coupling for the stabilization of long-lived triplet states on the organic chromophore, (iv) individual tunability, and (v) prolonged excited-state lifetimes through the reservoir effect.^{34–36,45–47} The latter is particularly attractive as the excited-state lifetime is a crucial factor for photocatalytic applications.^48,49 However, synthesizing and analyzing such bichromophoric systems can be challenging and typically requires an interdisciplinary approach.

In a previous study, Doettinger et al. revealed by steady-state and time-resolved spectroscopy together with quantum-chemical computations that additional pyrene chromophores act as an energy sink for CuPS MLCT states.⁵⁰ This concept enabled efficient singlet oxygen sensitization, with yields depending on the position (4,7 vs. 5,6) where the pyrene was attached to the neo ligand.⁵⁰ In another study, Takeda et al. attached a variety of phenyl, thiophenyl and furan units to the 4,7-positions of neo.¹⁵ Most complexes retained their MLCT behavior, but heterocycles bearing a heteroatom in 2-position displayed altered excited states and reactivities.¹⁵

To expand the catalog of diimines and bichromophoric systems, three new ligands were developed in this study (Fig. 1) in which the 4,7-positions of neo are symmetrically disubstituted with polycyclic aromatic hydrocarbon (PAH) chromophores: 9,9-dimethyl-9H-fluorene (F), phenanthrene (P) and anthracene (A). To suppress potential deactivation channels, e.g. through n–π* states, heteroatoms were avoided in the substituents. According to the El-Sayed rule, these states allow higher crossing rates between systems.^28,51 Furthermore, F, P and A are rigid and modifiable chromophores with well-defined absorption characteristics and sufficient photostability.^52–58 For these reasons, they were used in diverse applications including fluorescence microscopy dyes,⁵⁸ dye lasers,⁵⁹ DSSCs,⁶⁰ two-photon absorption^59,61 and photon upconversion.^54,57,62,63

Fig. 1 Schematic representation of the diimine ligands and their respective heteroleptic Cu(I) complexes investigated in this study. The influence of different PAH chromophores (F = 9,9-dimethyl-9H-fluorene, P = phenanthrene, A = anthracene) on key photophysical and electrochemical properties is assessed. neo = neocuproine, also 2,9-dimethyl-1,10-phenanthroline, bcp = bathocuproine, also 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, nF = 4,7-di-(9,9-dimethyl-9H-fluoren-2-yl)-2,9-dimethyl-1,10-phenanthroline, nP = 2,9-dimethyl-4,7-di(phenanthren-9-yl)-1,10-phenanthroline and nA = 4,7-di(anthracen-2-yl)-2,9-dimethyl-1,10-phenanthroline.

While PAHs have many desirable applications, this work focuses on the synthesis and in-depth characterization of the novel neo derivatives 4,7-bis(9,9-dimethyl-9H-fluoren-2-yl)-neocuproine (nF), 4,7-di(phenanthren-9-yl)-neocuproine (nP), 4,7-di(anthracen-2-yl)-neocuproine (nA) and their respective heteroleptic Cu(I) complexes CunF, CunP, and CunA (Fig. 1, P^P: xantphos, anion: PF₆⁻). As references, neo and bathocuproine (bcp) as well as the complexes Cuneo and Cubcp were selected.

The following sections present structural, electrochemical and spectroscopic analysis supported by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, providing insights into the interplay between the different chromophores.

Results and discussion

Synthesis

The reference ligands neo and bcp were acquired commercially. The syntheses of the three novel ligands nF, nP and nA were conducted via Suzuki–Miyaura cross-coupling reactions^64,65 from the common substrate 4,7-dichloro-2,9-dimethyl-1,10-phenanthroline⁶⁶ and their corresponding boronic acids (for nF: 9,9-dimethyl-9H-fluoren-2-yl-2-boronic acid, for nP: phenanthrene-9-boronic acid, for nA: anthracene-2-boronic acid; further details see SI Chapter 2). The precursor 4,7-dichloro-2,9-dimethyl-1,10-phenanthroline was synthesized on gram-scale as described elsewhere.^65,66 Subsequent cross-coupling reactions were conducted in a THF/water mixture using K₃PO₄ as base (0.5 M aqueous solution). To ensure efficient cross coupling, the XPhos-Pd-G2 precatalyst was employed, which under basic conditions releases a highly reactive XPhos-Pd⁰ species.⁶⁷ Compared to the more commonly used [Pd(PPh₃)₄], the XPhos-Pd-G2 precatalyst is often a more convenient and reliable choice for transformations involving 4,7-dichloro- or 5,6-dibromo-1,10-phenanthroline derivatives, as shown in earlier studies.^23,50,65 The target ligands nF, nP and nA were obtained after column chromatography on basic aluminum oxide in yields of 91%, 77% and 80%, respectively (for further details SI Chapter 2, 3 and 4).

