Mauro Maestri

Photoinduced processes in 4′-(9-anthryl)-2,2′:6′,2″-terpyridine, its protonated forms and Zn(II), Ru(II) and Os(II) complexes

Abstract We investigated the absorption spectra and the luminescence properties of 2,2′:6′,2″-terpyridine(tpy), 4′-(9-anthryl)-2,2′:6′,2″-terpyridine (An-tpy), their protonated forms, and their Ru(II), Os(II), and Zn(II) metal complexes ([M(tpy) 2 ] 2+ and [M(An-tpy) 2 ] 2+ ). For both tpy and An-tpy, the addition of CF 3 SO 3 H to acetonitrile (or dichloromethane) solutions causes changes in the absorption spectra that indicate the formation of monoprotonated (tpyH + and An-tpyH + ) and diprotonated (tpyH 2 2+ and An-tpyH 2 2+ ) forms. The weak fluorescence of tpy ( λ max = 338 nm ) becomes much stronger and moves to lower energy ( λ max = 410 nm ) upon the first protonation, and then moves back to higher energy ( λ max = 360 nm ) upon the second protonation. An-tpy shows a strong anthracene-type fluorescence band with λ max = 422 nm which disappears upon the first protonation: the diprotonated form shows a very weak emission with λ max = 505 nm . Both the absorption and emission spectra of the protonated forms of An-tpy indicate that the lowest excited state can be assigned to a charge-transfer transition from the anthracene moiety to the protonated tpy moiety. In contrast, [Zn(tpy) 2 ] 2+ shows a very intense, ligand-centered fluorescence band with λ max = 353 nm , whereas [Zn(An-tpy) 2 ] 2+ shows that a much weaker emission with λ max = 543 nm , assigned to an An → [Zn(tpy)) 2 ] 2+ charge-transfer transition. The very weak metal-to-ligand charge transfer (MLCT) phosphorescence of [Ru(tpy) 2 ] 2+ is no longer present in [Ru(An-tpy) 2 ] 2+ because the 1 MLCT level of the Ru-based unit is quenched by the lower lying T 1 excited state of the anthracene unit, as shown by the appearance of the characteristic T 1 transient absorption band with λ max = 420 nm in flash spectroscopy experiments. In aerated solutions the T 1 excited state of the anthracene unit of [Ru(An-tpy) 2 ] 2+ is quenched by dioxygen with formation of 1 Δ (O 2 ), whose emission can be observed in the near-IR region ( λ max = 1270 nm. Continued irradiation of [Ru(An-tpy) 2 ] 2+ in aerated solution causes the destruction of the anthracene moiety because of its reaction with 1 Δ (O 2 ). [Os(An-tpy) 2 ] 2+ shows the same 1 MLCT emission as [Os(tpy) 2 ] 2+ ( λ max = 728 nm) since the 1 MLCT level lies below the T 1 excited state of the anthracene moiety. The absorption and excitation spectra of [Os(An-tpy) 2 ] 2+ are almost coincident, showing that the S 1 excited state of the anthracene unit is converted efficiently to the 1 MLCT level of the [Os(tpy) 2 ] 2+ unit.

Luminescence of ortho-metallated platinum(II) complexes

Abstract The absorption spectra, emission spectra, and emission lifetimes of Pt(Phpy)2, Pt(Thpy)2, and Pt(Bhq)2 complexes (Phpy−, Thpy−, and Bhq− are the ortho C-deprotonated forms of 2-phenylpyridine, 2-(2-thienyl)-pyridine, and benzo(h)quinoline) have been studied and compared with those of the C-protonated neutral ligands. For all complexes examined the low-energy absorption bands in the near UV and visible region are assigned to metal-to-ligand charge-transfer transitions. The strong and structured luminescence emissions observed in the 500–600 nm region (lifetime in the microsecond range at 77 K) are assigned to metal-to-ligand charge-transfer excited states.

Multistate/Multifunctional Molecular-Level Systems: Light and pH Switching between the Various Forms of a Synthetic Flavylium Salt

An optical memory device with multiple storage in two different memory levels and nondestructive readout capacity requires the properties exhibited by the 4′-hydroxyflavylium ion, which can exist in several forms (multistate) that can be interconverted by more than one type of external stimulus (multifunctional), as depicted on the right. Its intricate network of light- and/or pH-induced transformations form a basis for simple logic operations.

Dendrimers with a cyclam core. Absorption spectra, multiple luminescence, and effect of protonation

Abstract We have synthesized two dendrimers ( 4 and 5 ) consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam) core appended with four dimethoxybenzene and eight naphthyl units ( 4 ) and 12 dimethoxybenzene and 16 naphthyl units ( 5 ). The absorption and luminescence spectra of these compounds and the changes taking place upon protonation of their cyclam core have been investigated. In acetonitrile-dichloromethane 1:1 v/v solution they exhibit three types of emission bands, assigned to naphthyl localized excited states ( λ max =337 nm), naphthyl excimers ( λ max ca 390 nm), and naphthyl-amine exciplexes ( λ max =480 nm). The tetraamine cyclam core undergoes only two protonation reactions, whose constants have been obtained by fitting the spectral changes. Protonation not only prevents exciplex formation for electronic reasons, but also causes strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers.

