Excited State Dynamics of a Pt^IIDiimine Complex bearing a Naphthalene-Diimide Electron Acceptor

Abstract

A combination of picosecond time-resolved infrared spectroscopy, picosecond transient absorption spectroscopy, and nanosecond flash photolysis was used to elucidate the nature and dynamics of a manifold of the lowest excited states in Pt(phen-NDI)Cl 2 ( 1), where NDI = strongly electron accepting 1,4,5,8-naphthalene-diimide group. 1 is the first example of a Pt (II)-diimine-diimide dyad. UV/vis/IR spectroelectrochemistry and EPR studies of electrochemically generated anions confirmed that the lowest unoccupied molecular orbital (LUMO) in this system is localized on the NDI acceptor group. The lowest allowed electronic transition in Pt(phen-NDI)Cl 2 is charge-transfer-to-diimine of a largely Pt-->phen metal-to-ligand charge-transfer (MLCT) character. Excitation of 1 in the 355-395 nm range initiates a series of processes which involve excited states with the lifetimes of 0.9 ps ( (1)NDI*), 3 ps ( (3)MLCT), 19 ps (vibrational cooling of "hot" (3)NDI and of "hot" NDI ground state), and 520 mus ( (3)NDI). Excitation of 1 with 395 nm femtosecond laser pulses populates independently the (1)MLCT and the (1)NDI* excited states. A thermodynamically possible decay of the initially populated (1)MLCT to the charge-transfer-to-NDI excited state, [Pt (III)(phen-NDI (-*))Cl 2], is not observed. This finding could be explained by an ultrafast ISC of the (1)MLCT to the (3)MLCT state which lies about 0.4 eV lower in energy than [Pt (III)(phen-NDI (-*))Cl 2]. The predominant decay pathway of the (3)MLCT is a back electron transfer process with approximately 3 ps lifetime, which also causes partial population of the vibrationally hot ground state of the NDI fragment. The decay of the (1)NDI* state in 1 populates vibrationally hot ground state of the NDI, as well as vibrationally hot (3)NDI. The latter relaxes to form (3)NDI state, that is, [Pt(phen- (3)NDI)Cl 2]*, which possesses a remarkably long lifetime for a Pt (II) complex in fluid solution of 520 mus. The IR signature of this excited state includes the nu(CO) bands at 1607 and 1647 cm (-1), which are shifted considerably to lower energies if compared to their ground-state counterparts. The assignment of the vibrational bands is supported by the density-functional theory calculations in CH 2Cl 2. Pt(phen-NDI)Cl 2 acts as a modest photosensitizer of singlet oxygen.

Influence of a Naphthaldiimide Substituent at the Diimine Ligand on the Photophysics and Reverse Saturable Absorption of PtII Diimine Complexes and Cationic IrIII Complexes

“…Thus, the observed fs TA spectrum of 2 should emanate from the NDI -· radical anion. This indicates that upon photoexcitation at 390 nm, a charge transfer occurs creating a radical anion on the NDI component, which is different from the Pt(phen-NDI)Cl 2 complex reported by Weinstein's group, [39] Figure S8) at 540 nm gave a time constant of 1215 Ϯ 106 ps, while fitting of the decay profile at 608 nm provided a time constant of 2052 Ϯ 263 ps.…”

“…Synthesis of the ligand 8 and the precursor complexes Pt(phen)Cl 2 (5) and Pt(phen-NDI)Cl 2 (7) followed the procedures reported by Weinstein's group. [39] The synthesis of ligand 6 was reported by our group previously. [24] Substitution of the Cl coligands in precursors 5 and 7 with the acetylide ligand 6 in the presence of CuI and diisopropylamine (DIPA) gave the Pt II complexes 1 and 2.…”

“…[18,27] In addition to the aforementioned work, Weinstein's group reported a detailed study on the excited state dynamics of a Pt II diimine chloride complex bearing a strong electron-withdrawing naphthaldiimide (NDI) substituent on the diimine ligand. [39] It was found that there is little electronic interaction between the Pt(phen)Cl 2 [14,18,20,23,24,27] In this work, we synthesized a Pt II diimine complex and two cationic Ir III complexes bearing the NDI motif on the diimine (N ∧ N) ligand (complexes 2-4 in Figure 1). Pt II complex 1, which contains the 1,10-phenanthroline ligand, was synthesized to be a reference compound for the Pt II complex 2 in order to better understand the role of NDI in the presence of the BTF acetylide ligands.…”

Ultrafast charge transfer dynamics in supramolecular Pt(ii) donor–bridge–acceptor assemblies: the effect of vibronic coupling

“…50 The Pt$acetylide bridge is decoupled from the acceptor as the latter is orthogonal to the intermediate phenyl ring, and no strong MLCT band is observed in visible region (Fig 2). We therefore use 380 nm, ~50 fs excitation pulse to populate the 1 NDI* singlet excited state to trigger subsequent charge transfer.…”

“…50,51,[53][54][55] A range of additional, overlapping, higher energy transitions observed for * ), especially two prominent features centred at 327 and 341 nm but not in ( allows their assignment to intra$ ligand transitions associated with the Pt(II)$centred / $acetylide bridging unit. An additional absorption band at >400 nm, with the extinction coefficient of an order of 100 dm 3 mol $1 cm $1 observed in *$) implies a presence of a charge$transfer transition. '…”