The photophysical properties of transition metal complexes have long attracted interest in the literature with significant research activity during the past two to three decades due to the potential ...exploitation of these materials in solar energy conversion, light-emitting technology, luminescence biological imaging and photodynamic therapeutic applications to name but a few. Since the advent of the facile preparation of 1,2,3-triazole-based compounds through copper(
i
)-catalysed cycloaddition, ligands based on this heterocycle have received widespread attention in coordination chemistry. Inevitably, their ability to be used as pyridine-like analogues has resulted in significant attention on the photophysical properties of their resultant complexes. There are, however, two sides to this tale; on the one hand, routes to 1,2,3-triazoles have enabled the realisation of highly tunable and efficient phosphors and photosensitisers. On the other hand, 1,2,3-triazole-based complexes have allowed highly novel photochemical processes to be explored offering insights into hitherto unappreciated excited state dynamics. This Perspective review covers the developments of photophysically active triazole-based complexes over the last decade, highlighting some of the key discoveries from our own laboratory as well as seminal contributions from other groups who are active in the area. We also identify possible new avenues for investigation and exploitation stemming from the work so far.
Complexes containing the humble 1,2,3-triazole ring moiety have enabled access to highly tunable with efficient phosphorescence but have also in facilitated access to novel photoreactive excited states yielding highly unusual photochemical reactivity.
The sensitivity of both nuclear magnetic resonance spectroscopy and magnetic resonance imaging is very low because the detected signal strength depends on the small population difference between spin ...states even in high magnetic fields. Hyperpolarization methods can be used to increase this difference and thereby enhance signal strength. This has been achieved previously by incorporating the molecular spin singlet para-hydrogen into hydrogenation reaction products. We show here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800-fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation. These polarized signals can be selectively detected when combined with methods that suppress background signals.
Spotlight collection on photoinduced redox chemistry Elliott, Paul I. P; Heinze, Katja; Teets, Thomas S
Dalton transactions : an international journal of inorganic chemistry,
07/2023, Letnik:
52, Številka:
27
Journal Article
Recenzirano
Guest Editors Paul I. Elliott, Katja Heinze and Thomas Teets introduce the spotlight collection, "Photoinduced redox chemistry".
Cellular uptake, luminescence imaging and antimicrobial activity against clinically relevant methicillin-resistant
S. aureus
(MRSA) bacteria are reported. The osmium(
ii
) complexes Os(
N
^
N
)
3
2+
...(
N
^
N
= 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (
1
2+
); 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (
2
2+
); 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (
3
2+
)) were prepared and isolated as the chloride salts of their meridional and facial isomers. The complexes display prominent spin-forbidden ground state to triplet metal-to-ligand charge transfer (
3
MLCT) state absorption bands enabling excitation as low as 600 nm for
fac
/
mer
-
3
2+
and observation of emission in aqueous solution in the deep-red/near-IR regions of the spectrum. Cellular uptake studies within MRSA cells show antimicrobial activity for
1
2+
and
2
2+
with greater toxicity for the meridional isomers in each case and
mer
-
1
2+
showing the greatest potency (32 μg mL
−1
in defined minimal media). Super-resolution imaging experiments demonstrate binding of
mer
- and
fac
-
1
2+
to bacterial DNA with high Pearson's colocalisation coefficients (up to 0.95 using DAPI). Phototoxicity studies showed the complexes exhibited a higher antimicrobial activity upon irradiation with light.
Cellular uptake, luminescence imaging and antimicrobial activity of facial and meridional isomers of Os(
ii
) triazole-based complexes against methicillin-resistant
S. aureus
, MRSA.
Multitarget synthetic strategies to access novel photochromic 3H-naphtho2,1-bpyrans decorated with pyridyl units are described. The new pyridyl-substituted 3H-naphtho2,1-bpyrans display good ...photochromic properties with reversible generation of photomerocyanines, which exhibit mainly orange/red hues. Photochromic parameters including photocolorability and persistence of color vary tremendously on structural modification of the naphthopyran core.