The complexes were synthesized via a well-established two-step, one-pot approach,^10,68,69 in which the xantphos ligand is first coordinated to the Cu(I) precursor [Cu(CH₃CN)₄]PF₆ in deaerated dichloromethane at elevated temperatures (SI Chapter 2). Subsequently, the respective diimine ligand (nF, nP or nA) was added dropwise at 0 °C for at least one hour using an automatic syringe pump. Adding the reagent at low temperature prevents formation of the thermodynamically favored homoleptic [Cu(N^N)₂]PF₆ side product.^3,70–73 Typically, isolation of the complexes proceeds via slow precipitation using n-hexane to afford crystalline solids. However, due to the bulky π-extended chromophores, the complexes synthesized herein formed deep red oils rather than solids upon n-hexane addition. This behavior was also observed for pyrenyl-substituted analogs and is likely caused by competing intermolecular π–π-interactions hindering crystal formation.⁵⁰ Nevertheless, the target complexes CunF, CunP and CunA were successfully obtained in yields of 66%, 59% and 50%, respectively (see SI Chapter 2).

The identities and purities of all ligands and complexes were confirmed by nuclear magnetic resonance (NMR: ¹H, ¹³C{¹H}, DEPT 135, and ³¹P{¹H}, see SI Chapter 3) and electrospray ionization high-resolution mass spectrometry (ESI-HRMS, SI Chapter 4). nP exhibited distinct behavior in the ¹H NMR spectrum, revealing an unexpected pair of singlets for the methyl protons (3.08 ppm, Δ = 0.002 ppm), whereas a single singlet is typically observed. Combined with duplicate signals in the aromatic region of the ¹³C{¹H} and DEPT135 spectra, this strongly suggests the presence of stereoisomers. This phenomenon is also observed for CunP, but is absent in all other ligands or complexes. However, due to the minor structural differences and the absence of distinctive spectroscopic or electrochemical effects, the presence of these isomers was considered negligible for further discussions.

Structural characterization

To elucidate the main structural similarities and differences between the designed complexes, DFT calculations were performed and the predicted molecular structures were analyzed. Additionally, single crystals of a solvate of CunA were obtained by slow diffusion of n-hexane into a concentrated dichloromethane/diethyl ether solution at room temperature, enabling a direct comparison between the experimental solid-state structure and the calculated geometry (Table 1, for experimental details: SI Chapter 5). The experimental (Fig. 2) and computed molecular structures (Fig. S5.1 and S5.2) are in good agreement and consistent with previous reports on heteroleptic Cu(I) complexes.^{15,16,29,50,65,74} Cu–N bond lengths (206.5–212.3 pm) are consistently shorter than Cu–P bond lengths (224.3–232.6 pm), and bite angles fall within expected pseudo-tetrahedral geometry ranges (N–Cu–N: 78.3–80.4°; P–Cu–P: 116.5–119.7°). The interplane angles θ between the planes formed by the N–Cu–N and P–Cu–P atoms (83.2–87.7°) are typical for such heteroleptic Cu(I) complexes.^{15,16,29,50,65,75,76}

Fig. 2 Molecular crystal structure of CunA. Nitrogen atoms are depicted in blue, oxygen in red, phosphorus in pink, copper in orange, and hydrogen as well as solvent molecules are omitted for clarity. The carbon atoms in 4,7-position are labeled with C₄ and C₇. Torsion angles ϑ are measured along C₃–C₄–C_Sub1,1–C_Sub1,2 and C₈–C₇–C_Sub2,1–C_Sub2,2. The second substituent C_Sub−2 is on the same side as the stacking phenyl ring.

Table 1 Selected bond lengths (pm), bond angles (°), N–Cu–N and P–Cu–P interplane angle θ (°) and torsion angles ϑ (°) from DFT (calc.) and X-ray (exp.) data for all complexes. The C_neo–C_sub bond lengths describe the bond length between the respective substituent and the C₄/C₇ of neo. Torsion angles ϑ_sub1 and ϑ_sub2 express torsion angles along the substituent linkages from C₃–C₄–C_Sub1,1–C_Sub1,2 and C₈–C₇–C_Sub2,1–C_Sub2,2, where the second substituent carbon atom is on the same side as the stacking phenyl ring. Cubcp and CunA were analyzed both experimentally and computationally for validation (experimental data for Cubcp taken from ref. 16). Standard deviations are given in parentheses. Calculations at PBE0/def2-TZVP level (CPCM: CH₂Cl₂)

