The photophysical properties of quadruply bonded M 2 arylethynyl carboxylate complexes 64th International Symposium on Molecular Spectroscopy Carly Reed

Quadruply Bonded Dimetal Units Conjugated organic polymers potential applications: thin-film transistors, organic solar cells, and molecular memory devices. Incorporating quadruply bonded dimetal units into conjugated organic polymers is of interest to determine new tunable optoelectronic properties. M = Mo, W Macromol. Chem. Phys. 2008, 209, 1319

TiPB Dimetal units are brought into conjugation with conjugated ligand by carboxylate tethers Key orbital interactions involve M 2  and CO 2 combinations with the  system of the conjugated ligand  Out of phase combination of  * mixes strongly with M 2  orbitals  However in-phase has no symmetry match TiPB = PNAS, 2008, 105 (40), 15247

Absorption, Excitation, Emission (r.t.), …. Emission (77K) J. Am. Chem. Soc. 2005, 127, b Background Observed short lived visible emission originating from 1 MLCT (Mo 2  ) 1 (O 2 C-aryl  *) Visible emission decayed < 10 ns, however, in ns- TA a long lived excited state was observed (  s) Tentatively assigned as 3 MLCT non-emissive excited state

Motivation What is the nature and behavior of this long lived excited state; how can it be tuned? Can steric interactions be alleviated while maintaining conjugation? How will this effect the excited state charge distribution? Steric interactions between carboxylate oxygen and peri-H atoms on anthracene cause twisting of conjugated ligand out dimetal plane in complex shown to have longest lived excited state (76  s). J. Am. Chem. Soc. 2006, 128, The Chemical Record. 2005, 5, Dihedral  = ~45 o (oblique), ~85 o (perpendicular) Solution: Introduce ethynyl unit

DFT-Calculations 574 nm: MO 164 (HOMO)  165 (LUMO) 426 nm: MO 162  nm: MO 164  168 DFT calculations utilized B3LYP functional basis set 6-31G* for non-metal atoms and SDD energy consistent pseudo-potential for Mo 461 nm: MO 120 (HOMO)  121 (LUMO) 284 nm: MO 118  121

533 nm: 120 (HOMO)  121 (LUMO) 344 nm: 120  nm: 116    nm: 164 (HOMO)  165 (LUMO) 443 nm: 162  165; 164  nm: 164  nm: 159  165; 164  172 DFT-Calculations DFT calculations utilized B3LYP functional basis set 6-31G* for non-metal atoms and SDD energy consistent pseudo-potential for Mo

max = 570 nm, ~710 nm; THF Emissive Properties of Molybdenum Complexes Solvent Dependence NIR emission 1  em 2  em THF1072 nm/ 9,328 cm nm/ 9,259 cm -1 CH 2 Cl nm / 9,433 cm nm/ 9302 cm -1 MeCN1065 nm / 9,389 cm -1 Vibronic spacing cm -1. Indicative of M-M symmetric stretching frequencies.

 = 103  s fs-TAns-TA Mo 2 (TiPB) 2 (O 2 CC 2 C 6 H 4 CH 3 ) ps 103  s Mo 2 (TiPB) 2 (O 2 CC 2 C 14 H 9 ) 2 86  s  = 86  s Nd:YAG laser (fwhm ~ 8ns, ~ 5 mJ per pulse Long-lived triplet excited state on microsecond time-scale also indicates MM  * excited state, matching well with Mo 2 TIPB 4 long-lived excited state (43  s) Inorg. Chem. 2009, 48, 4394 ns-Transient Absorption

SOMO 1 SOMO 2 LUMO Molden plots of frontier orbitals plots showing the character of the lowest energy triplet state, T 1, for each complex. Mo 2 (TiPB) 2 (Tolyl) 2 Mo 2 (TiPB) 2 (Anthryl) 2 DFT calculations utilized unrestricted B3LYP (UB3LYP) functional basis set 6-31G* for non-metal atoms and SDD energy consistent pseudo-potential for Mo DFT Calculations

In molybdenum complexes long lived excited state assigned as 3  * Ligand Independent Solvent Independence Vibronic Features DFT Calculations 3  *

Compoundsl abs, nm and l em, nmStokes shift W 2 (TiPB) 2 (O 2 CC 2 C 6 H 4 CH 3 ) nm, ~ 670 nm, 875 nm ~1470 cm -1, 4965 cm -1 W 2 (TiPB) 2 (O 2 CC 2 C 14 H 9 ) nm, 830 nm 1109 cm -1 Emissive Properties of Tungsten Complexes Do not see vibronic features at low temp W 2 TiPB 4 3  * emission 815nm Inorg. Chem. 2009, 48, 4394

