Designing All‐Photonic Molecular Analogs for Electrical Components: A Reprogrammable Luminescent Filter Based on Ln3+ Ions

The increasing demand for computing power and downscaling is reaching the limits of the current lithographic methods, further precluding the shrinkage of the silicon chips using state‐of‐the‐art top‐down approaches. Moreover, the current chip shortage exposes the excessive world dependence on silicon, stressing the need for silicon‐free computing technologies, preferably operating at the molecular level. Here, a Eu3+/Tb3+ co‐doped organic‐inorganic di‐ureasil hybrid is used to demonstrate an illustrative example of an all‐photonic device based on the emission temporal dynamics of the Eu3+ and Tb3+ ions. An all‐photonic approach for temperature‐reprogrammable change from a low‐pass filter to a high‐pass filter is reported, showing a firm step toward the design and development of molecular analogs of conventional circuit electrical passive components.


Introduction
The observed advances in computer science and industry 4.0, based on artificial intelligence and machine learning methods, digitalization, and, especially, internet-of-things big-data analytics, are continuously demanding further computing power. [1,2] Despite the lithographic techniques experiencing an amazing evolution during the last decades, they are likely to reach their physical limits, signing that a turn in the paradigm of computation is becoming crucial, [3][4][5][6] as recognized by the worldwide semiconductor industry. [7] Several strategies are being exploited such as photonic [8][9][10][11][12] and quantum computation. [13][14][15][16][17][18] The former uses photons to carry the information (a single photon DOI: 10.1002/lpor.202200877 carries up to 10 bits of information [12] ) and can benefit from the current developments in optoelectronics technologies, whereas the latter exploits concepts of quantum mechanics to process the information as quantum bits. The major drawback of the quantum approach is that there is still a lack of quantum algorithms. [18] A third way to tackle the computing challenges is by developing molecular-or atomic-level equivalents of the current electronic integrated circuits, as recognized in early 1970 by Aviram [19] and pointed out as the scientific breakthrough of the year by the Science journal in 2000. [20] The first years were quite promising, and even a molecular rectifier operating as a traditional diode was reported, however, without effectively replacing any electronic counterpart. An important step forward was given by Forest Carter extending the original concept to molecular computing, in which molecules are the ultimate electronic components or elementary units where the bits of information can be processed and stored. [20] Passive molecular electronics such as tunnel junctions, rectifiers, single-molecule transistors, and molecular switch tunnel junctions were also reported. [21] A recent overview of the progress in the field has been provided by Mathew in 2018 stressing the potential applications of molecules to replace electronic counterparts. [22] Single organic-molecule devices [23] and carbon nanotube/molecular junctions [24,25] have been reported as equivalents to conventional electronic components, and new approaches based on metal−organic frameworks [26] and chemically modified silicon substrates [27] are being actively investigated. Very recently, a molecular transistor constituted of a single bi-nuclear ruthenium-diarylethene complex was reported demonstrating that ultra-miniaturized functional electrical circuits can operate beyond Moore's law. [28] Intriguingly, and as far as we know, there are no reports on molecular species for effectively replacing the electronic counterparts that can be interrogated using photons, despite reprogrammable systems using chemical stimuli, [29,30] and some other molecular equivalents for specific electronic counterparts have been reported. [31][32][33] The former examples report a photo-controlled shift of the pH titration curve, describing the acidochromic behavior of the water-soluble spiropyran, that was harnessed for the design of a molecular triode. The output of this device Flow-chart scheme of the designing of a) molecular optical filters versus b) conventional electronic filters. D refers to the time-resolved emission starting delay detection parameter which is the temporal distance between the excitation pulse and the detection (see Figure 2), whereas the frequency (defined in Section 2.2) is related to the integration window (see Figure 2) detection parameter that is the time period in which the emission signal is recorded. T is the temperature.
is read through the exclusive absorption signal and by the UV intensity. [33] The unique approach reported here combines the abovementioned advantages of photonic chips with the molecular design of the devices, and thus, the optical inputs can be remotely delivered. Here we exploit the benefits of light as a stimulus to develop an example of an all-photonic molecular analogue for the electrical passive filters using the luminescence properties of trivalent lanthanide (Ln 3+ ) ions.