		Cuneocalc.	Cubcpexp.	Cubcpcalc.	CunFcalc.	CunPcalc.	CunAexp.	CunAcalc.
Cu–N1	(pm)	210.9	207.26(19)	210.3	210.7	211.5	210.62(33)	211.5
Cu–N2		212.3	209.63(19)	211.4	211.1	211.1	211.14(38)	210.3
Cu–P1		226.3	223.99(6)	226.0	226.3	226.7	226.29(13)	225.5
Cu–P2		229.6	230.19(6)	229.9	229.2	229.1	231.68(12)	232.6
C4–Csub1		—	147.5(3)	147.6	147.4	148.1	149.16(61)	147.3
C7–Csub2		—	148.4(3)	147.6	147.5	148.1	149.77(65)	147.4
N–Cu–N	(°)	79.7	80.41(7)	79.2	79.2	79.4	78.253(142)	79.0
P–Cu–P		118.5	119.95(2)	118.7	118.8	119.0	116.789(47)	116.5
θ		85.4	85.280(6)	85.2	86.7	87.7	86.695(99)	83.8
ϑsub1		—	−129.414(246)	−122.3	−125.6	−82.4	50.330(613)	−126.5
ϑsub2		—	124.560(278)	55.0	55.1	104.6	50.183(618)	127.1

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In general, the geometry around the central Cu(I) atom is largely unaffected by the substituents, which is reasonable as the 4,7-positions are para to the copper-binding nitrogens. The C₄–C_Sub1,1 and C₇–C_Sub2,1 bond lengths (Table 1) confirm a carbon–carbon C–C single bond, with average values of 149.2 pm (exp.) and 147.7 pm (calc.). The observed torsion angles ϑ_sub1,2 along atoms C₃–C₄–C_Sub1,1–C_Sub1,2 and C₈–C₇–C_Sub2,1–C_Sub2,2 resemble those of biphenyl-like systems (ϑ_solid = 0–10°, ϑ_vapor = 44.4 ± 1.2°, θ_solute = 30–40°), where conjugation (ϑ ≈ 0°) and steric hindrance (ϑ ≈ 90°) compete.^77–80 For the present series, the effective torsion between the neo plane and the substituent plane is 50.2–57.7° for Cubcp, CunF, and CunA, i.e. moderate twisting and partial conjugation limited by steric hindrance. This is primarily due to the repulsion between the ortho-hydrogens and the 5,6-hydrogens of the neo backbone, as also noted by Takeda et al.⁸¹

In contrast, with ϑ_sub1,2 = 82.4 and 75.4° the phenanthrene units in CunP are nearly perpendicular to the diimine plane. This can be explained by the repulsion between the ortho-hydrogen and the adjacent methine bridge with the 5,6-hydrogen atoms of neo. As a result, through-bond electronic communication between neo and phenanthrene in CunP is significantly reduced, because the π-systems and p-orbitals cannot effectively overlap. Overall, torsion increases in the following order: Cubcp ≈ CunF ≈ CunA (50.2–57.7°) < CunP (75.4–82.4°) and the expected π-communication follows: Cubcp ≈ CunF ≈ CunA > CunP.

Molecular orbital trends

To better understand the electronic communication and redox behavior, the molecular orbitals (MO) of the Cu(I) complexes (Fig. 3 and SI Chapter 6) were investigated (PBE0/def2-TZVP, CPCM: dichloromethane). The frontier molecular orbitals of Cuneo and Cubcp (Fig. S6.12 and S6.14) serve as reference systems: their highest occupied molecular orbital (HOMO) is predominantly d_Cu- and p_P-based, and the lowest unoccupied molecular orbital (LUMO) corresponds to a π_neo* orbital on the neo ligand, consistent with literature.^{73,75,76,82–84}

Fig. 3 Molecular orbital energy diagram calculated with PBE0/def2-TZVP. Energy differences are given relative to the Cuneo HOMO (−6.15 eV) for the respective molecule columns (in eV). Encircled markers denote the d_Cu-HOMO, diamonds denote the π_neo*-LUMOs. Representative molecular orbitals of the latter are shown on the lefthand side for Cuneo, while the HOMO, LUMO and LUMO+2 of CunA are shown on the righthand side.

The HOMO (SI Chapter 6) is only marginally affected (<0.1 eV) upon 4,7-disubstitution of neo, and spin density analysis of the singly-reduced species (Fig. S6.22) corroborate a neo-centered LUMO. CunP and CunF retain π_neo* LUMOs (Fig. S6.16 and S6.18) comparable in energy and character to the references Cuneo and Cubcp (Fig. 3, S6.12 and S6.14), reflecting minimal substituent contribution to π_neo*-MOs (SI Chapter 6).

