Two-photon absorption properties of proquinoidal D-A-D and A-D-A quadrupolar chromophores

Abstract

We report the synthesis, one- and two-photon absorption spectroscopy, fluorescence, and electrochemical properties of a series of quadrupolar molecules that feature proquinoidal π-aromatic acceptors. These quadrupolar molecules possess either donor-acceptor-donor (D-A-D) or acceptor-donor-acceptor (A-D-A) electronic motifs, and feature 4-N,N-dihexylaminophenyl, 4-dodecyloxyphenyl, 4-(N,N-dihexylamino)benzo[c][1,2,5]thiadiazolyl or 2,5-dioctyloxyphenyl electron donor moieties and benzo[c][1,2,5]thiadiazole (BTD) or 6,7-bis(3',7'-dimethyloctyl)[1,2,5]thiadiazolo[3,4-g]quinoxaline (TDQ) electron acceptor units. These conjugated structures are highly emissive in nonpolar solvents and exhibit large spectral red-shifts of their respective lowest energy absorption bands relative to analogous reference compounds that incorporate phenylene components in place of BTD and TDQ moieties. BTD-based D-A-D and A-D-A chromophores exhibit increasing fluorescence emission red-shifts, and a concomitant decrease of the fluorescence quantum yield (Φ(f)) with increasing solvent polarity; these data indicate that electronic excitation augments benzothiadiazole electron density via an internal charge transfer mechanism. The BTD- and TDQ-containing structures exhibit blue-shifted two-photon absorption (TPA) spectra relative to their corresponding one-photon absorption (OPA) spectra, and display high TPA cross sections (>100 GM) within these spectral windows. D-A-D and A-D-A structures that feature more extensive conjugation within this series of compounds exhibit larger TPA cross sections consistent with computational simulation. Factors governing TPA properties of these quadrupolar chromophores are discussed within the context of a three-state model.

Figure 1

Structures of D-A-D and A-D-A chromophores, along with their respective abbreviations; note that t and d signify connectivities utilizing respectively C-C triple and double bonds.

Figure 2

Electronic absorption spectra of: (A) (DHAt)₂BTD, (B) (DHAd)₂BTD, (C) (ROPht)₂TDQ, (D) (ROPhd)₂TDQ, (E) (DHA-BTDt)₂BTD (F) (BTDt)₂ROPh, (G) (DHAtPht)₂ROPh, (H) (DHAtBTDt)₂ROPh, and (I) (DHAtROPht)₂BTD, recorded in cyclohexane, toluene, tetrahydrofuran (THF), CH₂Cl₂, acetone, and dimethylformamide (DMF) solvents.

Figure 3

Fluorescence spectra of: (A) (DHAt)₂BTD, (B) (DHAd)₂BTD, (C) (ROPht)₂TDQ, (D) (ROPhd)₂TDQ, (E) (DHA-BTDt)₂BTD, (F) (BTDt)₂ROPh, (G) (DHAtPht)₂ROPh, (H) (DHAtBTDt)₂ROPh, and (I) (DHAtROPht)₂BTD, recorded in cyclohexane, toluene, THF, CH₂Cl₂, acetone, and DMF solvents. Sharp peaks evident at ~500 and 800 nm derive from scattered light from the excitation source.

Figure 4

Lippert plots determined for the (DHAt)₂BTD, (DHAd)₂BTD, (ROPht)₂TDQ, (ROPhd)₂TDQ, (DHA-BTDt)₂BTD, (BTDt)₂ROPh, (DHAtPht)₂ROPh, (DHAtBTDt)₂ROPh, and (DHAtROPht)₂BTD chromophores.

Figure 5

Potentiometrically determined E_1/2^0/+ and E_1/2^−/0 values measured in CH₂Cl₂ solvent for: (A) (DHAt)₂BTD, (B) (DHAd)₂BTD, (C) (ROPht)₂TDQ, (D) (ROPhd)₂TDQ, (E) (DHA-BTDt)₂BTD, (F) (BTDt)₂ROPh, (G) (DHAtPht)₂ROPh, (H) (DHAtBTDt)₂ROPh, and (I) (DHAtROPht)₂BTD. Experimental conditions are described in Table 3. Redox potentials shown are relative to the ferrocene/ferrocenium (Fc/Fc⁺) redox couple, which was used as an internal standard in these experiments.

Figure 6

Two-photon fluorescence excitation spectra determined in toluene solvent for: (A) (DHAt)₂BTD, (B) (DHAd)₂BTD, (C) (ROPht)₂TDQ, (D) (ROPhd)₂TDQ, (E) (DHA-BTDt)₂BTD, (F) (BTDt)₂ROPh, (G) (DHAtPht)₂ROPh, (H) (DHAtBTDt)₂ROPh, and (I) (DHAtROPht)₂BTD. Systematic error for δ is ±30% for each point.

Figure 7

Normalized one-photon absorption and two-photon excitation spectra of: (A) (DHAt)₂BTD, (B) (DHAd)₂BTD, (C) (ROPht)₂TDQ, (D) (ROPhd)₂TDQ, (E) (DHA-BTDt)₂BTD, (F) (BTDt)₂ROPh, (G) (DHAtPht)₂ROPh, (H) (DHAtBTDt)₂ROPh, and (I) (DHAtROPht)₂BTD. The solid line represents the one-photon absorption spectrum. The two-photon fluorescence excitation spectral data are displayed as solid circles (the dotted line serves only as a guide to the eye). Except for (ROPhd)₂TDQ (D), the two-photon fluorescence excitation spectral data are plotted as a function of the total transition energy (twice the photon energy). Note that the (ROPhd)₂TDQ fluorescence excitation data is plotted as a function of the photon energy to highlight the extent to which the TPA magnitude rises near the low energy absorption edge.

Figure 8

The dependence of the two-photon excited fluorescence intensity, plotted logarithmically as a function of the logarithm of the laser power P, for chromophores (A) (DHAt)₂BTD, (B) (DHAd)₂BTD, (C) (ROPht)₂TDQ, and (D) (DHAtPht)₂ROPh.

Scheme 1

Synthetic routes to D-A-D and A-D-A chromophores and their precursors.

Scheme 1

Synthetic routes to D-A-D and A-D-A chromophores and their precursors.

Scheme 1

Synthetic routes to D-A-D and A-D-A chromophores and their precursors.

Scheme 1

Synthetic routes to D-A-D and A-D-A chromophores and their precursors.