Fundamental insights into the mechanism of triplet-excited-state interligand energy transfer dynamics and the origin of dual emission for phosphorescent iridium(III) complexes are presented. The ...complexes Ir(C∧N)2(N∧N)+ (HC∧N = 2-phenylpyridine (1a–c), 2-(2,4-difluorophenyl)pyridine (2a–c), 1-benzyl-4-phenyl-1,2,3-triazole (3a–c); N∧N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room-temperature fluid solutions from triplet metal-to-ligand charge transfer (3MLCT) states admixed with either ligand-centered (3LC) (1a, 2a, and 2b) or ligand-to-ligand charge transfer (3LL′CT) character (1c, 2c, and 3a–c). Particularly striking is the observation that pyrimidine-based complex 1b exhibits dual emission from both 3MLCT/3LC and 3MLCT/3LL′CT states. At 77 K, the 3MLCT/3LL′CT component is lost from the photoluminescence spectra of 1b, with emission exclusively arising from its 3MLCT/3LC state, while for 2c switching from 3MLCT/3LL′CT- to 3MLCT/3LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of 3MLCT/3LC states for 1a, 2a, and 2b which persist throughout the 3 ns time frame of the experiment. These 3MLCT/3LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their 3MLCT/3LL′CT states. Transient data for 1b reveals intermediate behavior: the spectral features of the initially populated 3MLCT/3LC state also undergo rapid evolution, although to a lesser extent than that observed for 1c and 2c, behavior assigned to the equilibration of the 3MLCT/3LC and 3MLCT/3LL′CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both 3MLCT/3LC and 3MLCT/3LL′CT states of 1a–c and 2a–c. Indeed, two distinct 3MLCT/3LC minima were optimized for 1a, 1b, 2a, and 2b distinguished by upon which of the two C∧N ligands the excited electron resides. The 3MLCT/3LC and 3MLCT/3LL′CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the 3MLCT/3LC and 3MLCT/3LL′CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimized excited-state geometries enable the key structural differences between the 3MLCT/3LC and 3MLCT/3LL′CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Furthermore, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.
Diimine metal complexes have significant relevance in the development of photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) applications. In particular, complexes of the TAP ligand ...(1,4,5,8-tetraazaphenanthrene) are known to lead to photoinduced oxidation of DNA, while TAP- and triazole-based complexes are also known to undergo photochemical ligand release processes relevant to PACT. The photophysical and photochemical properties of heteroleptic complexes Ru(TAP) n (btz)3–n 2+ (btz = 1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl, n = 1 (1), 2 (2)) have been explored. Upon irradiation in acetonitrile, 1 displays analogous photochemistry to that previously observed for Ru(bpy)(btz)22+ (bpy = 2,2′-bipyridyl) and generates trans-Ru(TAP)(btz)(NCMe)22+ (5), which has been crystallographically characterized, with the observation of the ligand-loss intermediate trans-Ru(TAP)(κ2-btz)(κ1-btz)(NCMe)2+ (4). Complex 2 displays more complicated photochemical behavior with not only preferential photorelease of btz to form cis-Ru(TAP)2(NCMe)22+ (6) but also competitive photorelease of TAP to form 5. Free TAP is then taken up by 6 to form Ru(TAP)32+ (3) with the proportion of 5 and 3 observed to progressively increase during prolonged photolysis. Data suggest a complex set of reversible photochemical ligand scrambling processes in which 2 and 3 are interconverted. Computational DFT calculations have enabled optimization of geometries of the pro-trans 3MC cis states with repelled btz or TAP ligands crucial for the formation of 5 from 1 and 2, respectively, lending weight to recent evidence that such 3MC cis states play an important mechanistic role in the rich photoreactivity of Ru(II) diimine complexes.
Re(
i
) complexes bearing thermally reversible photochromic naphthopyran axial ligands undergo highly efficient, reversible phosphorescence quenching actuated by either visible or UV irradiation. The ...photoinduced quenching of the triplet metal-to-ligand charge-transfer (
3
MLCT) emission is interpreted based on changes in the relative energies of the excited states.
UV or visible light induced cycling of the photochromic pyridylnaphthopyran ligand induces reversible switching of the Re(
i
)-centred phosphorescence.