Nd:YAG laser (fwhm ~ 8ns, ~ 5 mJ per pulse fs-TAns-TA W 2 (TiPB) 2 (O 2 CC 2 C 6 H 4 CH 3 ) 2 < 1 ps< 10 ns W 2 (TiPB) 2 (O 2 CC 2 C 14 H 9 ) 2 ?< 10 ns Longest lived excited state indicated lowest energy excited state is something other 3  * for these tungsten compounds because W 2 (TiPB) 4 lowest energy long lived excited state existed with  = 1.6  s Inorg. Chem. 2009, 48, 4394 ns - Transient Absorption

Molden plots of frontier orbitals plots showing the character of the lowest energy triplet state, T 1, for each complex. LUMO SOMO 2 SOMO1 HOMO

In tungsten complexes long lived excited state is not 3  * DFT Calculations Emission energies differ from W 2 (TiPB) 4 and show no vibronic features Shorter triplet lifetime compared to W 2 (TiPB) 4 (1.6  s) Future Work Part: Further explore nature of long-lived excited state with time resolved IR and Raman

Thank You! Thanks to:  Prof. Malcolm Chisholm  Prof. Claudia Turro  Chisholm Group Members  Turro Group Members  NSF  Wright Center for Photovoltaics Innovation and Commercialization  Ohio Supercomputing Center

Dalton Trans., 2004, Synthesis

+ 2 LCO 2 H L = M = Mo, W Characterized by 1 H NMR, MALDI-TOF

3 4 12

Absorption, Excitation, Emission (r.t.), …. Emission (77K) J. Am. Chem. Soc. 2005, 127, b Observed short lived visible emission originating from 1 MLCT (Mo 2  ) 1 (O 2 C-aryl  *) Stokes shift larger than 1  * M 2 complexes ( cm-1) smaller than previously reported for 3  * Re 2 ( cm -1 ) Vibronic progressions at 77K consistent with vibrations of aromatic carboxylic acid ligands Solvent dependence (  1200 cm-1 from THF to DMSO) Background: Part I

 abs 1 abs 2 THF440 nm520 nm CH 2 Cl nm507 nm MeCN427 nm504 nm 1 2

 abs 3 abs 4 THF610 nm760 nm CH 2 Cl nm690 nm MeCN596 nm732 nm (2:1 MeCN/THF mix) 3 4

Sonogashira Coupling

em (W 2 TiPB 4 ): 815 nm W 2 TiPB 4 Dihedral angles between carboxylates and C 6 plane: 29 o and 67 o

TRIR Time-resolved infrared (TRIR) spectroscopy  pump pulse: UV region (Nd:YAG laser)laser  probe beam: infrared region. Operates down to the picosecond time regime surpasses transient absorption and emission spectroscopy by providing structural information on the excited-state.

Questions EDIT Re Quad Bond, and look at orbital looks like delta star? Why do Mo 3 MMCT lifetimes differ?  Have not mapped trends Why introducing thiophene to series? Why are ligand abs bound to W show less vibrations?  Perhaps because tungsten is coupling more – therefore less pure “ligand” transition W interaction with ligands – orbital?  Energetics of tungsten closer to ligand orbital energy – therefore more overlap Explanation for lower energy W ex states having shorter lifetimes?  Since it’s a forbidden process (Triplet to ground state) – tungsten with greater spin orbit coupling makes it more allowable and therefore faster? Have we done emission decay of singlet emission to match with fs- TA?  In yagna IC papers just say decays in less than 10 ns

Mo 2 Triplet  trends Mo 2 (TiPB) 4 = 43 ms Mo 2 (ThCO 2 ) 2 = 77 ms Mo 2 (ThCOS) 2 = 50 ms Mo 2 (Th 2 CO 2 ) 2 = 83 ms Mo 2 (Th 3 CO 2 ) 2 = 72 ms Mo 2 (Tolyl 2 CO 2 ) 2 = 103 ms Mo 2 (AnthCO 2 ) 2 = 83 ms Mo 2 (BenzCN) 2 = 93 ms Mo 2 (BenzNO 2 ) 2 = 79 ms Mo 2 (Benz 2 NO 2 ) 2 = 83 ms Dimer-Dimers Mo 2 (TT) = 69 ms Mo 2 (DTT) = 60 ms Mo 2 (BT) = 72 ms