V-Shaped Tröger Oligothiophenes Boost Triplet Formation by CT Mediation and Symmetry Breaking

Abstract

A new family of molecules obtained by coupling Tröger's base unit with dicyanovinylene-terminated oligothiophenes of different lengths has been synthesized and characterized by steady-state stationary and transient time-resolved spectroscopies. Quantum chemical calculations allow us to interpret and recognize the properties of the stationary excited states as well as the time-dependent mechanisms of singlet-to-triplet coupling. The presence of the diazocine unit in Tröger's base derivatives is key to efficiently producing singlet-to-triplet intersystem crossing mediated by the role of the nitrogen atoms and of the almost orthogonal disposition of the two thiophene arms. Spin-orbit coupling-mediated interstate intersystem crossing (ISC) is activated by a symmetry-breaking process in the first singlet excited state with partial charge transfer character. This mechanism is a characteristic of these molecular triads since the independent dicyanovinylene-oligothiophene branches do not display appreciable ISC. These results show how Tröger's base coupling of organic chromophores can be used to improve the ISC efficiency and tune their photophysics.

Scheme 1. Structures of the a and b Moieties and That of Diazocine (DA) (i.e., the Bisphenyl-Fused DA Compound Is Known as Tröger’s Base Highlighted in the Dotted Line Box)

In a, b, and DA, orbital representations of TBI and TSI (red arrows) together with their first-order perturbation couplings are shown. Some known examples of Tröger’s base derivatives and bisarene Tröger-based chromophores are shown in the solid line box. Note that the signs of the p_z orbitals are just to qualitatively describe the interaction (i.e., they do not follow a nodal pattern).

Scheme 2. Chemical Structure of Some Symmetrically Dicyanovinylene-Substituted Oligothiophenes (DCVTn) as Well as Donor–Acceptor Dicyanovinylenes with Substitution Pattern through the α Positions of the Thiophenes

The chemical structures of the bis(dicyanovinylene) oligothiophenes segmented by the DA unit or thiophenic Tröger’s bases are also shown together with the nomenclature used in this investigation.

Scheme 3. Syntheses of the New Oligothiophene Congeners of Tröger’s Base Analogues TB1, TB2, and TB3 and the Model Compound DCVT2

Figure 1

Frontier molecular orbitals (a) and their energy diagram (b) for the DCVT1 moiety (black), DCVT1 dimer (orange) and TB1, TB2, and TB3 compounds (blue), computed at the CAM-B3LYP/6-311+G(d,p) level. H: HOMO, L: LUMO.

Figure 2

UV–vis electronic absorption spectra of TB3 (panels a and d), TB2 (panels b and e), and TB1 (panels c and f) at room temperature (black lines) and at 80 K (blue lines) in 2-MeTHF (left) and at room temperature (black lines) upon treatment with TFA (pink lines) in CH₂Cl₂ solution (right).

Figure 3

Vertical excitation energies (a) and their oscillator strengths (b) for the S₁ (empty circles) and S₂ (filled circles) transitions for the following systems: the DCVT1 moiety (black) and its through-space dimer, DCVT dimer (orange), and the target molecules TB1, TB2, and TB3 compounds (blue). Quantum chemical calculations are performed at the CAM-B3LYP/6-311+G(d,p) level.

Figure 4

Left: excitation (dotted lines) and emission (solid lines) of TBn at room temperature (black lines) and at 80 K (blue lines) in 2-MeTHF. Right: absorption (dotted lines) and emission (solid lines) spectra of TBn at room temperature in toluene (dark blue lines), CHCl₃ (red lines), THF (green lines), CH₂Cl₂ (orange lines), acetone (gray lines), and CH₃CN (yellow lines). From top to bottom: (a and d) TB3, (b and e) TB2, and (c and f) TB1. Sharp peaks at ca. 450–470 nm in panels (b) and (c), which are not present in the absorption spectra, are ascribed to scattering artifacts.

Figure 5

Microsecond (top) and picosecond (middle) time-resolved transient absorption spectra in CH₂Cl₂ at 298 K. Excited state spectra comparisons (bottom) from global analysis. (a, d, and g) TB1, (b, e, and h) TB2, and (c, f, and i) TB3. Microsecond TAS was obtained upon excitation at 100 μJ/cm² at 415, 450, and 500 nm for TB1, TB2, and TB3, respectively. ps-TAS was obtained upon excitation at the same wavelength but with a power of 0.5 mW.

Figure 6

State energy (in eV) diagram of the TDDFT (CAM-B3LYP/6-311+G(d,p)) triplet states of TB1 with respect to the relaxed S₁ and associated S₁ → T_n SOC values (cm^–1). Electron/hole pair densities (orange/blue) are shown for the S₁ and T₄ states as obtained at the S₁ equilibrium geometry.

Figure 7

Jablonski diagram representing the photoinduced processes (SB: symmetry breaking, EL: electron localization, FL: fluorescence, Abs: absorption, ISC: intersystem crossing) in the TBn compounds. Decay lifetimes of the triplet excited states are for TB2 and TB3.