Materials

Unless otherwise noted, all commercial materials were used without further purification. Solvents for the synthesis were obtained from Sigma-Aldrich. Malononitrile (Aldrich, > 99%) was recrystallized from ether45. Pseudosaccharyl chloride was prepared following a described procedure46. 3,4-Dimethylanisole (dMA, Aldrich, > 99%), isopropylidenemalononitrile (iPN, TCI Europe, > 98%) were used as received. EAC (Carlo Erba, > 99%), chloroform (CHF, Carlo Erba ≥ 99.9%) and dichloromethane (DCM, Sigma Aldrich, ≥ 99.9%) were used without additional purification. IPE (Merck, 98%) was dried over CaSO 4 and passed by activated alumina column. NBE (NBE, Acros Organics, ≥ 99.9) passed through an activated silica column, and then distilled over CaCl 2 .

Synthesis of the estrone derivative 1

The estrone derivative 1 was prepared according the following scheme:

Synthesis of the estrone derivative 1b (3-methoxy-1,3,5 (10)-estratrien-17-one). In a pressure tube 1.0 g (3.7 mmol) of estrone (3-hidroxy-1,3,5 (10)-estratrien-17-one) (1a), 1.2 g (3.6 mmol) of potassium carbonate and 2.0 mL of methyl iodide (16 mmol) were added to 50 mL of acetone. The mixture was placed in an oil bath at 60 °C for 24 h. The formation of the methyl ether was monitored by NMR. Incomplete methylation led to the addition of more methyl iodide and the reaction was continued. The precipitate formed was filtered and the liquid evaporated at reduced pressure. The solid residue was recrystallized in dichloromethane/methanol originating 0.81 g (η = 78 %) of the estrone derivative 1b (m.p. = 168–169.5 °C).

Synthesis of the estrone derivative 1 (3-methoxy-1,3,5 (10)-estratrien-17-yliden)malononitrile). The introduction of the dicyano group was carried out using described procedures4,47. In a round flask 400 mg of 1b (1.4 mmol), 590 mg of ammonium acetate, 1.52 mL of acetic acid and 309 mg (4.68 mmol) of malononitrile were added to 30 mL of toluene. The solution was refluxed overnight in a Dean-Stark apparatus, under nitrogen. The solution was treated with a saturated solution of NaHCO 3 (50 mL), washed with water, dried over Na 2 SO 4 and the solvent evaporated under reduced pressure. The residue was subject to column chromatography (silica, DCM and then EAC). The product was recrystallized in ethanol originating 273 mg (η = 56 %) of 1 (m.p. = 188–189 °C; 190 °C)48.

Synthesis of the estrone derivative 2

The estrone derivative 2 was prepared according the following scheme

The estrone derivative 2b (3-[1,2-benzisothiazole-1,1-dioxide]-1,3,5 (10)-estratrien-17-one) was prepared following a described procedure49. In a round bottom flask 1.2 g of 1a (4.4 mmol), 0.7 mL of triethylamine and 1.0 g (4.9 mmol) of pseudosaccharyl chloride were added to 100 mL of toluene and refluxed for 2 h under N 2 . The hot solution was filtered and the liquid left to cool to room temperature. The precipitate 2b was filtered and dried. The material was chromatographed (silica-gel DCM/ethyl ether 3:1) to yield 0.70 g (η = 36 %) of 2b (m.p. = 261–263 °C (decomp.).

The estrone derivative 2c (1,3,5 (10)-estratrien-17-one) was prepared following a described procedure49. In a round bottom flask 0.45 g (0.10 mmol) of 2b and 1.0 g of C/Pd (10 %) were added to 100 mL of benzene and heated to reflux. A solution of 2.6 g (30 mmol) of sodium hypophosphite in 50 mL of water was added stepwise and the mixture was stirred under reflux during 4 h. After cooling to room temperature the catalyst was filtered. The solution was extracted with ethyl ether, washed with water and dried with anhydrous sodium sulfate. Solvent evaporation originates a solid material that was chromatographed (silica-gel, DCM/ethyl ether 10:1). The first fraction was collected and corresponded to 0.20 g (0.078 mmol) of 2c (η = 78 %). m.p. = 134–135 °C (134–135 °C)49.