The Ln 3+ ions are widely used due to their unique emission features (e.g., high emission quantum yield, narrow bandwidth, and long-lived emission). [34,35] However, these ions present poor absorption cross-sections and, thus, relatively low brightness. To circumvent this drawback, lanthanide complexes with organic molecules should be employed. These ligand chromophores typically present effective absorption cross sections 10 4 −10 5 times higher than the Ln 3+ corresponding ones. Moreover, the absorbed energy in the ligands is efficiently transferred to the Ln 3+ centers, which in turn undergo a radiative emitting process, the so-called lanthanide luminescence sensitization, or antenna effect, in analogy to light harvesting molecules found in photosyn-thetic systems. [36] Moreover, to improve the thermal stability, mechanical features, and light-emission properties (e.g., quantum yield, lifetime, photostability), of the Ln 3+ complexes they can be easily incorporated into organic-inorganic hybrids. [37] One intriguing example of an organic-inorganic hybrid is the amine-functionalized cross-linked sol-gel derived hybrids called di-ureasils, which comprise a siliceous backbone that is covalently bonded to poly(ethylene oxide) chains by urea crosslinks. [38,39] In the past, these materials were applied in luminescence thermometry, [40,41] M-optical, [42] QR-codes, [43][44][45] LSCs, [46,47] and light-emitting diodes. [48,49] More recently, we reported the application of these same materials for molecular logic applications, reporting spectrally reconfigurable and temperature-reprogrammable logic gates. [50] In this work, we describe the proof-of-concept of a fully reconfigurable all-photonic molecular device based on a Tb 3+ /Eu 3+ di-nuclear complex incorporated into a di-ureasil hybrid host (dU6EuTb) that can mimic conventional electronic passive pass filters. The system yields full reversibility, contrasting with the conventional electronic components for which a new function implies a new circuitry (see Figure 1). This is the first example of an all-photonic device that mirrors the transfer function of the conventional electronic passive circuit components.

Implementing Time-Resolved Spectroscopy
The temporal dynamics of the luminescence of the Ln 3+ ions (commonly in the millisecond order) and the host material (in the millisecond and nanosecond order at 10 and 298 K, respectively) are well-known and already reported in the literature. [51] In addition, the temperature dependence of the 5 D 0 (Eu 3+ ) and 5 D 4 (Tb 3+ ) levels decay lifetimes in the 12-320 K range was previously reported. [43] Essentially, it was demonstrated that the temporal dynamics related to the 5 D 0 excited state are virtually temperature-independent, whereas that of the Tb 3+ state notably decreases with the temperature increase. [43] The relevant detection parameters for recording the time-resolved emission are the integration window (W), defined as the time interval in which the emission is recorded and the starting delay (D), which refers to the time distance between the excitation pulse and the detection (Figure 2). Figure 3 shows the dependence of the time-resolved emission spectra of the dU6EuTb (synthesis details in Experimental Section) with W recorded at 12 and 298 K for D = 0.05 × 10 −3 s and 1.00 × 10 −3 s. The narrow emission lines associated with both Tb 3+ and Eu 3+ are observed and regardless of the D value set, as W increases the intensity of the transitions of the Ln 3+ ions increases, reaching a maximum for W > 10 × 10 −3 s at both temperatures.
The emission spectra exhibit evident changes with the temperature and W for shorter or longer D that can be further quantified in terms of a ratiometric parameter involving the integrated intensity of the transitions originated in both Ln 3+ ions, that is, Δ = I Tb /I Eu . The ratiometric approach was adopted as it compensates for the eventual experimental stochastic fluctuations. [52] Figure 4a-d displays the evolution of the normalized integrated areas of the emission bands related to the 5 D 4 → 7 F 5 (Tb 3+ , I Tb ) and 5 D 0 → 7 F 2 (Eu 3+ , I Eu ) transitions with W recorded at 12 and 298 K for fixed D = 0.05 × 10 −3 s and 1.00 × 10 −3 . Normalizing the first experimental data point for each transition provides useful information regarding the increasing rate. On one hand, for D = 0.05 × 10 −3 s, increasing W, I Tb undergoes a higher increasing rate than I Eu for lower temperatures whereas the opposite trend is observed at higher temperatures (Figure 4a,b). On the other hand, for D = 1.00 × 10 −3 s, a similar trend with the temperature is observed for I Tb and I Eu (Figure 4c,d), but the changes in I Tb and I Eu are significantly lower than those observed for the lower D value.
