Synthesis and Photophysical Properties of Laterally Asymmetric Digold(I) Alkynyls and Triazolyl: Ancillary Ligand and Organic Functionality Dictate Excited-State Dynamics

Functionality Dictate Excited-State Dynamics

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Synthesis and Photophysical Properties of Laterally Asymmetric

Digold(I) Alkynyls and Triazolyl: Ancillary Ligand and Organic

Joseph J. Mihaly, Alexis T. Phillips, Jacob T. Malloy, Zachary M. Marsh, Matthias Zeller, Joy E. Haley,

Kimberly de La Harpe, Tod A. Grusenmeyer, and Thomas G. Gray*

Cite This: https://dx.doi.org/10.1021/acs.organomet.9b00768

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sı Supporting Information

ABSTRACT: Four new laterally asymmetric dinuclear gold(I)

complexes made from a new chromophoric ligand (Au-DiBTF0−

) have been characterized. Changing the organic ancillary ligand

from a phosphine ((Au-DiBTF0 (PMe ) and Au-DiBTF1

(

PCy )) to an N-heterocyclic carbene (Au-DiBTF2) with an

alkynyl linking moiety or modifying the alkynyl linkage to a

triazolyl moiety with the ancillary ligand being an N-heterocyclic

carbene (Au-DiBTF3) impacts the ground- and excited-state

properties of these systems. All four complexes exhibit structured

absorption and emission spectra with weak phosphorescence. A

bathochromic shift is observed in both the ground-state absorption

and luminescence spectra as the series varies from Au-DiBTF0 to

Au-DiBTF3. The dinuclear alkynyl complexes (Au-DiBTF0−2)

display decreased ﬂuorescence lifetimes and ﬂuorescence quantum yields along with more eﬃcient intersystem crossing when the

capping ligand is changed from a trialkylphosphine to an N-heterocyclic carbene. This change results in comparable rates of radiative

decay and intersystem crossing and negligible rates of nonradiative decay. Changing the π-bridging moiety (Au-DiBTF3) results in a

diminished ﬂuorescence quantum yield, shorter ﬂuorescence lifetime, and increased intersystem quantum yield, resulting in faster

intersystem crossing accompanied by slower radiative decay and more eﬃcient nonradiative decay relative to the alkynyl-bridged

complexes. Density functional theory calculations are in accord with the observed photophysics, with nearly identical S -to-T energy

gaps for the dinuclear alkynyl complexes (Au-DiBTF0 and -2) and a smaller energy gap for Au-DiBTF3. Experimentally, Au-

DiBTF3 has the highest rate constant and quantum yield of intersystem crossing of the new gold(I) organometallics.

olymetallynes have long been studied as photoactive

carbenes, solubilize many gold(I) organometallics and allow

steric protection of an otherwise open metal site.

1,2,11,12,3−10,13−16

35−37,39

materials.

The ease of modifying the

alkynyl ligands, coupled with triplet-state photophysics in

solution and the solid state, lends them to a variety of

applications, including organic light emitting diodes

To date, investigations of dinuclear gold(I) complexes have

emphasized chromophoric bridging alkynyls attached to

38−40

(phosphine)gold(I) centers.

Wu and co-workers reported

7−21

18−27

13,28,29

(

OLEDS),

optoelectronics,

sensors,

and non-

Much of this

the synthesis and photophysics (including ﬂuorescence

lifetimes and excited-state quantum yields) of laterally

symmetric dinuclear gold(I) phosphines with variations in

0−33

32,33

linear optics (NLO),

among others.

work has focused on polymeric and multinuclear Pt(II)

complexes. In these d species, the heavy-metal atom facilitates

the bridging alkynyl chromophore. Schanze and co-workers

conducted two separate photophysical studies comparing

laterally symmetric dinuclear gold(I) and platinum(II)

intersystem crossing to the triplet excited state manifold. Gold

presents special opportunities in the design of photoactive

triplet states. Gold(I), having a d¹conﬁguration, is

diamagnetic, and its ﬁlled d subshell circumvents deactivation

by ligand-ﬁeld excited states. Gold is also the most electro-

negative metal, with a Pauling electronegativity near that of

carbon, rendering the carbon−gold bond eﬀectively nonpolar.

Additionally, the spin−orbit coupling constant of gold (5104

42,43

alkynyls.

