Cyclometalated Platinum(II) 6-Phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine Complexes: Synthesis, Photophysics, and Nonlinear Absorption

Abstract

A series of mononuclear and dinuclear cyclometalated platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2'-bipyridine complexes (F-1-F-5) were synthesized and their photophysical properties were systematically investigated. All complexes exhibit strong (1)pi,pi* absorption bands in the UV region, and a broad, structureless charge transfer band in the visible region. The charge-transfer band is broadened and red-shifted for F-3-F-5 compared to those for F-1 and F-2 because of the electron-donating acetylide ligand and the involvement of the ligand-to-ligand charge transfer character. The molar extinction coefficients for the dinuclear complex F-5 are much higher than those for the mononuclear complexes F-1-F-4, indicating the electronic coupling through the bridge ligand. All complexes are emissive in solution at room temperature and in glassy matrix at 77 K. When excited at the charge transfer absorption band, the complexes exhibit a long-lived red/orange emission around 600 nm, which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission ((3)MLCT/(3)ILCT). For emission at 77 K, the emitting state is tentatively assigned as (3)MLCT for F-2-F-4, and (3)MLCT/(3)pi,pi* for F-1 and F-5 taking into account the emission energy, the shape of the spectrum, the lifetime, and the thermally induced Stokes shift. F-1-F-4 exhibit broad triplet transient difference absorption in the visible to the near-IR region, with a lifetime comparable to those measured from the decay of the (3)MLCT/(3)ILCT emission. Therefore, F-1-F-4 give rise to a strong reverse saturable absorption for ns laser pulses at 532 nm. Z-scan experiments were carried out at 532 nm using both ns and ps laser pulses, and the experimental data was fitted by a five-band model to extract the singlet and triplet excited-state absorption cross sections. The degree of reverse saturable absorption follows this trend: F-1 = F-2 > F-3 > F-4 > F-5, which is mainly determined by the ratio of the triplet excited-state absorption cross-section to that of the ground-state and the triplet excited-state quantum yield. Comparison of the photophysics of F-1, F-2, and F-3 to those of their corresponding Pt complexes without the fluorenyl substituent discovers that F-1-F-3 exhibit larger molar extinction coefficients for their low-energy charge transfer absorption band, longer triplet excited-state lifetimes, higher emission quantum yields, and increased ratios of the excited-state absorption cross-section to that of the ground-state.

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

“…This causes the blue-shift of the UV-vis absorption spectrum. This phenomenon is also in accordance with that observed in many of the Pt(II) bipyridyl or terpyridyl complexes reported in the literatures [19,20].…”

“…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.…”

Luminescent PtII Complexes of Tridentate Cyclometalating 2,5‐Bis(aryl)‐pyridine Ligands

“…[1,2] Having the same shape and electronic configuration as 2,2′:6′,2′′-terpyridine (terpy), [3] the (Phbpy) -anion provides very stable tridentate binding and the cyclometalation, requiring C-H activation of the phenyl core, can be carried out at moderate conditions, usually by heating suitable Pt II or Pd II precursors and the C-H functionalised protoligands (ligand precursors, prior to metalation). [1,2,[4][5][6][7][8][9][10][11][12][13][14] Very soon after their initial use, it was found that many Pt II derivatives of the Phbpy ligand show intense luminescence, some even at ambient temperature. [11,[15][16][17][18][19] One reason lies in the destabilisation of dark metal-centred excited states associ-to 0.44 in PMMA matrices.…”