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Article

The Rhodamine–Perylene Compact Electron Donor–Acceptor Dyad: Spin-Orbit Charge-Transfer Intersystem Crossing and the Energy Balance of the Triplet Excited States

by
Muhammad Imran
†,
Dongyi Liu
†,
Kaiyue Ye
,
Xue Zhang
and
Jianzhang Zhao
*
State Key Laboratory of Fine Chemicals, Frontiers Science Center for Smart Materials Oriented Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Rd., Dalian 116024, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Photochem 2024, 4(1), 40-56; https://doi.org/10.3390/photochem4010004
Submission received: 30 December 2023 / Revised: 23 January 2024 / Accepted: 24 January 2024 / Published: 29 January 2024
(This article belongs to the Special Issue Exclusive Papers of the Editorial Board Members of the Journal Photochem)
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Abstract

:
We prepared a rhodamine (RB)–perylene (Pery) compact electron donor/acceptor dyad (RB–Pery) to study the spin-orbit charge-transfer intersystem crossing (SOCT–ISC). The UV–vis absorption spectrum indicates a negligible electronic interaction between the donor and acceptor at ground state. However, the fluorescence of both the RB and Pery units are quenched in the dyad, which is attributed to the photoinduced electron transfer, supported by the electrochemical studies. Nanosecond transient absorption (ns-TA) spectra show delocalized triplet states, i.e., there is an excited-state equilibrium between Pery and the RB triplet states. The triplet state lifetime was determined as 109.8 μs. With intermolecular triplet–triplet energy transfer, monitored using ns-TA spectra, the triplet-state energy balance between RB and Pery in RB–Pery was confirmed. The proposed cascade photophysical processes of the dyad are 1RB*-Pery→RB–Pery+•→[3RB*-Pery↔RB-3Pery*]. Moreover, long-lived rhodamine radical cation (in milliseconds) was detected in both deaerated/aerated non-polar or low-polarity solvents (i.e., p-xylene, toluene). The potential energy curve of the dyad against the variation in the dihedral angle between the two units indicates large torsional freedom (53°~128°) in RB–Pery, which leads to inefficient SOCT–ISC; consequently, low singlet-oxygen quantum yields (ΦΔ = 2~8%) were observed.