In those reports, variations in the bridging

chromophore or ancillary ligand were not explored. Lastly,

Patra and co-workers described dinuclear gold(I) alkynyls with

Received: November 9, 2019

−

−1 34

cm ) exceeds that of platinum (4481 cm ). Finally, gold(I)

tends strongly toward linear twofold coordination. Ancillary

ligands, such as organophosphines and N-heterocyclic

XXXX American Chemical Society

Organometallics XXXX, XXX, XXX−XXX

Organometallics

DiBTF0 −3 in toluene appear in Figure 1 . All of the absorption

Communication

ﬂuorenyl-based organic frameworks whose photophysical

DiBTF3. The results of the TGA experiments appear in Figure

S11.

Ground-state absorption and emission spectra for Au-

properties could be tuned by introducing donor/acceptor

moieties on the alkynyl backbone.

Understanding of the fundamental relationship between

structure and photophysical properties in dinuclear gold(I)

alkynyls remains limited. Of primary importance are excited-

state lifetimes and quantum yields of emission and intersystem

crossing. We have therefore investigated the excited-state

properties of laterally asymmetric dinuclear gold(I) complexes

with attention to the eﬀects of the capping ligand and the

bridging chromophore.

The new benzothiazole-2,7-ﬂuorenyl dialkyne (BTF dia-

lkyne) ligand precursor was prepared by a modi ﬁ cation of a

previously reported literature procedure (Scheme S1 ).

Experimental details appear in the Supporting Information.

Metalation with gold(I) was achieved by in situ deprotonation

of the BTF dialkyne ligand and addition of this solution to a

suspension of the chlorogold(I) precursor. This procedure

yielded the new compounds Au-DiBTF0−2 after 16 h

(

Scheme 1). Au-DiBTF2 was further functionalized through

Scheme 1. Synthesis of Au-DiBTF0−3

Figure 1. Ground-state absorption and emission spectra of Au-

DiBTF0 (black), Au-DiBTF1 (red), Au-DiBTF2 (blue), and Au-

DiBTF3 (green) in toluene. Emission measurements were collected

following three freeze−pump−thaw cycles. Inset: detail of the

emission spectra from 500 to 700 nm.

spectra show similarly structured band shapes. The spectra

display two distinct lower-energy maxima with a higher-energy

shoulder. Altering the linking moiety (alkyne vs triazole) or the

capping ligand (phosphine vs N-heterocyclic carbene) shifts

the ground-state absorption spectra of the complexes. Au-

DiBTF0 and Au-DiBTF1, which contain a trialkylphosphine

ancillary ligand, possess the highest-energy absorption maxima

(

λ_a_b_s) with values of 383 and 384 nm, respectively. Changing

the ancillary ligand from a phosphine to a carbene in Au-

DiBTF2 red-shifts the absorption maximum to 387 nm. Lastly,

altering the organic alkynyl to a triazole functionality further

red-shifts the absorption maxima of Au-DiBTF3 to 390 nm.

All of the complexes exhibit similar extinction coeﬃcients (ε)

−1

of (5−7) × 10 M cm in dichloromethane.

The emission spectra of these complexes show structured

ﬂuorescence at 400−550 nm and weak phosphorescence at

50−700 nm (Figure 1). The ﬂuorescence maxima (λ ) follow

a previously published copper-catalyzed click reaction to yield

the same energy trends as the absorption spectra, with a

bathochromic shift observed in going from Au-DiBTF0 to Au-

DiBTF3. Because of the weak phosphorescence in these

systems, it is diﬃcult to assign the emission maxima (Figure 1

inset). The structured absorption and emission proﬁles

indicate that the transitions are mainly of π−π* character in

Au-DiBTF3 (Scheme 1). An X-ray diﬀraction crystal

structure of Au-DiBTF3 as well as experimental procedures

for the synthesis of all four compounds are given in the

Supporting Information.

The new gold(I) organometallics were subjected to

thermogravimetric analysis (TGA). The mass loss as a function

of temperature was dependent on the chemical identity of the

ligands. Mass loss began at 230 °C in Au-DiBTF0, 260 °C in

Au-DiBTF1, 350 °C in Au-DiBTF2, and 310 °C in Au-

all four complexes.

