Extended shortwave infrared absorbing antiaromatic fluorenium-indolizine chromophores

Abstract

Shortwave infrared (SWIR, 1000-1700 nm) and extended SWIR (ESWIR, 1700-2700 nm) absorbing materials are valuable for applications including fluorescence based biological imaging, photodetectors, and light emitting diodes. Currently, ESWIR absorbing materials are largely dominated by inorganic semiconductors which are often costly both in raw materials and manufacturing processes used to produce them. The development of ESWIR absorbing organic molecules is thus of interest due to the tunability, solution processability, and low cost of organic materials compared to their inorganic counterparts. Herein, through the combination of heterocyclic indolizine donors and an antiaromatic fluorene core, a series of organic chromophores with absorption maxima ranging from 1470-2088 nm (0.84-0.59 eV) and absorption onsets ranging from 1693-2596 nm (0.73-0.48 eV) are designed and synthesized. The photophysical and electrochemical properties of these chromophores, referred to as FluIndz herein, are described via absorption spectroscopy in 17 solvents, cyclic voltammetry, solution photostability, and transient absorption spectroscopy. Molecular orbital energies, predicted electronic transitions, and antiaromaticity are compared to higher energy absorbing chromophores using density functional theory. The presence of thermally accessible diradical states is demonstrated using density functional theory and EPR spectroscopy, while XRD crystallography confirms structural connectivity and existence as a single molecule. Overall, the FluIndz chromophore scaffold exhibits a rational means to access organic chromophores with extremely narrow optical gaps.

Fig. 1. Absorption maxima of indolizine xanthene dyes containing oxygen-based cores (blue), silicon-based cores (purple), and fluorene-based cores (red), and alkyl amine donor-based fluorene dye (green), along with computed orbital energy levels of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) with the HOMO–LUMO energy gap (E^H−L_g). Energy levels obtained from density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level of theory with dichloromethane (DCM) implicit solvation.

Scheme 1. Synthesis of FluIndz dyes (top) and 1,7DBA indolizine donor (bottom).

Fig. 2. Molar absorptivity of FluIndz dyes in CS₂ (left) and electrochemical potentials (E_S+/S, E_S/S−, and E_S+/S*) with optical energy gaps in DCM (right). Electrochemical potentials shown are referenced to Fc⁺/Fc at 0.00 V in DCM with a 0.1 M Bu₄NPF₆ supporting electrolyte.

Fig. 3. Frontier molecular orbitals including HOMO (bottom) and LUMO (top) of FluIndz dyes ^2PhFluIndz (left), ^1PhFluIndz (center left), ^7DMAFluIndz (center right), and ^1,7DBAFluIndz (right).

Fig. 4. NICS_ZZ(avg) (in ppm) for both the singlet ground state (S₀) and first triplet state (T₁) as the excited state. Ar = o-tolyl (^tolRosIndz) or o-xylyl (SiRos1300 and ^2PhFluIndz).

Fig. 5. Variable temperature EPR experiment: spectrum of the first derivative absorption of ^2PhFluIndz in PhCl solution from 25 °C (g = 2.0034) up to 127 °C (g = 2.0035).

Fig. 6. XRD crystal structure of ^1PhFluIndz dye (the counteranion and solvent molecules have been omitted to enhance clarity).

Fig. 7. Photostability of FluIndz dyes under 1 sun irradiation in DCM solution. Trials were done in triplicate (error bars given as the standard deviation in % absorbance remaining at each time point) and each data set is fit with an exponential function.