Electron transfer (ET) is among the most fundamental and ubiquitous chemical reactions, including oxidation-reduction, electrochemical, and photo-induced charge separation reactions.1 This process involves changes in oxidation states as well as energy transfer. Since the 1950s, extensive experimental and theoretical studies have delved into the mechanisms of ET and the factors affecting its rate.2 The ability to manipulate and exploit ET for donor-bridge-acceptor (D-B-A) molecules has opened up significant avenues in various fields, ranging from nanotechnology to renewable energy and biochemistry.3, 4 Particularly, the bridge plays a significantly role in influencing the electron trans ET fer process. For example, superexchange can mediate ET over σ-type bridge,5, 6 whereas π-type bridge provides opportunity for electron hopping.7, 8 Many chemical methods have been proposed to regulate the ET process by changing the type or length of the bridge.9, 10 However, the effect of excited bridge on the ET process remains unknown (Scheme 1).
Usually, the excitation energies of traditional σ-, or π-type bridges fall within the high-energy ultraviolet region, often exceeding the first excitation energy of the donor or acceptor. Therefore, the contribution of excited bridges cannot be determined. It presents a challenge when attempting to regulate the bridge in an excited state while concurrently maintaining the donor and acceptor in their ground states. The key is to find an example where the excitation energy of the bridge is lower than the first excitation energy of the donor or receptor. Recently, stemming from our rigorous exploration into the photochromic characteristics of a series of new crystalline inorganic-organic hybrid organophosphate-Ln-polyoxoniobates (PONbs), we have proposed an unprecedented D-f-A ET mechanism, where the 4f orbitals of Ln ions serve as ET bridges.11 Due to the shielding effect of the 4f orbitals of Ln ions by the 5s and 5p orbitals, electronic transitions between the 4f levels are hardly influenced by external environmental factors. The absorption bands of characteristic transitions exhibit strong monochromaticity and are located in the visible or near-infrared region. This feature allows us to circumvent the problem of the excitation energy of the bridge overlapping with that of the electron donor or acceptor. Therefore, the photochromic PONbs with f-type bridge can serve as ideal proof-of-concept models for studying the effects of excited bridge on the ET process.
Here, a representative crystalline organophosphate-Ln-PONb, H48K8Na4{[Dy4(CO3)4(RA)2]2[Nb32O92(H2O)4]2}·108H2O (RA-Dy-PONb, RA = risedronic acid), is selected as a case study. Through the excited bridge, the photoinduced electron transfer (PET) coloration rate increased by ∼3.3 times, and the excited bridge can sustain at least 3 h through naked eye observation. Interestingly, the colored samples can also accelerate the bleaching through the excited bridge, which is the first case that the reverse ET can be accelerated under light assistance, breaking the traditional reverse ET limited by thermal mode. The light-assisted bleaching rate is about 494 times faster than the traditional thermal relaxation mode, and the half-life (τ) of bleaching is shortened from 91.58 min to 11.13 s at room temperature (RT). Further results indicate that the excited f-type bridge significantly increases the electronic coupling, leading to accelerate ET process. Crucially, regulating the electronic state of the bridge opens up a new possibility for photochromic application in anti-counterfeiting.
The RA-Dy-PONb is synthesized by the reported procedure.11 The phase purity of their crystalline samples is confirmed by PXRD, IR and ICP analyses (see the Experimental Section and Figure S1,2 in the Supporting Information (SI)). As show in Fig. 1a, four Dy3+ ions arranged in a nearly square configuration are joined together by two RA donors and four CO32− ions to form an inorganic-organic hybrid tetranuclear {Dy4(CO3)4(RA)2} (Dy4) lanthanide clusters. Then two such square Dy4, positioned in a face-to-face fashion (Figure S3), are sandwiched by the two square PONb macrocycles {Nb32O92(H2O)4} (γ-Nb32) via sixteen corner-sharing Nb = O-Dy bonds to form a hamburger-like tri-layer hybrid composite {[Dy4(CO3)4(RA)2]2[Nb32O92(H2O)4]2} (Dy8Nb64) cluster with the dimensions ca. 2.32 × 1.86 × 1.85 nm3.