Fluorescence lifetimes (τ ), ﬂuorescence quantum yields

(φ ), and intersystem crossing quantum yields (φ_t_r_i_p_l_e_t) were

obtained for the new gold(I) complexes (Table 1). Data from

Organometallics XXXX, XXX, XXX−XXX

Organometallics

DiBTF2 ′ (Figure 2 ). However, Au-DiBTF3 ′ has a much

Communication

Table 1. Photophysical Properties of Digold Alkynyl and Triazolyl Complexes at 298 K in Toluene

complex

Au-DiBTF0

Au-DiBTF1

Au-DiBTF2

Au-DiBTF3

λ_a_b_s(nm)

383

6.00

384

5.10

387

6.76

390

6.99

ε (10 M cm⁻¹

−1

)

λ_f_l(nm)

φ_f_l

413

418

427

0.55 ± 0.02

0.51 ± 0.01

438 ± 5

1.3 × 10

≤2.3 × 10

0.55 ± 0.02

0.43 ± 0.02

442 ± 1

1.2 × 10⁹

0.44 ± 0.01

0.57 ± 0.02

380 ± 1

1.2 × 10⁹

0.28 ± 0.02

0.68 ± 0.01

369 ± 1

7.6 × 10⁸

1.08 × 10

2.4 × 10⁹

φ_t_r_i_p_l_e_t

τ (ps)

k_r(s⁻¹)

k_n_r(s⁻¹)

k_I_S_C(s⁻¹)

≤4.5 × 10

≤2.6 × 10

9.7 × 10⁸

1.5 × 10⁹

1.2 × 10

All of the reported molar extinction coeﬃcients were measured in dichloromethane because of the minimal solubility of Au-DiBTF3 in toluene.

The estimated error is ±5%. Details are given in the Supporting Information.

these experiments are shown in Figures S12 −S15. Au-DiBTF0

and Au-DiBTF1 display similar ﬂuorescence quantum yields,

ﬂuorescence lifetimes, and intersystem crossing quantum

yields. Comparatively, Au-DiBTF2 shows a lower ﬂuorescence

quantum yield, shorter ﬂuorescence lifetime, and increased

intersystem crossing quantum yield. This trend continues for

Au-DiBTF3, which has the lowest ﬂuorescence quantum yield,

shortest ﬂuorescence lifetime, and highest intersystem crossing

quantum yield. The combination of the ﬂuorescence lifetime,

ﬂuorescence quantum yield, and intersystem crossing quantum

yield allows for the calculation of the rate constants for

radiative decay (k ), nonradiative decay (k ), and intersystem

Au(I) in character. These results are in accord with the ligand-

centered emission observed for all of the complexes.

The results of time-dependent DFT (TDDFT) calculations

on Franck−Condon singlet and triplet states are consistent

with the similar excited-state decay pathways for the dialkynyl

^c^o^m^p^l^e^x^e^s^,^wⁱ^t^hⁿ^e^a^r^l^yⁱ^d^eⁿ^tⁱ^c^a^l^eⁿ^e^r^g^y^g^a^p^s^b^e^t^w^e^eⁿ^t^h^e^S1

and T states for the model complexes Au-DiBTF0′ and Au-

crossing (k_I_S_C). For the the alkynyl complexes (Au-DiBTF0−

), the rate constants for radiative decay and intersystem

−1

crossing have magnitudes on the order of 1 × 10 s . The rate

of nonradiative decay for these compounds is roughly 2 orders

of magnitude lower. In Au-DiBTF3, the intersystem crossing

rate constant is a factor of 2 larger than those in Au-DiBTF0−

2, the radiative decay rate constant is a factor of 2 smaller than

those of Au-DiBTF0−2, and the nonradiative decay rate

constant is appreciably larger than those of Au-DiBTF(0−2).

Despite subtle changes in their ﬂuorescence lifetimes,

ﬂuorescence quantum yields, and intersystem crossing

quantum yields, the decay pathways of the excited-singlet

states in Au-DiBTF0−2 are essentially identical. Conversion of

the alkynyl linker to a triazolyl linker in Au-DiBTF3 results in

entirely diﬀerent excited-state dynamics relative to Au-

DiBTF0−2.

Figure 2. Energies of low-lying Franck−Condon singlet and triplet

states calculated for model complexes with TDDFT.

Density functional theory (DFT) calculations were

performed on models of Au-DiBTF0, Au-DiBTF2, and Au-

DiBTF3. The trimethylphosphine ligand of Au-DiBTF0 was

retained, but methyl groups were substituted for the mesityl

groups on the N-heterocyclic carbene, methyl groups for the

ethyl groups on the ﬂuorenyl moiety, and methyl groups for

the benzyl groups on the triazolyls in Au-DiBTF3. The

computational results obtained for the (trimethylphosphine)-

gold(I) alkynyl complex are taken as representative of both

phosphine-containing complexes on the basis of their structural

similarity. Frontier Kohn −Sham orbital diagrams of model

complexes appear in Figures S16 −S18. The calculations show

that both the highest occupied Kohn−Sham molecular orbital

smaller S -to-T energy gap, in accordance with the k value

ISC

that is 2 times larger than those for Au-DiBTF0−2, along with

the highest intersystem crossing quantum yield, lowest

ﬂuorescence quantum yield, and shortest ﬂuorescence lifetime

among all of the complexes (Figure 2).