Graphical Abstract

1. Introduction

The formation of long-lived triplet states in triplet photosensitizers (PSs) is of great importance due to their potential applications in photocatalysis [1,2,3,4], optical limiting [5], molecular probes [6], and triplet–triplet annihilation upconversion [3,4,7,8]. Conventional triplet PSs contain heavy atoms to enhance the intersystem crossing (ISC), for instance, Pt, Ir, Ru or I, Br, etc. [3,9,10,11]. However, these triplet PSs suffer from the drawbacks of high cost and toxicity and a shortened triplet-state lifetime due to the strong spin-orbit coupling effect. In order to address these challenges, in recent years, numerous heavy-atom-free triplet PSs have been developed. Some methods used in designing heavy-atom-free triplet PSs to show predetermined efficient ISC include using an electron spin converter [12,13,14], singlet fission [15,16], exciton coupling [17], or radical enhanced ISC [8,18]. The charge recombination (CR)-induced ISC via radical pair ISC (RP-ISC) mechanism in electron donor/acceptor dyads is also known, but the electron donor and acceptor are separated by a large distance and the ISC efficiency is usually low [19,20]. However, most of these triplet PSs are synthetically demanding; for instance, a special orientation of the chromophores for the exciton coupling-induced ISC is required [21], whereas the dyads showing RP-ISC require long and rigid linkers, which make the synthesis challenging. Therefore, a simple molecular structural motif for the design of triplet PSs showing efficient ISC is highly desired.
Recently, it was found that efficient ISC may occur in compact electron donor/acceptor dyads if the electron donor and acceptor adopt orthogonal geometry. Under this circumstance, the CR is accompanied by molecular orbital angular momentum change, which offsets the electron spin angular momentum change of the ISC. Consequently, the ISC is enhanced by the CR in these orthogonal compact electron donor/acceptor dyads [22,23]. This so-called spin-orbit charge transfer ISC (SOCT–ISC) mechanism actually complies with the spirit of El Sayed’s rule for ISC. These compact electron donor/acceptor dyads showing SOCT–ISC have advantages of simple molecular structures, feasible preparation, and high ISC yields. The molecular structures and the photophysical properties can be feasibly tuned by using different electron donors/acceptors. Previously, a bodipy−anthracene dyad was reported to show efficient triplet formation (triplet quantum yield~90%) and a long-lived triplet state (85 μs) via SOCT–ISC [24]. This SOCT–ISC was also studied with other electron donor–acceptor systems, for instance, anthryl/phenothiazine [25], Bodipy/phenothiazine [26], perylene (Pery)/Bodipy [27], Pery/phenothiazine [28], perylenemonoimide/phenothiazine [29], perylenemonoimide/carbazole [30], and naphthaimide/Pery [31]. However, rhodamine B (RB) is rarely used in electron donor/acceptor dyads showing SOCT–ISC [32,33,34].
The RB moiety is a well-known and versatile fluorophore that shows strong absorption and emission in the visible spectral region; its derivatives have been used extensively for fluorescent molecular probes and fluorescent bioimaging [35,36,37,38,39]. Recently, rhodamine moiety has been used as a light-harvesting unit in transition-metal complexes [40,41,42], as well as for the application of the triplet state in TTA upconversion [43], photodynamic therapeutic studies [44], or one-photon excitation molecular upconversion [45,46,47]. However, rhodamine was not used for the preparation of electron donor/acceptor dyads showing SOCT–ISC. Since rhodamine shows strong absorption of visible light and is used as a strong electron acceptor (oxidation potential (EOX) = +0.87 V vs. Fc/Fc+), it will be interesting to study the amplification of rhodamine in electron donor/acceptor dyads showing SOCT–ISC.
Herein, we use Pery as an electron donor and cationic RB, a well-known xanthene-based dye, as an electron acceptor, to synthesize an RB–Pery compact dyad with favourable geometry to achieve SOCT–ISC. Note that the rhodamine part is positively charged, the electron transfer is actually a charge shift, and there is no Coulombic interaction in the resulting state. Steady-state absorption/emission spectroscopies, time-resolved transient absorption spectroscopy, and electrochemical and density functional theory (DFT) computations have been used to study the photophysical properties of the compounds. Triplet-state equilibrium was observed with nanosecond transient spectra showing a delocalized triplet state. Also, a long-lived rhodamine radical cation (~10 ms) was detected in non-polar solvents.

2. Materials and Methods

2.1. General Methods

UV–vis absorption spectra were measured on an Agilent 8453 UV–vis spectrophotometer (Agilent Ltd., Santa Clara, CA, USA). Fluorescence emission spectra were recorded on an RF-5301PC spectrofluorometer (Shimadzu Ltd., Kyoto, Japan). Luminescence lifetimes were measured on an OB920 fluorescence/phosphorescence lifetime spectrometer (Edinburgh Instruments Ltd., Livingston, UK). The fluorescence quantum yields were measured with an absolute photoluminescence quantum yield spectrometer (Quantaurus-QY Plus C13534-11, Hamamatsu Ltd., Hamamatsu, Japan).

2.2. Synthesis of Compound RB–Pery

A mixture of 3-perylenecarboxaldehyde (140 mg, 0.5 mmol), 3-(diethylamino)phenol (200 mg, 1.2 mmol), p-TsOH (18 mg, 0.1 mmol), and AcOH (7 mL) was heated at 90 °C and stirred for 12 h. Then, the reaction mixture was cooled to room temperature and the pH of the mixture was adjusted to above 7 with a 10% NaOH solution. The precipitate was filtered and washed with water (20 mL). Then, the solid was dissolved in CH2Cl2 (10 mL) and chloranil (62 mg, 0.25 mmol) was added. The mixture was stirred for 4.5 h. After the removal of the solvent, the residue was purified via column chromatography (silica gel; CH2Cl2/methanol, 20:1, v/v) to give a purple solid, 15 mg (yield: 5.0%). 1H NMR (400 MHz, CDCl3, ppm) δ 8.37–8.38 (d, J = 4.0 Hz, 1H), 8.32–8.34 (d, J = 8.0 Hz, 1H), 8.26–8.27 (d, J = 4.0 Hz, 2H), 7.78–7.82 (dd, 2H), 7.54–7.60 (q, 2H), 7.40–7.45 (m, 2H), 7.22–7.25 (d, J = 8.0 Hz, 2H), 7.15–7.17 (d, J = 8.0 Hz, 1H), 6.95 (s, 2H), 6.79–6.82 (d, J = 12.0 Hz, 2H), 3.65–3.66 (q, 8H), 1.33 (t, J = 12.0 Hz, 12H); 13C NMR (CDCl3, 100 MHz): δ 158.0, 156.7, 155.7, 134.6, 133.8, 133.0, 132.0, 131.9, 130.4, 130.1, 129.2, 128.8, 128.7, 128.6, 128.5, 128.1, 126.9, 126.9, 124.9, 121.4, 121.3, 121.0, 119.4, 114.2, 114.1, 96.6, 77.4, 77.3, 77.1, 76.8, 46.3, 29.7, 12.7. ESI–HRMS (C41H37N2O+): Calcd, m/z = 573.2900; found, m/z = 573.2909.

