Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C^N)2(N^N)]+ Complexes

Paul Scattergood, Anna Ranieri, Luke Charalambou, Adrian Comia, Daniel Ross, Craig Rice, Samantha Hardman, Jean-Louis Heully, Isabelle M. Dixon, Massimiliano Massi, Fabienne Alary, Paul Elliott

Research output: Contribution to journalArticle

Abstract

Fundamental insights into the mechanism of triplet excited state interligand energy transfer dynamics and origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C^N)2(N^N)]+ (HC^N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N^N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room temperature fluid solutions from triplet metal-to-ligand charge transfer (3MLCT) states admixed with either ligand-centred (3LC) (1a, 2a & 2b) or ligand-to-ligand charge transfer (3LL’CT) character (1c, 2c and 3a-c). Particularly striking is the observation that the pyrimidine-based complex 1b exhibits dual emission from both 3MLCT/3LC and 3MLCT/3LL’CT states. At 77 K the 3MLCT/3LL’CT component is lost from photoluminescence spectra of 1b, with emission exclusively arising from its 3MLCT/3LC state, whilst for 2c switching from 3MLCT/3LL’CT to 3MLCT/3LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of 3MLCT/3LC states for 1a, 2a and 2b which persist throughout the 3 ns timeframe of the experiment. These 3MLCT/3LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their 3MLCT/3LL’CT states. Transient data for 1b reveals intermediate behaviour: the spectral features of the initially populated 3MLCT/3LC state also undergo rapid evolution, although to a lesser extent than observed for 1c and 2c, behaviour assigned to the equilibration of the 3MLCT/3LC and 3MLCT/3LL’CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both 3MLCT/3LC and 3MLCT/3LL’CT states of 1a-c and 2a-c. Indeed, two distinct 3MLCT/3LC minima were optimized for 1a, 1b, 2a & 2b distinguished by upon which of the two C^N ligands the excited electron resides. The 3MLCT/3LC and 3MLCT/3LL’CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally-resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the 3MLCT/3LC and 3MLCT/3LL’CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimised excited state geometries enable the key structural differences between the 3MLCT/3LC and 3MLCT/3LL’CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Further, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.
Original languageEnglish
Number of pages19
JournalInorganic Chemistry
Early online date14 Jan 2020
DOIs
Publication statusE-pub ahead of print - 14 Jan 2020

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Excited states
Triazoles
Energy transfer
energy transfer
engineering
Ligands
ligands
excitation
Charge transfer
Iridium
charge transfer
Photoexcitation
spectral signatures
pyrimidines
Density functional theory
Absorption spectra
iridium
photoexcitation
Photoluminescence
Metals

Cite this

Scattergood, Paul ; Ranieri, Anna ; Charalambou, Luke ; Comia, Adrian ; Ross, Daniel ; Rice, Craig ; Hardman, Samantha ; Heully, Jean-Louis ; Dixon, Isabelle M. ; Massi, Massimiliano ; Alary, Fabienne ; Elliott, Paul. / Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C^N)2(N^N)]+ Complexes. In: Inorganic Chemistry. 2020.
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title = "Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C^N)2(N^N)]+ Complexes",
abstract = "Fundamental insights into the mechanism of triplet excited state interligand energy transfer dynamics and origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C^N)2(N^N)]+ (HC^N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N^N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room temperature fluid solutions from triplet metal-to-ligand charge transfer (3MLCT) states admixed with either ligand-centred (3LC) (1a, 2a & 2b) or ligand-to-ligand charge transfer (3LL’CT) character (1c, 2c and 3a-c). Particularly striking is the observation that the pyrimidine-based complex 1b exhibits dual emission from both 3MLCT/3LC and 3MLCT/3LL’CT states. At 77 K the 3MLCT/3LL’CT component is lost from photoluminescence spectra of 1b, with emission exclusively arising from its 3MLCT/3LC state, whilst for 2c switching from 3MLCT/3LL’CT to 3MLCT/3LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of 3MLCT/3LC states for 1a, 2a and 2b which persist throughout the 3 ns timeframe of the experiment. These 3MLCT/3LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their 3MLCT/3LL’CT states. Transient data for 1b reveals intermediate behaviour: the spectral features of the initially populated 3MLCT/3LC state also undergo rapid evolution, although to a lesser extent than observed for 1c and 2c, behaviour assigned to the equilibration of the 3MLCT/3LC and 3MLCT/3LL’CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both 3MLCT/3LC and 3MLCT/3LL’CT states of 1a-c and 2a-c. Indeed, two distinct 3MLCT/3LC minima were optimized for 1a, 1b, 2a & 2b distinguished by upon which of the two C^N ligands the excited electron resides. The 3MLCT/3LC and 3MLCT/3LL’CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally-resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the 3MLCT/3LC and 3MLCT/3LL’CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimised excited state geometries enable the key structural differences between the 3MLCT/3LC and 3MLCT/3LL’CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Further, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.",
keywords = "iridium, excited-state, Photophysics",
author = "Paul Scattergood and Anna Ranieri and Luke Charalambou and Adrian Comia and Daniel Ross and Craig Rice and Samantha Hardman and Jean-Louis Heully and Dixon, {Isabelle M.} and Massimiliano Massi and Fabienne Alary and Paul Elliott",
year = "2020",
month = "1",
day = "14",
doi = "10.1021/acs.inorgchem.9b03003",
language = "English",
journal = "Inorganic Chemistry",
issn = "0020-1669",
publisher = "American Chemical Society",

}

Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C^N)2(N^N)]+ Complexes. / Scattergood, Paul; Ranieri, Anna; Charalambou, Luke; Comia, Adrian; Ross, Daniel; Rice, Craig; Hardman, Samantha; Heully, Jean-Louis; Dixon, Isabelle M.; Massi, Massimiliano; Alary, Fabienne; Elliott, Paul.