Four new laterally asymmetric dinuclear gold(I) complexes

originating from a benzothiazole ﬂuorenyl dialkynyl ligand

have been synthesized and photophysically characterized. Our

results demonstrate that modiﬁcation of the ancillary ligand

(

phosphine to carbene) or the linkage (alkynyl to triazole)

changes the ground- and excited-state properties. Slight

structural modiﬁcations in the phosphine capping ligands

exert muted eﬀects on the ground-state absorption spectra,

luminescence spectra, ﬂuorescence quantum yield, intersystem

crossing quantum yield, and ﬂuorescence lifetime. Slight

decreases in the ﬂuorescence quantum yield and ﬂuorescence

lifetime coinciding with a slight enhancement of the

intersystem crossing quantum yield were observed when the

(

HOMO) and the lowest unoccupied Kohn−Sham molecular

orbital (LUMO) are localized almost entirely on the BTF

dialkynyl ligand. Kohn −Sham orbital energy level diagrams are

also shown in Figures S16 −S18. In all three complexes, the

HOMO is composed of greater than 96% alkynyl ligand

electron density, with the remaining electron density largely

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Complete contact information is available at:

Communication

ancillary ligand was changed from a phosphine to an N-

heterocyclic carbene. This results in nearly identical singlet-

state dynamics for Au-DiBTF0−2. Changing the bridging

moiety results in the largest impact on the excited-state

dynamics, as observed in the photophysical behavior of the

triazolyl complex (Au-DiBTF3). The ground-state absorption

and luminescence spectra of this complex show the largest

bathochromic shifts in the series. These are accompanied by

greater variation in the ﬂuorescence quantum yield, inter-

system crossing quantum yield, and ﬂuorescence lifetime. The

kinetic evolution of the singlet excited state of Au-DiBTF3

diﬀers from that of the alkynyl-bridged counterparts. Computa-

tional results echo the observed excited-state behavior, as Au-

DiBTF0 has an S -to-T energy gap almost identical to that of

Matthias Zeller − Department of Chemistry, Purdue University,

West Lafayette, Indiana 47907, United States; orcid.org/

0000-0002-3305-852X

Joy E. Haley − Materials and Manufacturing Directorate, Air

Force Research Laboratory, Dayton, Ohio 45433, United States

Kimberly de La Harpe − Department of Physics, United States

Air Force Academy, U.S. Air Force Academy, Colorado 80840,

United States

Tod A. Grusenmeyer − Materials and Manufacturing

Directorate, Air Force Research Laboratory, Dayton, Ohio

45433, United States; orcid.org/0000-0002-1842-056X

https://pubs.acs.org/10.1021/acs.organomet.9b00768

Au-DiBTF2, whereas Au-DiBTF3 has the smallest S -to-T

Notes

energy gap of the complexes, suggesting a higher intersystem

crossing rate constant and quantum yield compared with Au-

DiBTF0−2. To conclude, this study demonstrates how

changes in both the ancillary ligands and the linkages to the

gold atoms alter the excited-state dynamics in a series of

dinuclear gold complexes. Research is currently underway in

this laboratory to further investigate the eﬀects of both the

ancillary ligand and the linkage to the gold atom on the

excited-state dynamics in a variety of systems containing gold.

The authors declare the following competing ﬁnancial

interest(s): Case Western Reserve University has ﬁled for

patent protection of the gold(I) organometallics described

herein.

ACKNOWLEDGMENTS

■

This work was supported by the Air Force Oﬃce of Scientiﬁc

Research (Contract FA9550-18-1-0247 to T.G.G.). All of the

AFRL-aﬃliated authors recognize the Air Force Oﬃce of

Scientiﬁc Research under AFOSR Award 9550-19-1-18RX056

and Air Force Research Laboratory/RXAP Contract FA8650-

ASSOCIATED CONTENT

sı Supporting Information

■

6-D-5402-0001. M.Z. acknowledges funding for the single-

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.organomet.9b00768.

crystal X-ray diﬀractometer from NSF through the Major

Research Instrumentation Program (Grant CHE 1625543).

Experimental details, synthetic procedures, and compu-

tational and crystallographic data (PDF)

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