Rather than forming one uniform π-system, the MOs of neo and the substituents (phenyl in bcp, phenanthrene in nP and 9,9-dimethyl-9H-fluorene in nF) remain largely separate. This is consistent with the non-coplanar geometries (see above) and an energetic mismatch between the π-systems. The substituent orbitals of the latter are outside the shown frontier orbital window (Fig. 3). These findings highlight the bichromophoric nature of CunF and CunP, where electronic independence of the two chromophores is largely preserved. A similar separation is observed for the ligands (SI Chapter 6). Such behavior has also been observed for related systems containing carbazole, aniline or pyrene substituents.^13,50,85

In contrast, CunA displays a unique MO behavior. Four additional MOs appear within the frontier orbital region, differentiating it from the other complexes (Fig. 3). Their assignment is complicated due to the energetically close MO energies of anthracene and neo as well as the torsion between the anthracene and diimine units, allowing some electronic interaction. The two highest occupied MOs (HOMO, HOMO−1) lie above the d_Cu MO (HOMO−2) and are clearly anthracene-centered (Fig. S6.20). The LUMOs, however, cannot unambiguously be assigned to either neo or A (Fig. S6.20). Thus, CunA differs from the other complexes in possessing notable LUMO stabilization and partial delocalization across both fragments, consistent with its distinct excited-state behavior discussed below.

Electrochemical behavior

Cyclic voltammetry (CV) was performed to evaluate the redox behavior and support the previously discussed MO assignments. For the unsubstituted complex Cuneo (Fig. S7.9), a reversible one-electron reduction is observed at −2.09 V vs. Fc/Fc⁺ in deaerated dimethylformamide (see Table 2).^11,50,69 The localization of the reduction at the neo ligand was confirmed by spin density analysis of the singly reduced species (Fig. S6.22). Oxidative events were not analyzed in detail, as oxidation in heteroleptic Cu(I) complexes typically leads to irreversible cleavage of the Cu–P bond at ≈0.9 V vs. Fc/Fc⁺.^{23,24,86–88}

Table 2 Reduction potentials of the substituents, ligands and complexes measured in deaerated dimethylformamide and referenced against the ferrocene/ferrocenium couple (Fc/Fc⁺). All irreversible events are simply listed at their peak (minimum) current without special notation. Reversible signals are given as the half wave potential (E_red,1/2)

	—	—	F	P	A
Ered/V	—	—	−3.13^b	−2.73	−2.41^b
				−2.91^b

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	neo	bcp	nF	nP	nA
Ered/V	ca. −2.7^a	−2.60	−2.42	−2.46	−2.23^b
			−2.46	−2.53	−2.68
			−2.70	−2.75
			−2.76

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	Cuneo	Cubcp	CunF	CunP	CunA
a No distinct minimum determinable.b reversible.
Ered/V	−2.09^b	−2.01^b	−1.99^b	−2.00^b	−1.94^b
	−2.83	−2.63	−2.47^b	−2.67	−2.25
			−2.86	−2.76	−2.50
			−2.94		−2.70

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The introduction of PAH chromophores at the 4,7-positions of neo leads to a slight anodic shift in the reduction potential (Table 2). For Cubcp, a small shift to −2.01 V (Δ = +80 mV) is observed (Fig. S7.10), in line with the computations (Table S6.11). For CunF and CunP, the reduction waves are shifted by ≈ +100 mV (Table 2 and Fig. S7.11, S7.12) and correspond to the same neo-centered reduction. The computed shift for CunF (+80 mV) matches well with the experiment, while the shift for CunP (+40 mV) is underestimated, likely due to the lack of computed contribution from the phenanthrene unit (Fig. S6.22).

CunA exhibits a more complex reduction behavior, with multiple reductions between −1.9 V and −2.7 V (Fig. S7.13). A comparison with related systems and its ligand nA (Fig. S7.8) suggests that the reduction at −1.94 V corresponds to the neo based reduction (Δ = +150 mV vs. Cuneo). The calculated shift (Δ = +230 mV) is overestimated due to strong orbital contributions from anthracene, but the overall trend is reproduced. Furthermore, CunA has two weaker reversible reductions around −1.8 V and −1.2 V, which are also present in nA (Fig. S7.8), but not in A itself (Fig. S7.3). These features were not predicted by DFT in the present setup and may arise from intermolecular and solvent interactions or side reactions.

In summary, the observed anodic shifts in neo-based reductions correlate with the size of the attached π-system (Δ ≈ +100 mV for F and P, +150 mV for A), but remain modest overall. This supports the concept that the Cu(I) redox core is largely decoupled from the chromophoric PAH substituents. The calculated reduction potentials follow the same trend (see SI Table S6.11). The disagreement with experiment likely arises from the minimal orbital overlap in CunP and significant orbital mixing in CunA (Fig. S6.22).

UV/vis absorption

To explore the bichromophoric character of the novel complexes, UV/vis absorption spectra were recorded. Substitution at the 4,7-positions of neo with PAH chromophores results in only small red-shifts, but a drastic increase in molar attenuation coefficients (Fig. 4 and Table 3). This trend has also been observed for related substitution patterns.^3,12,89

Fig. 4 UV/vis absorption spectra of CunF (top), CunP (middle), and CunA (bottom) compared to their individual fragments. Top: CunF (blue), nF (light blue), neo (black), F (dark blue). Middle: CunP (green), nP (light green), neo (black), P (dark green). Bottom: CunA (red), nA (light red), neo (black), A (dark red). Difference density plots in the top right of the insets show the migration of the electron density from blue (−) to yellow (+) during the MLCT excitations (S₁ ← S₀). An additional anthracene-localized π_A*–π_A excitation (S₈ ← S₀) is shown for CunA.