The effect of the temperature in our system points out the existence of a temperature value for which the inversion between I Tb and I Eu occurs. To probe into this matter, further analysis of the temperature dependence of the Δ values was performed. For a straightforward visualization of the data, we present in Figure 4e,f the normalized Δ as a function of W, recorded at different temperatures for a fixed D. As the temperature increases, the normalized Δ values undergo changes whose trend is inverted for temperatures above 262 K. This temperature value, denoted here by T c , is characterized by a dU6EuTb luminescence that remains unaltered upon W increase and thus, there is an independence of Δ with W. This corresponds to a turning point that is correlated to the above-mentioned distinct dependences of the temporal dynamics of the 5 D 4 (Tb 3+ ) and 5 D 0 (Eu 3+ ) energy levels with the temperature. [43] For temperature values higher than the turning point, Δ decreases with increasing W (Figure 4e). For a higher D, the same behavior with the temperature is observed, except for the fact that the changes in normalized Δ values are significantly lower (Figure 4f).
Our results pinpoint that the temperature plays an important role in the temporal dynamics of Eu 3+ and Tb 3+ related emission bands. Although the calculation of the ligand-to-Ln 3+ transfer rates is out of the scope of the present work, we stress that the temperature dependence of the Eu 3+ and Tb 3+ emissions within the di-ureasil hybrid matrix was already theoretically rationalized. [45,[53][54][55] The temporal dynamics of the luminescence of dU6EuTb and its relation with the detection parameters (W and D) will be addressed in future works.

Defining the Transfer Function of the All-photonic Molecular Analogue
The intriguing temperature-driven behavior displayed by the luminescence of dU6EuTb may be exploited for designing an allphotonic molecular optical passive pass filter. For this purpose, and convenience, we defined the detection frequency (f) and the transfer function parameter of dU6EuTb (H) as: where W is the integration window and  where Δ 0 denotes the Δ parameter evaluated at the lowest detection frequency, f = 0.03 × 10 3 Hz. Thus, the f parameter governs the processing information rate during the time-resolved spectroscopy measurements and H represents the relation between Δ and Δ 0 . Without losing generality, we choose two temperature values (250 and 298 K) to illustrate the working principle of the molecular device since the temperature-driven opposite behavior was demonstrated for temperatures below and above T c . In this sense, the selected temperatures are not arbitrary but, indeed, a convenient choice, in terms of real-world applications since the observed temperature behavior operates even for non-extreme temperature values. Figure 5 shows the dependence of the H with the logarithm of the detection frequency, f recorded at 250 and 298 K for a fixed D, a common representation in signal processing termed bode plot. The above-mentioned temperature-induced behavior can be easily observed in these figures. On one hand, when the temperature is 250 K, the H maximum value remains constant in the low detection frequency regime, decreasing 50% when the cut-off frequency (f c ) of ≈750 Hz is reached, and for f > f c , H markedly decreases (Figure 5a). On the other hand, setting the temperature at 298 K, H is initially damped for low detection frequencies, increasing by 50% when f c ≈ 1200 Hz. For f > f c the H value dramatically increases until reaching its maximum value (Figure 5c).
For higher D, the temperature dependence of H is kept, with the only difference that the f c values decrease (Figure 5b,d).
For a given temperature, the behavior depicted by H with f is intriguing, because it can be interpreted in terms of the well-known bode plots of conventional resistor-capacitor (RC) electrical circuit passive pass filters, that is, low-pass filter (LPH) and highpass filter (HPF). In this regard, for specific detection frequency regimes, it is possible to selectively "gate in/out" the Tb 3+ and/or Eu 3+ emissions of dU6EuTb. Hence, when T < T c (see Figure 5), dU6EuTb luminescence behaves as an LPF favoring the Tb 3+ emission (green photons) over the Eu 3+ one in the regime of low detection frequencies, whereas for T > T c and in the same regime of detection frequencies, it mimics an HPF for the Eu 3+ emission (see Figure 5). On the other hand, changing D increases the order of dU6EuTb as a passive pass filter, making steeper the stop band's roll-off slope.