Upon irradiation by a 500 W Hg lamp (UV, ca. 110 mW·cm− 2, default coloration light source hereafter), the as-synthesized crystalline RA-Dy-PONb undergoes a clear color change from colorless to blue under ambient condition (Fig. 1b). As reported, the photochromism originated from a PET process from the photosensitive RA to the NbV through the 4f orbitals of DyIII and the formation of stable NbIV (Fig. 1b). A new broad absorption band covering the 320–1200 nm region with the absorption center around 680 nm for the colored state, can be attributed to NbIV-to-NbV intervalence charge transfer (IVCT, Fig. 1b). Time-dependent electron absorption spectra monitored at λ = 680 nm indicate that the PET coloration process follows first-order reaction kinetics, with the coloration rate constant kobs of 1.07 × 10− 1 min− 1, and maximum colorability of 2.99.
Specifically, some strong absorption bands of bridge at near-infrared region are far lower than the first excitation energy of the donor or receptor ranging 200–300 nm (Fig. 1c). For example, the absorption center around 804, 903, and 1097 nm correspond to f-f transitions of DyIII bridge from the ground state 6H15/2 to the excited states 6F7/2, 6H5/2 and 6H7/2, respectively.12, 13 The 808 nm light, which falling in the absorption band of 6H15/2 to 6F7/2 transition, can excite DyIII bridge (Fig. 1c). As showed in Fig. 1d, after pre-excitation by 808 nm laser (ca. 5.00 W⋅cm− 2, default irradiance hereafter if not otherwise specified) for 15 min, the color and absorption spectrum remains unchanged (Fig. 1d, S4). Subsequently, the region previously exposed to 808 nm light becomes even bluer after UV irradiation. The coloration rate constant for the region exposed to 808 nm light is calculated to be 3.48 × 10− 1 min− 1, which is 3.3 times faster than that of the region without 808 nm pre-excitation. The maximum colorability of the bluer region is 4.79, which is 1.8 times higher than that of other regions, consistent with the naked eye observation. As a comparison, when first irradiated with a non-f-f transition 635 nm laser (ca. 0.85 W·cm− 2) and then exposed to UV, it is observed that the coloration rate and colorability remain unchanged (Figure S5). Electron absorption spectra data show that the 635 nm light cannot excite the bridge, whereas 808 nm light can excite the bridge.
Therefore, the observed increase in coloration rate and colorability can be attributed to the excited bridge. X-ray photoelectron spectroscopy (XPS) data further confirm that the sample with excited bridge produces more reduced NbIV after coloration. As shown in Fig. 2a, the core-level spectra for Nb 3d are remarkable difference for initial, direct colored and pre-excited colored samples. In the initial sample, there are only two peaks at ca. 207.08 and 209.83 eV, corresponding to the characteristic peaks for NbV 3d5/2 and NbV 3d3/2, respectively. For direct colored sample, a pair of new peaks appear at around 204.94 and 207.69 eV, indicating a 2.14 eV shift to lower binding energies, which represent the existence of NbIV. While for the pre-excited colored sample, a pair of new peaks show an even larger shift about 2.46 eV to lower binding energies and exhibit higher intensity.
To determine the optimal time for laser pre-excitation, the sample is irradiated with 808 nm for different times and then coloration with UV for 1 min. As shown in Fig. 2b, c, long pre-excited time is beneficial for improving the coloring speed, while sequentially extending the pre-excited time after 10 min does not significantly improve the PET rate. To investigate the duration of the excited bridge, the sample is pre-excited with an 808 nm laser for 15 min, then left in the dark for a variable period of time, followed by UV irradiation. As depicted in Fig. 2d, after placing in the dark for 3 h, a deeper colored area can still be observed upon UV irradiation, indicating that the duration of the excited bridge can maintain at least 3 h. However, after placing 12 h, no deeper blue is observed by UV irradiation (Figure S6), indicating that the excited state of the bridge has reverted to the ground state.