Table 3 Experimental absorption wavelengths λ_abs,exp (nm), extinction coefficients ε (10³ M⁻¹cm⁻¹), emission wavelengths λ_em,exp (nm), emission quantum yields Φ_em,exp (%), emission lifetime τ_em,exp (μs), transient absorption lifetime τ_tA,exp (μs), compared to calculated fluorescence wavelengths λ_fl,calc (relaxed S₁ → unrelaxed S₀, TDDFT) and phosphorescence wavelengths (relaxed T₁ → unrelaxed S₀, DFT) λ_ph,calc. All measurements were carried out in dichloromethane under inert conditions at 298 K. Computations were performed with implicit dichloromethane. The singlet–triplet energy gap ΔE_S1–T1,calc (eV) and interplane angles (θ, °) of the optimized singlet S₁ and triplet T₁ are given. Optimization of the S₁ state in CunA yielded two different excited states depending on the sophistication of the method (see computational details). The first and lowest one is the ¹MLCT and the second one was a ¹π_A–π_A*. The latter ended up as S₂ after convergence. Experimental data for Cubcp referenced from ref. 92

	λabs/nm (ε/10^3 M^−1 cm^−1)	λem,exp./nm	Φem,exp./%	τem,exp./μs	τtA,exp./μs	λfl,calc./nm	λph,calc./nm	ΔES1–T1,calc./eV	S1 θcalc./°	T1 θcalc./°
Cuneo	276 (47.4), 400 (3.29)	559	27.9	7.04	6.51	599.4	741.9	0.388	70.1	70.4
Cubcp	287(59.8), 400(5.58)	568	38.0 (ref. 92)	8.38	8.04	580.3	756.2	0.404	74.3	71.0
CunF	269(56.6), 290(57.7)	570	44.2	20.78	21.62	576.9	745.5	0.385	73.7	71.6
	325(49.2), 400 (10.3)
CunP	255(143.6), 278(83.2)	564	28.1	9.74	8.82	600.0	752.5	0.384	70.4	70.7
	325 (18.9), 400 (6.32)
CunA	252(114.6), 285(75.8)	442	0.4	n.d.	37.28	608.0/476.5	866.6	0.722/1.036	75.5/82.8	83.8
	335(24.6), 400 (11.1)				174.36

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All complexes exhibit a broad, structureless MLCT band between 350–450 nm, with ε ≈ 3–15 × 10³ M⁻¹ cm⁻¹. This assignment is supported by: (i) previous reports about Cuneo, Cubcp and related complexes,^{29,71,87,90,92} (ii) TDDFT calculations, which consistently predict that the S₁ state is an MLCT state (SI Chapter 6), and (iii) the absence of this band in the ligands (Fig. 4, SI Chapter 6 and Fig. S8.2).

As expected, Cuneo has the weakest MLCT absorption (Table 3), followed by Cubcp.^87,93 CunP exhibits a similar enhancement, suggesting that phenanthrene has a comparable influence to phenyl. In contrast, CunF and CunA both possess significantly higher attenuation coefficients.

TDDFT-derived difference densities confirm for all complexes that the lowest MLCT excitations (states 1–4, except CunA: 1, 4, 7, 11) have negligible substituent contribution (SI Chapter 6), consistent with the minimal red-shift. This indicates that the MLCT transitions are localized on the Cu(I)/diimine core, electronically decoupled from the substituents.^15,81 A comparable trend can be seen for the oscillator strength of the S₁ ← S₀ transition, suggesting that enhanced absorptivity stems from the elongated diimine system and a larger transition dipole moment.⁷⁵

In the UV region (250–350 nm), the absorption is dominated by π* ← π transitions within the diimine ligand and the substituents.^{29,71,87,90,91} Cuneo and Cubcp exhibit typical ligand-centered π_neo* ← π_neo transitions, corroborated by TDDFT (Fig. S6.2, S6.4, S6.13 and S6.15) and fragment spectrum (Fig. S8.4). Additional excitations involving xantphos and nonbonding MOs are present, but not discussed further.

Introducing PAH chromophores significantly alters the band shape and increases absorption (Fig. 4). A new shoulder at 300–350 nm appears experimentally (Fig. 4 and S8.5–8.7) and computationally (SI Chapter 6).