It is worth mentioning that the temperature ranges that dictate the behavior of dU6EuTb as high pass filter (for T > T c ) or low pass filter (for T < T c ) are well defined, ensuring that for a given temperature and small thermal fluctuations, the working parameters of the device remain unaltered. Thus, the luminescent properties of dU6EuTb permit the definition of molecular equivalents to passive pass filters, with the fundamental advantage of being remotely actuated by light. Indeed, the incident light  remained unaltered since dU6EuTb does not filter however it permits to define a transfer function of an on-demand passive filter when specific luminescent properties are analyzed. Such processing can be described using an analogue approach to that used in the electronic passive filter's theory.
Moreover, this illustrative system presents remarkable features and advantages over conventional filters. The nature and order of dU6EuTb as a filter can be freely modified with the temperature and D respectively while the optical signal of the material can be chosen with the detection frequency, all of this, using the same material without any addition of extra elements to the system. In contrast, changes in the nature (high pass or low pass) and order (filtering power) of conventional electronic RC circuit passive filters imply important modifications in their Laser Photonics Rev. 2023, 17, 2200877 circuitry configuration precluding any reconvertibility using the same circuit (Figure 1).
An immediate advantage of using dU6EuTb over other luminescent materials is that the lanthanide emissions (I Tb and I Eu ) can be estimated through a blend of theoretical methods such as density functional theory, intramolecular energy transfer, Judd-Ofelt theory, and rate equations modeling. [56,57] Furthermore, the thermal behavior of I Tb and I Eu emissions can be also forecasted. [45,[53][54][55] For example, Ramalho et al. successfully estimated I Tb and I Eu for similar organic-inorganic hybrids compounds with 2-thenoyltrifluoroacetone and salicylic acid as main ligands. [45] Recognizing that this device was not designed to perform rapid switches between low-and high-pass filters, we should however stress that this time can be modeled by the physical properties of the material such as the mass and the thermal capacity. [58]

Reprogrammable Logic Gates
As an added benefit, we also report the capabilities of our system as a platform for molecular logic. Very recently, we reported a molecular logic device using a similar material. [50] In our previ-ous work, the described logic device was required to work at two different temperatures (14 and 298 K), which was not a final engineering solution in terms of applications in real-world electronic devices. In this work, we use a different approach, exploiting the dependence of H with the f recorded at different temperatures for a fixed D (0.05 × 10 −3 s, Figure 6a). In this regard, for temperatures higher than T c , H has an increasing tendency, while for lower temperatures than T c , H decreases. To rationalize and implement the logic operations, we define the detection frequency as the only input taking the logic value of 0 in the condition of f = f 0 = 0.03 × 10 3 Hz and 1 for f > f 0 , and H as the output. The threshold to discriminate the 1 and 0 output is arbitrarily set at 1 (horizontal red dashed line in Figure 6a). When H is in the T < T c regime a NOT gate is described, while in the T > T c regime H output mimics a PASS 1 gate, the true table, and logic circuitry are depicted in Figure 6b,c. In this way, the system is defined as reprogrammable being possible to tune the output in two different temperature conditions. Since T c is the discriminant for defining the output as 0 or 1, the remarkable advantage compared to our previous work, is that the temperature volume working range is more acceptable for real-world applications. The same logic operations can be defined for D = 1.00 × 10 −3 s (see Figure S1, Supporting Information).