Ligand substitution patterns control photophysical properties of ruthenium(II)=-2,2':6',2-terpyridine complexes : room temperature emission from [Ru(tpy)2]2+ analogues

Ruthenium(II) complexes of 2,2′:6′,2″-terpyridine have not been widely investigated as photocatalysts; the introduction of electron-withdrawing substituents into the 4′-position of the ligand results in complexes [Ru(Xtpy)2]2+ (Xtpy = Cltpy or MeSO2tpy), which luminesce in fluid solution at room temperature.

Luminescence properties of cryptate europium (III) complexes incorporating heterocyclic N-oxide groups

Abstract The luminescence spectra, lifetimes, and quantum yields of the cryptate-type complexes [Eu ⊂ 1 ] 3+ and [Eu ⊂ 2 ] 3+ , where 1 and 2 are cryptand-type ligands containing two 2,2′-bipyridine (bpy) and one 2,2′-bipyridine-1,1′-dioxide (bpyO 2 ) units ( 1 ) or two bpy and one 3,3′-biisoquinoline-2,2′-dioxide (biqO 2 ) units ( 2 ), have been measured in H 2 ) and D 2 O at 77 and 300 K. The luminescence quantum yield upon excitation in the ligand-centered bands is much higher than that exhibited by the previously investigated [Eu ⊂ bpy.bpy.bpy] 3+ cryptate. The role played by various radiative and non-radiative paths is discussed.

A Dendritic Antenna for Near-Infrared Emission of Nd3+ Ions

An interior of 18 amide groups and a periphery functionalized with 24 dansyl groups forms a light-harvesting dendrimer which features intense absorption bands in the near-UV spectral region and a strong fluorescence band in the visible region. Upon encapsulation of Nd(3+) ions, the fluorescence of the dansyl groups is quenched and an intense sensitized near-infrared emission of Nd(3+) is observed. The associated energy transfer is shown in the cartoon.

Micelle effect on the ‘write–lock–read–unlock–erase’ cycle of 4′-hydroxyflavylium ion

In aqueous solution the 4′-hydroxyflavylium ion (AH + ) can be interconverted into several different neutral forms by light excitation and/or pH changes. All the observed processes are fully reversible and accompanied by strong changes in absorption and emission spectra. This system exhibits properties required by optical memory devices with multiple storage in two different memory levels and non-destructive readout capacity through a write-lock-read-unlock-erase cycle. The effect of micelles on the pH and light induced interconversion of AH + and its neutral forms has been investigated. Negatively charged sodium dodecyl sulfate micelles stabilize AH + , whereas the positively charged cetyltrimethylammonium bromide and neutral polyoxyethylene(10) isooctyl phenyl ether (Triton X-100) micelles stabilize the uncharged (basic) forms. Besides affecting the molar fraction distribution of the various species, the presence of micelles also influences their interconversion rates. Addition of micelles can therefore be considered as a third external stimulus (besides light excitation and pH jump) capable of changing the state of this multistate/multifunctional molecular-level system. Particularly interesting is the possibility to change the autolock pH of the photochromic reaction by addition of micelles.

PH-CONTROLLED PHOTOCHROMISM OF HYDROXYFLAVYLIUM IONS

The structural transformations and the photochromic properties of the 7-hydroxyflavylium ion have been investigated by means of the pH jump technique and continuous and pulsed light excitation. The various forms of flavylium compounds, trans- and cis-chalcone (Ct and Cc), quinoidal base A, and hemiacetal form B, are related by interconversions like communicating vessels containing a fluid. Since the amount of colored species formed upon irradiation depends on the pH of the solution, pH can be viewed as a “tap” that modulates the color intensity generated by light excitation.

Spectroscopic properties of Pt(II) and Pd(II) complexes with aromatic terdendate (C″>N^C) cyclometallating ligands

Abstract Emission spectra, emission lifetime and electrochemical behavior of the complexes Pt(H-dppy)2, Pt(dppy)S(Et)2, Pt(dppy)py, Pt(dppy)pyr, (dppy)Pt-pyr-Pt(dppy), and Pd(dppy)py (where dppy2− is the C− deprotonated form of 2,6-di-phenylpyridine and py and pyr are pyridine and pyrazine) have been studied. The structured luminescence spectra observed at 77 K have been assigned to transitions having i) MLCT character in the case of Pt(H-dppy)2, ii) mainly MLCT character with some LC contribution for the four tridentate trans Pt(II) complexes and iii) almost pure LC character for the Pd(II) complex. The luminescence emission involves in all cases the dppy ligand even though the electrochemical results suggest a LUMO orbital centred on the pyrazine ligand in the Pt(dppy)pyr complex and on the metal in the Pd(dppy)py complex.