For nP and CunP, TDDFT indicates intraligand charge transfer (ILCT) from P to neo (Fig. S6.8 transition 2 and Fig. S6.19 transitions 5,6) and an enhancement of the weak S₁ ← S₀ transition in P and neo (Fig. S6.8 state 1 vs. S6.2 state 3). The MLCT transitions remain centered on the core complex, supporting an electronic decoupling of the two chromophores.

A similar observation is made for nF and CunF: between 250–350 nm, strong deviations from the fragments F and neo occur (Fig. 4 or S8.5). TDDFT reveals enhanced π_neo* ← π_neo transitions (Fig. S6.6 states 1,2) and strong ILCT transitions from F to neo (π_neo* ← π_F, see Fig. S6.17 states 5–8,10). The bands at ≈290 and ≈310 nm in CunF and nF likely reflect fluorene-specific transitions, though an unambiguous assignment is not possible. Electronic communication between neo and F is somewhat stronger than in CunP, but a continuous π-system is still not formed.

For CunA, a subtle but distinct set of features is found: the MLCT band has two overlapping maxima at 385 and 366 nm. These are also present in nA and are attributed to vibronically structured anthracene-centered π_A* ← π_A transitions (Fig. 4 and S6.10 states 1,2). Additional transitions near the MLCT region display mixed local (π_A* ← π_A) and charge transfer (π_neo* ← π_A) character (S6.21 states 2, 3, 5, 6). Similar to the other systems, no significant red-shift of the MLCT band is observed, indicating negligible anthracene participation in the MLCT excitations. In the 300–350 nm range, however, CunA and nA display similar ILCT/π–π* patterns, suggesting an electronic communication between neo and A comparable to CunF.

Emission behavior

The ligands neo, bcp, nF, nP and nA emit broadly in the 350 to 500 nm range (Fig. S9.1–3) and their quantum yields (vs. pyrene⁹³) follow the order: nA (428 nm, ϕ_em = 46.4%) > nF (396 nm; ϕ_em = 29.6%) > nP (379 nm; ϕ_em = 6.2%) > bcp (385 nm; ϕ_em = 4.6%) > neo (364 nm; ϕ_em = 0.7%).

Upon excitation of the MLCT band at 390 nm, all complexes except CunA display yellow MLCT emission with a typically broad and featureless peak around 565 nm in deaerated dichloromethane (Fig. 5) as has been reported for Cuneo⁸⁷ and Cubcp.^11,29,76 CunF and CunP possess nearly identical emission wavelengths and band shapes compared to the references, differing mainly in emission intensity and quantum yield (referenced vs. Cubcp).⁹² The quantum yield ϕ_em order is: CunF (44.2%) > Cubcp (38.0%⁹²) > CunP (28.1%) ≈ Cuneo (27.9%, Table 3). A plausible explanation for this order combines different torsions (reduced overlap for CunP) with increased substituent rigidity in CunF (fluorene bridge), which disfavors non-radiative decay. Moreover, CunF exceeds Cubcp possibly due to the additional phenyl ring, which is forced in plane by the dimethylmethylene bridge.

Fig. 5 Emission spectra of Cuneo (black), Cubcp (gray), CunF (blue), CunP (green) and CunA (red) at excitation wavelength of 390 nm in deaerated dichloromethane. Samples were excited at an optical density of 0.1.

Time-resolved emission (SI Chapter 10) gives lifetimes of 8.38 µs for Cubcp (cf. 7.24 µs, Forero-Cortés et al.⁹²), 7.08 µs for Cuneo, 9.70 µs for CunP, indicating a negligible effect, and a significantly longer lifetime of 20.78 µs for CunF. Nevertheless, it is assumed that the final emissive state has MLCT characteristics, because the emission wavelength and shape are comparable to the references. Furthermore, computations reinforce a ¹MLCT assignment (Table 3): (i) TDDFT-optimized S₁ states converged to the flattened ¹MLCT state with fluorescence wavelengths closely matching experimentally observed values (Table 3); (ii) DFT-optimized T₁ states also converged to the flattened MLCT state, but the calculated phosphorescence wavelengths are strongly red-shifted due to underestimated triplet energies, which can be seen in the large calculated singlet–triplet gaps ΔE_S1–T1 (>0.3 eV, Table 3).^{3,29,87,91,94,95} Lastly, both singlet S₁ and triplet T₁ geometries demonstrate the characteristic MLCT flattening, where the interplane angle θ decreases from ≈86° to ≈72° on average (Table S5.2 and S5.3).²⁹

In CunA the ¹MLCT emission is absent and only a very weak band at 442 nm (Φ_em < 1%) attributed to the nA ligand is detected under deaerated conditions. TDDFT indicates two closely spaced singlet manifolds, a d_Cu–π_neo* ¹MLCT state and an anthracene-localized ¹π_A–π_A* state, whereby both can be obtained. The MLCT state yielded a theoretical emission wavelength of 608.0 nm with similar flattening of θ, while the anthracene-localized state yielded 476.5 nm with no geometric distortions to the Cu(I) core. Triplet–state ΔSCF optimizations reveal an anthracene-centered ³π_A–π_A* level lying well below the ³MLCT manifold, creating an energetically favorable, non-emissive energy sink. This enables efficient triplet–triplet energy transfer from the ³MLCT state, thereby quenching the ¹MLCT emission and explaining its absence in CunA. Alternatively, the closely-spaced ¹π_A–π_A* state may benefit from increased spin–orbit-coupling because of the Cu(I) chromophore to populate ³π_A–π_A* levels more easily.

