Symmetrical and Asymmetrical Thiophene-Coumarin-Based Organic Semiconductors

Abstract

Organic semiconductors are a valuable material class for optoelectronic applications due to their electronic and optical properties. Four new symmetric and asymmetric thiophene-coumarin derivatives were designed and synthesized via Pd-catalyzed Suzuki and Stille Cross-Coupling reactions. Single crystals of all synthesized thiophene-coumarin derivatives were obtained, and π···π interactions were observed among them. The π···π interactions were supported by UV-vis, transmission electron microscopy, and atomic force microscopy analyses. The photophysical and electrochemical properties of the coumarins were investigated and supported by density functional theory studies. Fluorescence quantum yields were recorded between 36 and 66%. Moreover, mega Stokes shifts (175 nm or 8920 cm^-1) were observed in these new chromophore dyes. The emission and absorption colors of the thiophene-coumarin compounds differed between their solution and film forms. Electrochemically, the highest occupied molecular orbital levels of the coumarins increased with the 3,4-ethylenedioxythiophene group, leading to a narrowing of the band gap, while the phenyl bridge weakened the donor-acceptor interaction, expanding the band gap.

Scheme 1. Synthesis Procedure of Thiophene-Coumarin Derivatives

Figure 1

(A) Ball–stick style drawings of the molecular structure showing the corresponding inter-ring dihedral angles (θ1 = 23.19° and θ2 = 6.14°) between the planes in ETQ (B) Perspective view of S···S and C–H···O intermolecular hydrogen bonding interactions (C) Perspective view of the molecular arrangement in ETQ (D) Intermolecular π···π-stacking interactions and C–H···O intermolecular hydrogen bonding interactions in ETQ (a: C16–H16···O4, d(H···O) = 2.470 Å, b: π···π, d(π···π) = 3.553(6) Å, c: S1···S1; d(S···S) = 3.296(16) Å, d: C18–H18C···O2, and d(H···O) = 2.594 Å).

Figure 2

(A) Ball–stick style drawings of the molecular structure showing the corresponding inter-ring dihedral angles (θ1 = 23.48° and θ2 = 4.25°, between the planes in DTQ (B) Perspective view of C–H···O intermolecular hydrogen bonding interactions (C) Perspective view of herringbone-like packing viewed down the c-axis in DTQ (D) Intermolecular π···π-stacking interactions and C–H···O intermolecular hydrogen bonding interactions in DTQ (a: C3–H3···O2, d(H···O) = 2.717 Å, b: C3–H3···O1, d(H···O) = 2.622 Å, c: C7–H7···O1, d(H···O) = 2.478 Å, d: π···π, d(π···π) = 3.608(3) Å, e: π···π, and d(π···π) = 3.562(3) Å)).

Figure 3

(A) Ball–stick style drawings of the molecular structure showing the corresponding inter-ring dihedral angles (∼θ1 = 51.72°:θ2 = 4.69° and ∼θ3 = 48.61°:θ4 = 26.42°) between the planes in TPQ. (B) Perspective view of C–H···O intermolecular hydrogen bonding interactions along the a-axis (C,D) Perspective view of intermolecular C–H···π interactions with various distances (c = C17–H17···π(thiophene), d(H···π) = 2.95 Å, d = C3–H3···π(phenyl), d(H···π) = 2.83 Å, e = C26–H26···π(phenyl), d(H···π) = 2.91 Å, f = C44–H44···π(thiophene), d(H···π) = 2.93 Å, g = C38–H38···π(phenyl), d(H···π) = 2.91 Å, h = C41–H41···π(coumarin), d(H···π) = 2.95 Å, i = C42–H42···π(coumarin), and d(H···π) = 2.93 Å) (E) Herringbone-like arrangement is viewed down the c-axis in TPQ.

Figure 4

(A) Ball–stick style drawings of the molecular structure showing the corresponding inter-ring dihedral angles (∼θ1 = 5.85°:θ2 = 45.35° and ∼ θ3 = 8.71°:θ4 = 19.13°) between the planes in EPQ (B) Perspective view of C–H···O intermolecular hydrogen bonding interactions (a = C38–H38···O4, d(H···O) = 2.692 Å, b = C42–H42···O4, d(H···O) = 2.665 Å, c = C12–H12···O9, and d(H···O) = 2.614 Å) along the a-axis (C) perspective view of the shortest intermolecular π···π interaction (d = π(coumarin)···π(thiophene) and d(π···π) = 3.574 Å in EPQ).

Figure 5

(A) UV–vis spectra of thiophene-coumarin derivatives (DTQ, ETQ, EPQ, and TPQ) in DCM. (B) Emission spectra of thiophene-coumarin derivatives (DTQ, ETQ, EPQ and TPQ) in DCM (λ_start^emission for DTQ = 357 nm; λ_start^emission for EPQ = 337 nm; λ_start^emission for ETQ = 350 nm; and λ_start^emission for TPQ = 347 nm). (C) Emission spectra of thiophene-coumarins (EPQ, TPQ, ETQ, and DTQ) on thin films. (D) UV–vis spectra and TEM-AFM images of thiophene-coumarins (EPQ, TPQ, ETQ, and DTQ) on thin films.

Figure 6

TCSPC trace for DTQ, EPQ, ETQ, and TPQ in DCM. (λ^em for DTQ = 490 nm; λ^em for EPQ = 515 nm; λ^em for ETQ = 550 nm; and λ^em for TPQ = 500 nm).

Figure 7

CV and DPV voltammograms of thiophene-coumarin derivatives in a 0.1 M TBAPF₆/DCM electrolyte solution at a scan rate of 100 mV/s Ag wire.

Figure 8

DFT-calculated energy levels and band gaps of thiophene-coumarin derivatives compared with experimental values.

Figure 9

DFT-calculated absorption spectrum of DTQ compared with the experimental spectrum in DCM.

Figure 10

Electron density maps of thiophene-coumarin derivatives (DTQ, EPQ, ETQ, and TPQ). Red areas represent electronegativity, and purple areas represent electropositivity.