Switching between Ligand‐to‐Ligand Charge‐Transfer, Intraligand Charge‐Transfer, and Metal‐to‐Ligand Charge‐Transfer Excited States in Platinum(II) Terpyridyl Acetylide Complexes Induced by pH Change and Metal Ions

Abstract

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2''-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.

Long‐Lived Room‐Temperature Near‐IR Phosphorescence of BODIPY in a Visible‐Light‐Harvesting N^C^N PtII–Acetylide Complex with a Directly Metalated BODIPY Chromophore

“…Pt II -acetylide complexes are ideal candidates because the aforementioned photophysical properties of these complexes can be readily tuned by changing the acetylide ligand, which is synthetically feasible. [27,28,38] Table 3. Electronic excitation energies (eV), oscillator strengths (f), main configurations, and CI coefficients of the low-lying electronically excited states of complex Pt-2.…”

Acid/base sensitive platinum terpyridyl complex: Switching between metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), and intraligand charge transfer (ILCT) states

“…For example, Wu and co-workers reported the photophysical properties of platinum terpyridyl phenylacetylide complexes bearing amino moieties on phenylacetylide or terpyridyl ligands [29]. Upon protonation, the lowest excited state was switched between LLCT, MLCT, and ILCT, which results in dramatic color change and luminescence intensity change [29]. Yam and co-workers has also developed colorimetric and luminescent pH sensors based on the same mechanism [28].…”

“…Upon structural modifications, the nature of the lowest excited state of these complexes can be tuned among the metal-to-ligand charge transfer (MLCT) state, ligand-to-ligand charge transfer (LLCT) state, intraligand charge transfer (ILCT) state, and intraligand (IL) p,p* state. For example, Wu and co-workers reported the photophysical properties of platinum terpyridyl phenylacetylide complexes bearing amino moieties on phenylacetylide or terpyridyl ligands [29]. Upon protonation, the lowest excited state was switched between LLCT, MLCT, and ILCT, which results in dramatic color change and luminescence intensity change [29].…”

“…Square-planar platinum(II) complexes have attracted a great interest in recent years due to their intriguing spectroscopic properties and potential applications in a variety of photonic devices, such as in organic light emitting diodes [1][2][3][4][5][6][7][8][9], photovoltaic cells [10][11][12][13], as nonlinear optical materials [14][15][16][17][18][19], photosensitizers [20][21][22][23], and chemical and biological sensors [24][25][26][27][28][29][30]. Platinum terpyridyl phenylacetylide complexes are particularly interesting because of their ease of synthesis and structural modifications.…”

Chromo- and Fluorogenic Organometallic Sensors

“…3) act as pH sensors, derivatives with crown ether pendants (3b; Fig. 3 [38][39][40][41][42]. The origins of the changes observed in the absorption and emission spectra vary, with some systems showing switching between different excited states in the Pt(II) complex on ion binding.…”