To sum it up, the emission data establish a clear structure–property relationship: 9,9-dimethyl-9H-fluorene and phenanthrene substitution retain bright, long-lived MLCT emission, whereas anthracene diverts excitation into a low-lying, non-radiative triplet. This provides a practical design guideline: choose F/P substituents to retain a long-lived MLCT excited state with high radiative yield, and A when efficient triplet generation/storage is sought.

Photostability and transient absorption spectroscopy

Prolonged irradiation with an intense, UV-rich light source (e.g. a 150 W Xe arc) facilitates a slow, steady photodecomposition (Fig. S12.1–S12.10). Both complexes and ligands in dichloromethane yield the same emissive photoproducts (Fig. S12.11, λ_em = 488 nm for Cuneo; 444 nm for Cubcp; 508 nm for CunF; 520 nm for CunP; 617 nm for CunA). This suggests transformations within the diimine ligand rather than the Cu/xantphos fragment. Two non-exclusive hypotheses are consistent with literature: (i) light-induced dimerization/oligomerization enlarging the π-system (reported for bcp dimers with green fluorescence),^96,97 or (ii) reactions with radicals generated from chlorinated solvents.^98,99

The transient absorption (tA, 355 nm pump) of Cuneo and Cubcp are characterized by very broad peaks around 521 and 534 nm (Fig. 6) with tA lifetimes (τ_tA) of 6.51 μs and 8.04 μs, respectively. These lifetimes are monoexponential and comparable to the emissive lifetime. The tA spectrum has been attributed to the triplet–triplet absorption of the ³MLCT state, which matches the spin density of the lowest computed triplet (Fig. S6.23) and the computed triplet–triplet absorption spectrum (Fig. S6.24).^16,65 The band at about 525 nm is characterized by two excitation types, namely one local π_neo* ← π_neo transition and ligand-to-metal charge transfers reducing the formal Cu(II) center (Fig. S6.24).

The PAH chromophore F could not be excited with 355 nm, but according to literature data of its derivative, 9H-fluorene has one sharp characteristic triplet–triplet absorption peak at around 375–390 nm.¹⁰⁰ However, the tA spectrum of CunF (Fig. 6) displays a broad maximum at 645 nm with a shoulder at 460 nm, which do not match the fluorene signature. Instead, the computed triplet–triplet spectrum of the ³MLCT state fits the spectral signature (Fig. S6.25). Compared to the reference complexes, the ³MLCT tA lifetime is effectively doubled to 21.62 μs with a new spectral signature.

Fig. 6 Nanosecond transient absorption spectra (top) of Cuneo (black), Cubcp (gray), CunF (blue), CunP (green) and CunA (red) in deaerated dichloromethane approximately 10 ns after excitation at 355 nm (absorbance of ≈0.3) with a detection gate-width of 3 μs. The decay kinetics (bottom) have been recorded at the transient absorption maxima: Cuneo (black, 521 nm), Cubcp (gray, 530 nm), CunF (blue, 645 nm), CunP (green, 505 nm) and CunA (red, 436 nm). See Fig S11.2 for full details on CunA. All complexes with the exception of CunA display a broad featureless transient spectrum that is attributed to the ³MLCT absorption and a representative spin density of the ³MLCT state of CunF is shown. CunA exhibits a sharp peak with a shoulder at 415 and 437 nm that is attributed to the absorption of the ³π_A–π_A* state of A as also shown by the spin density in the top left.¹⁰⁰

The tA spectroscopy of P reveals two distinct peaks at 460 and 492 nm with a shoulder at 432 nm¹⁰⁰ (Fig. S11.1). CunP (τ_tA = 8.82 μs), however, does not display these peaks and exhibits a similar spectral signature (Fig. 6) as Cuneo and Cubcp with one broad feature at ≈508 nm and another at around 700 nm. This, together with the matching computed ³MLCT-triplet absorption (SI Fig. S6.25), suggests that the triplet state of P is not populated. This is reasonable as the ³MLCT is energetically lower than the triplet states of P and F.^101–103

A also possesses two sharp characteristic triplet–triplet absorption bands at roughly 404 and 428 nm.¹⁰⁰ In contrast to both CunF and CunP, these characteristic peaks can also be found slightly shifted within CunA at 415 nm and 437 nm (Fig. 6 and S11.2). Furthermore, the tA spectrum of CunA agrees with the computed triplet–triplet absorption spectrum of the ³π_A–π_A* state (Fig. S6.26). This differentiates the PAH chromophores F and P from A, which efficiently funnels the excited state population into the low-lying anthracene-based ³π_A–π_A* T₁ state leading to vastly different photophysical properties. This is further evidenced in the significantly longer biexponential excited state lifetime (τ_tA = 37.28 and 174.36 μs).