The estrone derivative 2 (1,3,5 (10)-estratrien-17-yliden)malononitrile) was prepared following a described procedure4,47. In a round flask 0.25 g of 2c (0,098 mmol) 0.37 g (4.8 mmol) of ammonium acetate, 1.0 mL of acetic acid and 0.19 g (2.9 mmol) of malononitrile were added to 30 mL of toluene. The solution was refluxed overnight in a Dean-Stark apparatus, under nitrogen. The solution is treated with a saturated solution of NaHCO 3 (50 mL), washed with water, dried over Na 2 SO 4 and the solvent evaporated under reduced pressure. The residue is chromatographed (silica- DCM and then EAC). The product is recrystallized in ethanol originating 0.19 g (η = 65 %) of 2 (m.p. = 200–202 °C).

The characterization of estrone derivatives 1 and 2 is described in Supplementary Note 1 and Supplementary Figures 1-5.

Conformational searches and DFT calculations

A first search of conformers was made with OpenBabel using the confab model, and Marvin 17.2.27 2017 ChemAxon using the conformer plugin. We set a threshold of 50 kcal mol–1 in OpenBabel and did not find any conformers. In Marvin we used the strict optimization limit and the hyperfine post-processing step. All the conformers found differ by minor orientations of the methoxy group linked to the donor moiety, which are irrelevant for the electronic coupling between donor and acceptor moieties. The Supplementary Movie 1 offers various perspectives of the overlaid conformers and shows their similarity. It is very much likely that they collapse into one single conformer with more accurate energy minimization procedures.

Conformational search with GAMESS50 using the B3LYPV1R hybrid functional51,52,53 and the 6–31 G(d) Pople basis set for all atoms54, revealed the presence of only one low energy conformer in the ground state of 1. All singlet states were described using RHF formalism and the triplet state used UHF formalism. There was no relevant spin contamination on the UHF calculations, < S2 > = 2.022. It is clear that the molecules are not conformationally flexible in the region covering donor, spacer and acceptor, and the electronic coupling is unlikely to change with the temperature.

Geometries and conformations are presented in Supplementary Note 2, Supplementary Table 1 and Supplementary Movie 1).

Time-resolved measurements

Fluorescence decays in the time window between 250 ps to 20 ns were measured using a home-built TCSPC apparatus55 with a Horiba-JI-IBH NanoLED (λ ex = 282 nm) as excitation source. Fluorescence decays times with shorter time resolution were investigated using a picosecond time correlated single photon counting apparatus (TCSPC, λ ex = 272–273 nm)56. The excitation source consisted of a picosecond Spectra Physics mode-lock Tsunami laser (Ti:sapphire) model 3950 (repetition rate of about 82 MHz, tuning range 700–1000 nm), pumped by a Millennia Pro-10s, frequency-doubled continuous wave (CW), diode-pumped, solid-state laser (λ em = 532 nm). A harmonic generator model GWU-23PS (Spectra-Physics) was used to produce a third harmonic from the Ti:sapphire laser exciting beam frequency output. Deconvolution of the fluorescence decay curves was performed using the modulating function method, as implemented by G. Striker in the SAND program57. All the fluorescence decays were measured in 5 or 10 mm quartz cuvettes in the presence of oxygen. Temperatures control was achieved using a cryostat Optistat DN2 (188–308 K) or cuvette holder Flash 300 (253–328 K).

Flash photolysis employed excitation at 266 nm from the fourth harmonic of Nd:YAG laser (Spectra Physics) and the Applied Photophysics LKS.60 laser-flash-photolysis spectrometer58. Samples for flash photolysis were measured in the presence of oxygen, and in inert atmosphere (samples were bubbled with N 2 for 30 min prior to every experiment).