The bridge is further pre-excited to the excited states 6H5/2 and 6H7/2 by 915 and 1120 nm irradiation, to assess the impact of the different excited states of bridge on the ET process (Figure S7). Similarly, upon pre-excitation by 915 nm (ca. 2.22 W⋅cm− 2) or 1120 nm (ca. 2.20 W⋅cm− 2) lasers, followed by UV irradiation, the color change in the region with excited bridge is more pronounced compared to that with the ground bridge (Figure S7). The coloration rate constants are 2.64 × 10− 1 and 2.56× 10− 1 min− 1 for 6H5/2 and 6H7/2 excited bridges with maximum colorabilities being 4.45 and 4.41, respectively, outperforming those with ground bridge (Fig. 2e). These results show that the bridge in different excited states can all effectively promote the PET coloration process. The difference in coloration rate and colorability for various excited states of the bridge may be attributed to variations in oscillator strength of electron transition and laser irradiance levels, resulting in varying population of samples with excited bridge.
The colored sample will slowly restore to colorless initial state in thermal mode, with the NbIV-to-NbV IVCT peak fading in the absorption spectra (Fig. 3a, S8). Time-dependent electron absorption spectra indicate that the bleaching process follows first-order reaction kinetics, with the bleaching rate constant k− 1 of 7.57 × 10− 3 min− 1 and τ of 91.58 min at RT (Fig. 3b, S8). Comparing with the τ under different temperature T, the τ values quickly decreased to 5.16 min as T increased to 80°C (Fig. 3c, d, S8). This implies that the rise in temperature facilitates the thermally induced reverse ET, which aligns with the reports for T-type photochromic compounds.14, 15 The ET rate in the bleaching process follows the Arrhenius law with apparent activation energies (Ea) of 0.51 eV (Fig. 3c). We are then curious to know whether the excited bridge can also affect the reverse ET during the bleaching process?
Upon irradiation by 808 nm laser, the colored sample will rapidly bleach at a visually distinguishable rate (Fig. 3e, Video 1). Time-dependent electron absorption spectra indicate that the bleaching process also follows first-order reaction kinetics, with the bleaching rate constant of 3.74 min− 1 at RT, which is approximately 494 times faster than the traditional thermal relaxation mode (Fig. 3f). The calculated τ is as short as 11.13 s, and it only takes 35 s for complete bleaching. To determine the influence of laser thermal effects to bleaching process, the temperature change of the sample surface continuously illuminated by the 808 nm laser is monitored with an infrared thermometer. The temperature increased from RT to about 150°C within 10 s, indicating a contribution of thermal effect to the light-induced bleaching (Figure S9). Therefore, in order to investigate the influence of excited state bridges on the reverse ET process, it is necessary to deduct the contribution of thermal effects. The kinetic data (Fig. 3d, S9) shows that the bleaching rate of the thermal mode at 150°C is about 0.30 min− 1, which is one order of magnitude smaller than that of light-assisted bleaching. This result means that the excited bridge can effectively promote reverse ET.
To further demonstrate the important role of excited bridge in reverse ET, the laser irradiance of each wavelength, including 635, 808, 915, and 1120 nm, is unified to 100 mW⋅cm− 2. According to fitted bleaching kinetic data (Figure S10) for 635, 808, 915, and 1120 nm laser irradiation, the bleaching rates are 1.37, 4.56, 2.83, and 3.39 (× 10− 2 min− 1) with τ values being 41.98, 9.49, 19.56, and 19.00 min, respectively. The observed the increasing maximum temperature of the colored samples are 38.3, 53.4, 45.7 and 45.1°C after irradiation at these wavelengths, respectively (Figure S11). When only considering thermal effect, the corresponding bleaching rates, which can be obtained from above Arrhenius relationship (Fig. 3c), are 1.50, 3.60, 2.33 and 2.25 (× 10− 2 min− 1) with τ values being 46.08, 19.25, 29.80, and 39.77 min, respectively (Fig. 3h). It is worth noting that when irradiated with 635 nm that cannot achieve excited bridge, the bleaching rate is slower than that in thermal mode at 38.3°C. This is because as the sample bleaches, the efficiency of photothermal conversion also decreases to a large extent (Figure S12). Specifically, even though the decreasing efficiency of photothermal conversion, when irradiated with 808, 915, and 1120 nm lasers that can achieve excited bridges, the bleaching rates are significantly faster than those in thermal mode at 53.4, 45.7 and 45.1°C (Fig. 3h).
