Influence of Alkoxyl Substituent on 4,6-Diphenyl-2,2′-bipyridine Ligand on Photophysics of Cyclometalated Platinum(II) Complexes: Admixing Intraligand Charge Transfer Character in Low-Lying Excited States

Abstract

A series of platinum 4,6-diphenyl-2,2'-bipyridine complexes (6-10) with alkoxyl substituent on the 6-phenyl ring have been synthesized and characterized. The influence of the alkoxyl substituent on the nature of the low-lying excited states, and thus the photophysical properties, have been systematically investigated spectroscopically and theoretically. Complexes 6-10 exhibit a broad low-energy charge-transfer absorption band from 400 to 500 nm, which shows weak negative solvatochromic effect. They all emit at about 590 nm in fluid solutions at room temperature, with the emission energy essentially independent of the nature of the monodentate ligand and the polarity of solvent. The excited-state lifetimes of 6 and 7 are much longer (approximately 460-570 ns) than those of their corresponding "alkoxyl free" analogues 12 and 13 (approximately 40-100 ns) in CH(3)CN. Additionally, the emission quantum yields of 7-9 in CH(2)Cl(2) are quite high (0.15-0.21). Spectroscopic studies and Time-Dependent Density Functional Theory (TDDFT) calculations indicate that these unique photophysical properties are induced by the electron-donating ability of the alkoxyl substituent, which causes a mixture of the intraligand charge transfer (ILCT) with the metal-to-ligand charge transfer (MLCT)/ligand-to-ligand charge transfer (LLCT) in their low-lying excited states. Complexes 6-10 exhibit broad triplet transient difference absorption in the near-UV to the near-IR region, where reverse saturable absorption (RSA) could occur. Nonlinear absorption experiments at 532 nm for nanosecond laser pulses demonstrate that 6-9 are strong reverse saturable absorbers, while 10 exhibits weak RSA because of its larger ground-state absorption cross-section and its low triplet excited-state quantum yield.

Synthesis and photophysical properties of cyclometalated 4′-phenyl-2,2′:6′,2″-terpyridyl Pt(II) chloride complexes with different aryl substituents

“…Meanwhile, 1a and 1b show more structured bands, which are indicative of an emitting state mixed with some intraligand 3 π,π* character. These assignments are also consistent with the literature report for similar complexes [19][20][21][22][23][24][25][26][27][28]. Furthermore, because 1a-1f all contain the same 4'-phenyl-2,2':6',2″-terpyridyl Pt(II) chloride component, the observed difference in emission should be attributed to the different natures of the substituents attached on the terminal phenyl unit, especially the electron-donating or electron-withdrawing ability of the substituents.…”

“…The spectra of the complexes 1a-1f consist of intense absorption bands below 370 nm and a broad tail above 400 nm. The intense absorption bands below 370 nm can be assigned to the 1 π-π* transition within the terpyridyl ligands, while the broad tail absorption bands in the visible region could be assigned to the 1 MLCT (metal-to-ligand charge transfer) transition [19][20][21][22][23][24][25]. In addition, the energies of these absorption bands are similar to those observed from the protonated ligands, implying that these bands possess an intraligand charge transfer ( 1 ILCT) character from the aromatic rings component to the terpyridine component.…”

“…To date, the widely investigated nonlinear optical materials are metallophthalocyanines [3], organic dendrimers [4,5], bis(styryl)benzene derivatives [6] and metalloporphyrins [7]. Besides these, cyclometalated Pt(II) complexes have attracted great interest in recent years due to their intriguing spectroscopic properties as well as potential applications in the fields of optoelectronic devices [8][9][10][11][12], photocatalysis [13][14][15], chemosensors [16,17] and nonlinear optics [18][19][20][21][22]. Their rich photophysical properties and applications are intrinsically based on the presence of multiple charge transfer excited states, such as metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), intraligand charge transfer (ILCT), in addition to the ligand localized π,π* transitions.…”

Thiophene‐Inserted Aryl–Dicyanovinyl Compounds: The Second Generation of Fluorescent Molecular Rotors with Significantly Redshifted Emission and Large Stokes Shift

“…[35][36][37][38][39] In this work we used DFT calculations to study the absorption and emission properties of the new molecular rotors to investigate the role of the thiophene moiety in the π conjugation of the molecular framework.…”

Structure and optical spectra of bis(pyrrol‐2‐ylmethyleneamine) complexes: A DFT and TDDFT study of the self‐assembly complexes of bis(pyrrol‐2‐ylmethyleneamine) ligands linked by alkyl spacers with Cu(II)

“…For a closed-shell molecule, TDDFT can be used to calculate the whole manifolds of singlet as well as triplet excited states, provided that the transition energies are smaller than the vertical ionization potential. TDDFT has been successfully applied to open-shell and metal complexes [23][24][25][26][27][28][29][30], as well as triplet excited states [31][32][33].…”