Conclusion
The development of computing systems and devices made entirely or mostly of molecular materials is the goal of research into molecular computing. The molecular systems or devices to process information using molecular substrates are still to be demonstrated as an effective alternative to conventional electronic devices. The present work shows a Tb 3+ /Eu 3+ -doped organic-inorganic di-ureasil hybrid as an all-photonic device based on the temporal dynamics of the luminescence of the two Ln 3+ ions for the implementation of new strategies to molecular photonics. The potential of the Ln 3+ -doped di-ureasil hybrid as an all-photonic device is presented here as a proof of concept, mimicking the behavior of RC circuit passive pass filters, actuated exclusively by physical stimuli. Through time-resolved emission analysis, we demonstrated how the temperature along with detections parameters (starting delay and integration window), the spectroscopic features of dU6EuTb can be modulated, and further quantified in terms of the integrated intensity of the Ln 3+related emission bands. Exploiting the integrated intensity ratio of the Tb 3+ and Eu 3+ emissions, the molecular device is demonstrated to be an all-photonic molecular-based temperature reprogrammable passive pass optical filter, which "strength" as a filter can be modulated through starting delay using the same material, pointing out the advantages of using molecular analogues of conventional electronic passive components. Temperaturereprogrammable logic gates have been demonstrated using the detection frequency and the transfer function as input and output, respectively. The molecular counterparts of particular electronic elements are a clear step forward toward the development of future computing systems using molecules as the fundamental building blocks.
Synthesis of d-UPTES(600) precursor: The d-UPTES(600) organicinorganic hybrid precursor was prepared according to the method reported previously, [59] and its molecular structure is shown in Figure S2, Supporting Information.
Synthesis of Eu 0. 25 [60] Typically, 0.74 mL (6.0 mmol) of tfac was transferred to 10.0 mL of distilled water, followed by the addition of 6.0 mL (6.0 mmol) of 1.0 mol·L −1 NH 3 ⋅H 2 O. The resulting mixture was stirred at room temperature until a homogenous clear solution was obtained. Then a mixture of an aqueous solution containing 183.2 mg (0.5 mmol) of EuCl 3 ⋅6H 2 O and 560.1 mg (1.5 mmol) of TbCl 3 ⋅6H 2 O was added with the molar ratio of Eu and Tb to tfac of 1:3. The mixture was further stirred at 50°C and then placed at ambient condition overnight. The resultant precipitate was filtered off, washed with water, and dried. The product was purified by crystallization with chloroform.
Synthesis of d-U(600) doped with Eu 0.25 Tb 0.75 (tfac) 3 ⋅H 2 O (dU6EuTb): For the synthesis of the Tb 3+ /Eu 3+ co-doped di-ureasil host-based system (labeled as dU6EuTb), typically, 6.0 g (5.484 mmol) of d-UPTES(600) was mixed with 8 mL of EtOH under stirring. Then, 168.0 mg of Eu 0.25 Tb 0.75 (tfac) 3 ·H 2 O was added, and the mixture was treated under ultrasonic conditions until a clear solution was obtained. Next, 0.592 mL of HCl acidified water (pH = 2) was added under stirring to catalyze the hydrolysis and condensation reactions. The molar ratio of d-UPTES(600):H 2 O was 1:6. The resulting solution was stirred at room temperature for a further 2 h and then was deposited into a plastic cuvette for drying overnight in an oven at 60°C. The resulting hybrid was designed as dU6EuTb, and the doping concentration was 3.39 wt%.
Characterization of Pure Complex Eu 0.25 Tb 0.75 (tfac) 3 ·H 2 O and the Hybrid of dU6EuTb: The FT-IR spectra of the pure complex Eu 0.25 Tb 0.75 (tfac) 3 ·H 2 O and the hybrid of dU6EuTb were measured in the range of 4000-400 cm −1 on Bruker Optics Tensor 27 spectrometer (Bruker Corporation, Billerica, MA, USA) using KBr pellet technique with 64 scans and 2 cm −1 resolution and displayed in Figure S3, Supporting Information.
Photoluminescence Spectroscopy: The excitation spectra were recorded using a modular double grating excitation spectrofluorometer with an emission monochromator (Fluorolog-3 2-Triax, Horiba Scientific) coupled to a photomultiplier (R928 Hamamatsu), using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. The time-resolved emission spectra were measured using the same setup as the setup described for the Laser Photonics Rev. 2023, 17,2200877 luminescence spectra using a pulsed Xe-Hg lamp (6 μs pulse at half-width and 20-30 μs tail) as the excitation source. The temperature measurements were performed with a helium-closed cycle cryostat, connected to a vacuum system (4 × 10 −4 Pa), and an autotuning temperature controller with 0.1 K accuracy (Lakeshore 330) equipped with a resistance heater.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.