So what is the reason behind the increase in ET rate due to the excited bridge? According to the kinetic theory of ET reactions, the ET rate is directly proportional to the square of the electronic coupling (Vrp) between the initial and colored states.9, 16 The Vrp is defined as half of the avoided crossing energy level splitting between the potential energy surfaces of the initial and colored states. The smaller the apparent activation energy Ea for ET, the larger the Vrp value, which favors ET reactions (Fig. 4a-b). Taking the reverse ET during the bleaching process as an example, under thermal mode, the fitted Ea for electron transfer is 0.51 eV (Fig. 3c). Under light mode, the bleaching rate also follows Arrhenius law, and the Ea reduces to 0.30 eV (Fig. 3g, S13). This indicates that the excited bridge can enhance the Vrp between the initial and colored states, achieving efficient electron transfer.
Based on above all kinetic data obtained from time-dependent absorption spectra, an energy diagram is proposed to attribute the states involved in the coloring and bleaching processes (Fig. 4c). During the coloring process, the initial state RA-Dy-PONb is sensitized by UV to form the excited state RA*-Dy-PONb, followed by ET to generate the charge-separated colored state RA+⦁-Dy-PONbIV, reaching saturation in 16 min (Fig. 1b). However, when the initial state is pre-excited by 808 nm to form RA-Dy*-PONb, subsequent UV irradiation results in the formation of the RA*-Dy*-PONb, further leading to rapid ET to form the colored state RA+⦁-Dy*-PONbIV, reaching saturation in just 9 min with a higher colorability (Fig. 1d). The RA+⦁-Dy*-PONbIV can thermally relax to the RA+⦁-Dy-PONbIV, and spontaneous reverse ET occurs in the dark, completely reverting to the colorless bleached state RA-Dy-PONb in about 12 h (Figure S14). Under 808 nm irradiation, the colored state RA+⦁-Dy-PONbIV is excited to form RA+⦁-Dy*-PONbIV, followed by efficient reverse ET, resulting in the bleaching state of RA-Dy*-PONb, with complete bleaching occurring in just 35 s (Fig. 3e). Finally, RA-Dy*-PONb relaxes to the initial state RA-Dy-PONb with at least 3 h.
Motivated by the lifetime of excited bridge being up to 3 h for RA-Dy-PONb, along with the ability of the excited bridge to promote the ET process, the multiple anti-counterfeiting application is provided. As shown in the Fig. 4d, the region with 808 nm pre-excitation exhibits a faster coloring rate and a greater colorability under subsequent UV irradiation compared to the area without 808 nm pre-excitation. The colored state can completely bleach after 35 s of 808 nm irradiation, and immediately after, upon UV irradiation, the bleaching area shows a significantly deeper colorability. However, after being left for 12 h and then subjected to UV irradiation, all areas exhibit consistent colorability. This unique photochromic phenomenon is difficult to replicate with other materials and can used for multiple anti-counterfeiting application.17
In summary, we take a D-f-A ET photochromic polyoxoniobate with f-type bridge as the proof-of-concept model, to study the influence of excited bridge on the ET process for the first time. It is found that the excited bridge can significantly enhance the electronic coupling between the bleached state and the charge separated state, and promote the ET process. In particular, we firstly achieve accelerated reverse ET with light assistance, breaking through the limitation of traditional reverse ET to thermal mode, with an increase bleaching rate of 494 times and a half-life of 11 s. The important results of the excited bridge to improve the ET rate have important reference significance for the development of photochemistry, photoelectric, energy conversion and other interdisciplinary related to the ET process.
This work studied the effect of excited bridge on the electron transfer process for the first time, and found that the excited bridge can significantly enhance the electronic coupling between the bleached and colored state with promoting the bidirectional electron transfer.