In a systematic survey of luminescent bis(terdentate) osmium(II) complexes, a tipping point involving a reversal in photophysical tuning is observed whereby increasing stabilization of the ...ligand-based lowest unoccupied molecular orbital (LUMO) results in a blue shift in the optical absorption and emission bands. The complexes Os(N^N′^N″)22+ N^N′^N″ = 2,6-bis(1-phenyl-1,2,3-triazol-4-yl)pyridine (Os1), 2,6-bis(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os2), 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2′-bipyridyl (Os3), 2-(pyrid-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os4), 2-(pyrazin-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyridine (Os5), and 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2′-bipyrazinyl (Os6) have been prepared and characterized, and all complexes display phosphorescence ranging from the orange to near-IR regions of the spectrum. Replacement of the central pyridine in the ligands of Os1 by the more π-accepting pyrazine in Os2 results in a 55 nm red shift in the triplet metal-to-ligand charge-transfer-based emission band, while a larger red shift of 107 nm is observed for the replacement of one of the triazole donors in the ligands of Os1 by a second pyridine ring in Os3 (λem max = 702 nm). Interestingly, replacement of the central pyridine ring in the ligands of Os3 by pyrazine (Os4, λem max = 702 nm) fails to result in a further red shift in the emission band. Reversal of the relative positions of the pyridine and pyrazine donors in Os5 (λem max = 733 nm) compared to Os4 does indeed result in the expected red shift in the emission with respect to that for Os3 based on the increased π-acceptor character of the ligands present. However, an inversion in emission tuning is observed for Os6, in which the incorporation of a second pyrazine donor in the ligand architecture results in a blue shift in the optical absorption and emission maxima (λem max = 710 nm). Electrochemical studies reveal that while incorporating pyrazine in the ligands indeed results in an expected anodic shift in the first reduction potential through stabilization of the ligand-based LUMO, there is also a concomitant anodic shift in the OsII/OsIII-based oxidation potential. This stabilization of the metal-based highest occupied molecular orbital (HOMO) thus nullifies the effect of stabilization of the LUMO in Os4 compared to Os3, resulting in these complexes having coincident emission maxima. For Os6, stabilization of the HOMO through the incorporation of two pyrazine donors in the ligand structure now exceeds stabilization of the LUMO, resulting in a larger HOMO–LUMO gap and a counterintuitive blue shift in the optical properties in comparison with those of Os5. While it is known that the replacement of ligands (e.g., replacing bipyridyl with bipyrazinyl) can result in a larger HOMO–LUMO energy gap through greater stabilization of the HOMO, these results importantly allow us to capture the tipping point at which this inversion in photophysical tuning occurs. This therefore enables us to explore the limits available in emission tuning with a relatively simple and minimalist ligand structure.
The series of osmium(ii) complexes Os(bpy)3-n(btz)nPF62 (bpy = 2,2'-bipyridyl, btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, n = 0, n = 1, n = 2, n = 3), have been prepared and characterised. The ...progressive replacement of bpy by btz leads to blue-shifted UV-visible electronic absorption spectra, indicative of btz perturbation of the successively destabilised bpy-centred LUMO. For , a dramatic blue-shift relative to the absorption profile for is observed, indicative of the much higher energy LUMO of the btz ligand over that of bpy, mirroring previously reported data on analogous ruthenium(ii) complexes. Unlike the previously reported ruthenium systems, heteroleptic complexes and display intense emission in the far-red/near-infrared (λmax = 724 and 713 nm respectively in aerated acetonitrile at RT) as a consequence of higher lying, and hence less thermally accessible, (3)MC states. This assertion is supported by ground state DFT calculations which show that the dσ* orbitals of to are destabilised by between 0.60 and 0.79 eV relative to their Ru(ii) analogues. The homoleptic complex appears to display extremely weak room temperature emission, but on cooling to 77 K the complex exhibits highly intense blue emission with λmax 444 nm. As complexes to display room temperature luminescent emission and readily reversible Os(ii)/(iii) redox couples, light-emitting electrochemical cell (LEC) devices were fabricated. All LECs display electroluminescent emission in the deep-red/near-IR (λmax = 695 to 730 nm). Whilst devices based on and show inferior current density and luminance than LECs based on , the device utilising shows the highest external quantum efficiency at 0.3%.
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