2.3. Singlet Oxygen Quantum Yield Measurements

The determination of the singlet oxygen quantum yields (ΦΔ) of the compounds was carried out using the chemical trapping method; 1,3-diphenylisobenzofuran (DPBF) was used as the singlet oxygen (1O2) scavenger. The air-saturated mixture solution of the compounds and DPBF were irradiated at an appropriate wavelength (with a Xe lamp/monochromator) and the 1O2 production was monitored by following the absorbance of DPBF at ca. 414 nm, which was recorded with the UV–2550 spectrophotometer. Rose Bengal (ΦΔ = 76% in MeOH) and Ru(bpy)3(PF3)2 (ΦΔ = 57% in dichloromethanee, DCM) were used as standards. The absorbance at the excitation wavelength was the same for the reference and compounds; for this purpose, optically matched solutions were used. The ΦΔ values were calculated using Equation (1).
Φ Δ , sample = Φ Δ , standard 1 − 10 − A standard 1 − 10 − A sample m sample m standard η sample η standard 2
where A is the absorbance at the excitation wavelength, m is the slope of the plot of absorbance of DPBF at 414 nm versus the irradiation time, and η is the refractive index of the solvent used for the measurements.

2.4. Nanosecond Time-Resolved Transient Absorption Spectroscopy

An LP980 laser flash photolysis spectrometer (Edinburgh Instruments Ltd., Livingston, UK) was used to record the ns-TA spectra of the compounds. An optical parametric oscillator (OPO, tuneable between 210 and 710 nm) was used as the pulse laser excitation source, and the probe source was a 150 W ozone-free xenon arc lamp (pulsed mode up to 10 Hz). The typical laser pulse energy was ca. 5 mJ per pulse. The transmission properties of the sample before, during, and after the exciting pulse were converted by the detector into electrical signals that were measured with an oscilloscope (TDS 3012C, 100 MHz). For all measurements, the sample was placed in a quartz cuvette (10 mm optical path). Before measurements, all sample solutions were purged with N2 for ca. 15 min. The recorded kinetic traces and transient spectra were analysed using the L900 software. (https://www.edinst.com/products/l900-software/, access on 15 December 2023. The triplet–triplet energy transfer (TTET) method was used to verify the triplet excited-state equilibrium with the photosensitizer mixed with the compounds, and the transient spectra were analysed with global fitting using the Glotarn software, version 1.5.1 (sequential model).

2.5. Electrochemical Measurements

Cyclic voltammetry (CV) curves were obtained on a CHI610D electrochemical workstation (CHI Instruments, Inc., Shanghai, China). The electrochemical setup consists of two main parts, i.e., (i) an electrochemical workstation and (ii) an electrolytic cell compartment. The electrolytic cell compartment has three electrodes: one working electrode, a reference electrode, and a counter electrode. Under deaerated conditions, a glassy carbon electrode was used as the working electrode. A silver nitrate-containing (Ag/AgNO3) silver electrode as the reference electrode and a platinum electrode was used as a counter electrode. DCM was used as the solvent and ferrocene was the internal reference for all the CV measurements. The tetrabutyl hexafluoroammunium phosphate (Bu4N[PF6], 0.10 M) was used as a supporting electrolyte. After connecting the electrolytic cell to the electrochemical workstation, a scan was performed at a scan rate of 50 mV s−1. Then, by recording the current value under different voltages, the cyclic voltammetry curves were obtained. For reversible peaks, the potential values were taken as the average value of the anode peak potential and the cathode peak potential, and the ferrocene peak was adjusted at 0 V in the cyclic voltammograms.

2.6. DFT Calculations

The geometries of the compounds were optimized by using DFT with the B3LYP functional and 6–31G (d) basis set. The excitation energy and energy gaps between the S0 state and the excited triplet states of the compounds were computed using time-dependent density functional theory (TD-DFT), which is based on the optimal ground-state geometry. All the calculations were performed with the Gaussian 09W program [48].