In: Inorganic Chemistry, 14.01.2020.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C^N)2(N^N)]+ Complexes

AU - Scattergood, Paul

AU - Ranieri, Anna

AU - Charalambou, Luke

AU - Comia, Adrian

AU - Ross, Daniel

AU - Rice, Craig

AU - Hardman, Samantha

AU - Heully, Jean-Louis

AU - Dixon, Isabelle M.

AU - Massi, Massimiliano

AU - Alary, Fabienne

AU - Elliott, Paul

PY - 2020/1/14

Y1 - 2020/1/14

N2 - Fundamental insights into the mechanism of triplet excited state interligand energy transfer dynamics and origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C^N)2(N^N)]+ (HC^N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N^N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room temperature fluid solutions from triplet metal-to-ligand charge transfer (3MLCT) states admixed with either ligand-centred (3LC) (1a, 2a & 2b) or ligand-to-ligand charge transfer (3LL’CT) character (1c, 2c and 3a-c). Particularly striking is the observation that the pyrimidine-based complex 1b exhibits dual emission from both 3MLCT/3LC and 3MLCT/3LL’CT states. At 77 K the 3MLCT/3LL’CT component is lost from photoluminescence spectra of 1b, with emission exclusively arising from its 3MLCT/3LC state, whilst for 2c switching from 3MLCT/3LL’CT to 3MLCT/3LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of 3MLCT/3LC states for 1a, 2a and 2b which persist throughout the 3 ns timeframe of the experiment. These 3MLCT/3LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their 3MLCT/3LL’CT states. Transient data for 1b reveals intermediate behaviour: the spectral features of the initially populated 3MLCT/3LC state also undergo rapid evolution, although to a lesser extent than observed for 1c and 2c, behaviour assigned to the equilibration of the 3MLCT/3LC and 3MLCT/3LL’CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both 3MLCT/3LC and 3MLCT/3LL’CT states of 1a-c and 2a-c. Indeed, two distinct 3MLCT/3LC minima were optimized for 1a, 1b, 2a & 2b distinguished by upon which of the two C^N ligands the excited electron resides. The 3MLCT/3LC and 3MLCT/3LL’CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally-resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the 3MLCT/3LC and 3MLCT/3LL’CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimised excited state geometries enable the key structural differences between the 3MLCT/3LC and 3MLCT/3LL’CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Further, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.

AB - Fundamental insights into the mechanism of triplet excited state interligand energy transfer dynamics and origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C^N)2(N^N)]+ (HC^N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N^N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room temperature fluid solutions from triplet metal-to-ligand charge transfer (3MLCT) states admixed with either ligand-centred (3LC) (1a, 2a & 2b) or ligand-to-ligand charge transfer (3LL’CT) character (1c, 2c and 3a-c). Particularly striking is the observation that the pyrimidine-based complex 1b exhibits dual emission from both 3MLCT/3LC and 3MLCT/3LL’CT states. At 77 K the 3MLCT/3LL’CT component is lost from photoluminescence spectra of 1b, with emission exclusively arising from its 3MLCT/3LC state, whilst for 2c switching from 3MLCT/3LL’CT to 3MLCT/3LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of 3MLCT/3LC states for 1a, 2a and 2b which persist throughout the 3 ns timeframe of the experiment. These 3MLCT/3LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their 3MLCT/3LL’CT states. Transient data for 1b reveals intermediate behaviour: the spectral features of the initially populated 3MLCT/3LC state also undergo rapid evolution, although to a lesser extent than observed for 1c and 2c, behaviour assigned to the equilibration of the 3MLCT/3LC and 3MLCT/3LL’CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both 3MLCT/3LC and 3MLCT/3LL’CT states of 1a-c and 2a-c. Indeed, two distinct 3MLCT/3LC minima were optimized for 1a, 1b, 2a & 2b distinguished by upon which of the two C^N ligands the excited electron resides. The 3MLCT/3LC and 3MLCT/3LL’CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally-resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the 3MLCT/3LC and 3MLCT/3LL’CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimised excited state geometries enable the key structural differences between the 3MLCT/3LC and 3MLCT/3LL’CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Further, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.

KW - iridium

KW - excited-state

KW - Photophysics

U2 - 10.1021/acs.inorgchem.9b03003

DO - 10.1021/acs.inorgchem.9b03003

M3 - Article

JO - Inorganic Chemistry

JF - Inorganic Chemistry

SN - 0020-1669

ER -