The experimental setup for the ultrafast spectroscopic and kinetics measurements consisted of a broadband (340–1600 nm) HELIOS pump-probe femtosecond transient absorption spectrometer from Ultrafast Systems, equipped with an amplified femtosecond Spectra-Physics Solstice-100F laser (displaying a pulse width of 128 fs and 1 kHz repetition rate), coupled with a Spectra-Physics TOPAS Prime F optical parametric amplifier (195–22 000 nm) for pump pulse generation. Samples of dMA, 1 and 2 were excited with 283, 287, or 273 nm laser pulses at pulse energies of 3, 1, or 1.5 µJ respectively. The probe light in the UV range was generated by passing a small portion of the 795 nm light from the Solstice-100F laser through a computerized optical delay (with a time window of up to 8 ns) and then focusing in a vertical translating CaF 2 crystal to generate a white-light continuum (340–650 nm). All the measurements were made in 1 or 2 mm quartz cuvettes, with absorptions in the range 0.2–0.5 at the pump excitation wavelength. To avoid photodegradation, the sample was kept in movement using a motorized translating sample holder or stirred.

Steady-state spectroscopic measurements are described in Supplementary Note 3.

Analysis of kinetic data

The transient absorption data were analyzed using the Surface Xplorer PRO program from Ultrafast Systems and Glotaran for global and target analysis59. The results from several scans of freshly prepared samples were averaged. Each scan collected around 1000 time points at 310 different wavelengths. Global and target analysis simultaneously analyzed at least 100 wavelengths. A strong nonresonant signal with a relaxation time below 1 ps was observed in all samples and assigned to the solvent and cuvette. In order to eliminate this signal, for each experimental condition employed to study the samples, an experiment was performed just with the solvent in the cuvette. The normalized solvent response was subtracted from sample data point measured under exactly the same conditions. Transient spectra were also corrected for the dispersion of the probe light resulting from propagation through the crystal and sample (chirp correction).

A sequential kinetic scheme with species of increasing lifetimes was used to fit transient spectra collected for each sample, resulting in Evolution-Associated Spectra (EAS). The number of EAS (2 or 3 after the hot state) required to fit the spectra was estimated by inspection of the residuals. The EAS correspond to true SAS when the initially prepared Franck-Condon state of dMA decays to the relaxed singlet state and then to the triplet state (peak at 310 nm and shoulder at 380 nm), or when the relaxed singlet state is quenched by iPN and leads to the aromatic cation (absorption band ≈460 nm). The decays of 1 and of 2 follow the mechanism of Fig. 1, schematically represented in Eq. (5). When the triplet energy is close or higher than that of the charge-transfer (CT) species, which is the case for 1 in DCM, EAC and CHF, the decays can be fit with a sequential kinetic scheme with two species (in addition to the hot state), Eq. (5A), and the EAS are the true SAS. In the other cases, we attempted to use Target Analysis with the mechanism of Eq. (5B), with branching and equilibrium, which yields SAS. For 1 in NBE and IPE and for 2 in CHF the Target Analysis with 3 species (in addition to the hot state) gives all positive spectra, which are true SAS. For 2 in DCM, EAC, and NBE the spectra were very weak for a reliable Target Analysis, and Global Analysis with sequential kinetic scheme involving three species (in addition to the hot state) was performed. This revealed the need for an additional lifetime and the difficulty to associated it with a spectrum. In this case the EAS are not the true SAS

Femtosecond transient absorption (time resolution between 500 fs and 10 ns) allowed us to measure the formation and decay of the 1CT state but the locally excited triplet is formed and decays outside this time window. This is the reason why T 1 is not considered in Eqs. (5A) and (5B) used to obtain the EAS and SAS, respectively. Single photon counting (SPC, two excitation sources were used, a nanoLED and a laser with instrumental responses of 1 ns and 22 ps, respectively) could not be used to see the initial charge separation. Laser flash photolysis (instrumental response of 20 ns) allowed for the direct observation of T 1 .

SPC data were interpreted with an adaptation of the Birks excimer mechanism presented in Eq. (5C). The difference is that in the Birks mechanism two monomers have to diffuse to yield the excimer and this is a bimolecular reaction, whereas in our mechanism intersystem crossing between singlet and triplet states of the charge transfer (CT) species are first-order reactions. The important consequence of this difference is that the rate of the decay of the monomer in the Birks mechanism (corresponding to 1k CR in our mechanism) can be obtained at high dilution of the monomer, when it becomes the only relevant decay of the monomer (corresponding to 1CT in our mechanism), whereas in our mechanism there is no independent experimental measurement to obtain 1k CR . As shown in Supplementary Note 6, this can be circumvented with a reasonable estimate of the ratio between 1k isc and 3k isc .