In agreement with tA and the aforementioned findings (i.e. HOMO–LUMO gaps, electrochemistry, absorption and emission spectra) an energy level diagram of the novel complexes is proposed in Fig. S13.1 to illustrate possible photophysical decay pathways. To summarize, the lowest excited state in the four systems Cuneo, Cubcp, CunF and CunP has been identified to be of ³MLCT character. From a purely energetic perspective, the ³π–π* states of the substituents phenyl, F and P lie too high in energy in order to function as a triplet reservoir. Despite the latter, a significant extension of the lifetime has been observed for CunF which is about twice as large as in Cubcp, whereas that of CunP stayed similar to Cuneo. It is likely that this difference is attributed to geometric differences (e.g. more favorable dihedral angles). CunA on the other hand possesses very low-lying ³π_A–π_A* states that act as an energy sink, so that a repopulation of the ³MLCT state is no longer possible.

Conclusions and outlook

In this study, three novel bichromophoric Cu(I) complexes CunF, CunP, and CunA were successfully synthesized and fully characterized. The diimine ligands are based on neocuproine (neo) and selectively functionalized at the 4,7-positions with polycyclic aromatic hydrocarbon chromophores – 9,9-dimethyl-9H-fluorene, phenanthrene, and anthracene. Structural analysis by X-ray diffraction (CunA) and DFT optimization confirms pseudo-tetrahedral geometries around the copper center. Due to steric hindrance, the attached organic chromophores adopt twisted, non-coplanar conformations relative to the diimine core. Electrochemical and spectroscopic analyses, supported by (TD-)DFT computations, evidence that the MLCT characteristics of the Cu(I) scaffold are largely retained for CunF and CunP, while CunA exhibits pronounced electronic mixing in the excited-state manifold. All complexes display strong MLCT absorption in the visible range with significantly increased molar attenuation coefficients compared to the references (Cuneo and Cubcp). CunF and CunP exhibit MLCT-based emission in the yellow region (λ_em ≈ 565 nm) with quantum yields of 44.2% and 28.1%, respectively, and excited-state lifetimes up to 20.8 μs (CunF). In strong contrast, CunA has no detectable MLCT emission. Instead, a non-emissive, anthracene-localized ³π_A–π_A* state is populated, as confirmed by transient absorption spectroscopy and supported by TDDFT. Cyclic voltammetry revealed only moderate anodic shifts in the first reduction upon substitution with PAH chromophores. This indicates that the LUMO, at least for CunF and CunP, remains neo-centered and that the ground-state redox properties are largely preserved.

Three concise conclusions emerge: (i) structural torsion limits conjugation and preserves the Cu(I) redox core while increasing absorption strength; (ii) CunF/CunP retain MLCT excited states with long lifetimes, (iii) anthracene redirects the excited state into a low-lying, dark ³π_A–π_A* state that suppresses MLCT emission. Thus, substitution with PAH chromophores enables systematic tuning of absorption properties and excited-state dynamics without fundamentally altering the redox behavior, if the correct chromophore is chosen. CunF combines strong absorption, long lifetime and favorable redox potential, making it a promising candidate for photoredox catalysis. Similarly, CunP's excited-state and redox properties compare directly to Cuneo and Cubcp, but with significantly stronger UV absorption. The unique photophysical behavior of CunA enabled by a very long-lived locally excited triplet state opens up perspectives for its application in energy transfer catalysis and photon upconversion, e.g. via triplet–triplet annihilation (TTA).

In future studies, both directions will be explored. Moreover, the modular approach of combining Cu(I)-MLCT chromophores with rigid, non-coplanar π-systems may be extended to other platforms, offering a versatile strategy to design photoactive metal complexes with tailored excited-state landscapes.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional experimental, synthetic, computational, spectroscopic and electrochemical details. See DOI: https://doi.org/10.1039/d5qi02274f.

CCDC 2491672 (CunA) contains the supplementary crystallographic data for this paper.¹⁰⁴

Acknowledgements

We are grateful to Cengiz Özmen (TU Braunschweig) for help with solvent purification. L. E. B. is thankful for a doctoral scholarship by the Deutsche Bundesstiftung Umwelt (DBU, AZ: 20024/029). J. P. Z. expresses gratitude to the Cusanuswerk e.V. for a doctoral scholarship.

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Footnote

† These authors share the first authorship of this paper.

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