How the Way a Naphthalimide Unit is Implemented Affects the Photophysical and -catalytic Properties of Cu(I) Photosensitizers

Abstract

Driven by the great potential of solar energy conversion this study comprises the evaluation and comparison of two different design approaches for the improvement of copper based photosensitizers. In particular, the distinction between the effects of a covalently linked and a directly fused naphthalimide unit was assessed. For this purpose, the two heteroleptic Cu(I) complexes CuNIphen (NIphen = 5-(1,8-naphthalimide)-1,10-phenanthroline) and Cubiipo (biipo = 16H-benzo-[4′,5′]-isoquinolino-[2′,1′,:1,2]-imidazo-[4,5-f]-[1,10]-phenanthroline-16-one) were prepared and compared with the novel unsubstituted reference compound Cuphen (phen = 1,10-phenanthroline). Beside a comprehensive structural characterization, including two-dimensional nuclear magnetic resonance spectroscopy and X-ray analysis, a combination of electrochemistry, steady-state and time-resolved spectroscopy was used to determine the electrochemical and photophysical properties in detail. The nature of the excited states was further examined by (time-dependent) density functional theory (TD-DFT) calculations. It was found that CuNIphen exhibits a greatly enhanced absorption in the visible and a strong dependency of the excited state lifetimes on the chosen solvent. For example, the lifetime of CuNIphen extends from 0.37 µs in CH2Cl2 to 19.24 µs in MeCN, while it decreases from 128.39 to 2.6 µs in Cubiipo. Furthermore, CuNIphen has an exceptional photostability, allowing for an efficient and repetitive production of singlet oxygen with quantum yields of about 32%.

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

“…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. , This solvent effect in our pyrene-based systems differs from other results regarding the solvent-tuning effect of MLCT triplet energies. , In these studies, both MLCT emission and solvatochromic behavior of the MLCT states were observed. , Even a switch between both the excited state could be achieved when changing the solvent environment . In contrast, the small impact on the emission wavelength in CuNeo (only 4 nm, Table ) 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.…”

Rich or poor: the impact of electron donation and withdrawal on the photophysical and photocatalytic properties of copper(i) complexes

“…52,53 Hence, the catalytic generation of 1 O 2 can be accessed by measuring the NIR emission of 1 O 2 in reference to phenalenone as a universal standard. 85–89 From previous studies on related copper compounds, it is known that the singlet oxygen quantum yield in acetonitrile is usually comparatively low, 45,46 which is also the case here. The values were therefore determined in dichloromethane instead (see Table 3).…”

“…1) came into focus due to their long excited state lifetimes, beneficial excited state potentials and broad tunability. 6,17,31,37,38,40 Besides rather strong variations of the ligand scaffold, by fusing additional polyaromatic rings at different positions of the phenanthroline moiety, or by attaching an additional chromophore to create bichromophoric systems, [41][42][43][44][45][46] the impact of comparably small changes with respect to electron donating 47,48 or withdrawing units [47][48][49][50] has only scarcely been studied. This, however, provides detailed insights into the structure-property relationships with presumably predictable effects on the photophysical properties and photocatalytic activity of the envisioned photosensitizer.…”

Photoactive Copper Complexes: Properties and Applications

“…They were shown to display acceptable levels of activity for the reductive debromination of an alkyl bromide but negligible activity in PCET or ET processes. The influence of the incorporation of a naphthalimide unit to the diimine ligand in [Cu­(phen)­(Xantphos)]­PF 6 ([Cu­( NN 1 )­( PP 4 )]­PF 6 ) was reported and revealed the importance of the way this unit is incorporated (covalently linked or fused) . Upon covalent linking, a greatly enhanced absorption in the visible, a high photostability, and a strong dependency of the excited state lifetimes on the chosen solvent were noted.…”