The charge recombination rates independently measured by femtosecond transient absorption and single photon counting are in very good agreement. Figure 5 present the rates from single photon counting because they are better accommodated in the time window of this technique.

Electron transfer model

When λ s → 0, the dominant factor of the low temperature limit for a symmetrical radiationless transfer between two electronic states is40

$$k = \frac{2\pi}{\hbar }\left| {V^2} \right|\frac{1}{{\hbar \omega_s}}\exp \left[ { - \frac{{d\sqrt {2\mu_{\mathrm{DA}}\Delta E^{\ddagger}}}}{\hbar }} \right]$$ (6)

which shows that the nuclear Franck-Condon factor for symmetrical reactions and for exoergic processes can be recast in a form which is practically identical with the Gamov formula. For exoergic processes, the displacement d between the minima of the oscillators must be replaced by the barrier width ∆x, which is the horizontal distance between the turning points of vibration of the oscillator in the initial and final states of a radiationless transition,

$$\Delta x = d - \sqrt {\frac{{2\left| {\Delta E} \right|}}{f}}$$ (7)

where the harmonic force constant of the vibrational mode is given by its angular frequency of oscillation, \(\omega _

u = \sqrt {f/\mu }\), knowing the oscillator reduced mass µ. For example, the asymmetric CC stretching mode of benzene is observed at 1309 cm–1 and can be reproduced with a harmonic force field using a CC stretching mode with a force constant f CC ≈ 1000 kcal mol–1 Å–2 60. This vibration of the benzene ring is also described as the Kekulé mode and corresponds to three CC oscillators being simultaneously displaced from their equilibrium positions in the transfer of benzene from the initial to the final state. Hence, in a first approximation, the effective reduced mass in Eq. (3) should be µ benzene = 3µ CC . However, Eq. (3) was derived for radiationless transitions within a given molecule39, while in ET two molecules are involved (or two independent moieties is the same molecule). Within the approximations used to derive Eq. (3), the identical and similarly displaced oscillators involved in the transitions in these moieties have the same frequencies and reduced masses. Hence, the barriers ∆E‡ are the same for all the oscillators and the total reduced mass for the hypothetical case of two benzene molecules exchanging an electron is µ DA = [(µ benzene )1/2 + (µ benzene )1/2]2. For the case of molecules 1 and 2, one of the moieties can be approximated as the benzene ring and the other as dicyanoethylene. We have shown before that the average of the force constants of the relevant oscillators gives f CC ≈ 1.15 × 103 kcal mol–1 Å–2, and that the effective reduced masses are µ benzene = 3µ CC , µ dicyanoethylene = µ CC + 2µ CN 30,43,44. In general the effective reduced mass of the donor-acceptor system is

$$\mu _{{\rm{DA}}} = \left( {\sqrt {\mu _{{\rm{donor}}}} + \sqrt {\mu _{{\rm{acceptor}}}} } \right)^2$$ (8)

The nuclear tunneling rate constants calculated with Eq. (3) employed ∆x calculated with Eqs. (7) and (4), effective reduced masses calculated with Eq. (8) and ν≈2 × 1011 s–1. The tunneling rates are larger than thermal activation rates calculated over the same energy barrier

$$k_{{\rm{th}}} =

u \exp \left( { - \frac{{\Delta E^\ddagger }}{{{\mathrm{RT}}}}} \right)$$ (9)

for ∆G0 < –20 kcal mol–1. However, thermal activation dominates the rates in the normal region. Figure 5 combines the thermal activates rates in the normal region with the tunneling rates elsewhere.

In summary, ISM calculations employed the following set of parameters: a′ = 0.156, n‡ = 1.75, l eq = 1.37 Å, f = 1.14 × 103 kcal mol–1 Å–2, Λ = 70 kcal mol–1, µ donor = 19 amu and µ acceptor = 18 amu. These parameters are entirely consistent with those employed in our earlier applications of ISM to ET reactions35,44.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Correspondence and requests for materials should be addressed to Prof. Luis Arnaut (lgarnaut@ci.uc.pt).