3. Results and Discussion

3.1. Molecular Structure Design and Synthesis

Previously, a neutral spiro lactam rhodamine moiety was used as an electron donor in the construction of SOCT–ISC dyads [49,50]. However, the positively charged xanthene moiety is rarely used in SOCT–ISC dyads. On the other side, arenes can be used as electron donors to achieve decent SOCT–ISC efficiency [31,51]. Herein, we directly connected the cationic rhodamine moiety with perylene unit via a C–C bond and obtained an RB–Pery dyad (Scheme 1). In this molecular system, the positively charged rhodamine unit acts as an electron acceptor/visible-light-harvesting chromophore, and the perylene unit is the electron donor. Unsubstituted perylene and rhodamine substituted with a phenyl group (RB-Ph) are the two reference compounds for this study.
The synthesis of the target compounds was based on the reported methods [43]; all the chemicals used in the synthesis were analytically pure. The molecular structures of the compounds were verified with 1H, 13NMR, and HRMS characterization (see experimental section and supporting information).

3.2. Density Functional Theory (DFT) Calculations

The ground-state geometry of the dyad was optimized with DFT (Figure 1a). The dihedral angle between perylene and rhodamine moieties is 69.0°, which is supposed to be beneficial for the occurrence of SOCT–ISC [25,52,53,54]. However, low singlet-oxygen quantum yields of RB–Pery were obtained only in two solvents, i.e., 2.7% in toluene (TOL), 8% in p-xylene (p-XYL), and none in other solvents (see Table S3). In this case, the related potential energy curve (PEC) of RB–Pery against the torsion angle between the perylene and the rhodamine units was constructed with DFT optimization (Figure 1b), showing that the energy remains minimum under the room temperature thermal energy range (<0.026 eV) when the rotational dihedral angles (∠C4-C7-C51-C53) vary from 53° to 128°, indicating large rotation freedom in RB–Pery, and this may lead to inefficient SOCT–ISC.
The frontier molecular orbitals of compounds at the ground state were studied (Figure 2). The highest occupied molecular orbital (HOMO) of RB–Pery is completely confined on the perylene unit; in contrast, the lowest unoccupied molecular orbital (LUMO) is localized on the xanthene moiety. Therefore, the transition from HOMO→LUMO of RB–Pery is a CT feature that shows the possibility of intramolecular electron transfer (charge or hole shift) in the dyad upon photoexcitation. The result also indicates that perylene is an electron donor and xanthene is an electron acceptor in the dyad. On the other side, the HOMO and LUMO of the reference, RB-Ph, are mostly localized on the xanthene unit; thus, electron transfer is unlikely, although the phenyl moiety acts as electron donor in naphthalenediimide derivatives [55]. The triplet excited-state spin density of RB–Pery is delocalized on the entire molecule (Figure 2), indicating the possibility of energy balance or interconversion between the rhodamine triplet state (3RB*) and perylene triplet state (3Pery*) in RB–Pery.

3.3. UV–Vis Absorption and Fluorescence Emission Spectra

The steady-state UV–vis absorption and emission spectra of the compounds were studied (Figure 3a). A typical structured absorption profile was observed for Pery in the range of 350–450 nm. Similar absorption bands were observed in RB–Pery but with smaller magnitudes. The absorption of the rhodamine moiety at 575 nm is similar to the reference, RB-Ph, indicating the negligible interaction between two moieties at the ground state. The fluorescence emission spectra of RB–Pery and Pery were compared (Figure 3b; optically matched solutions were used). The fluorescence of Pery is at 443 nm with significant vibration progress. For RB–Pery, however, the emission of the Pery unit is strongly quenched (note that the perylene part is selectively excited). This quenching may be due to singlet energy transfer or charge transfer to the acceptor unit. Similarly, the fluorescence of RB–Pery and RB-Ph is compared (Figure 3c). In this case, the fluorescence of the rhodamine unit in RB–Pery at 598 nm is weaker than that of RB-Ph. The fluorescence quantum yields of the two compounds are 2.4% and 14.2%, respectively (Table 1). This reduced fluorescence quantum yield for the rhodamine unit in the dyad is attributed to the intramolecular charge transfer process.
The emission intensity of RB–Pery decreased as the solvent polarity increased (Figure S7a). The normalized emission spectra are shown in Figure 3d. Two emission bands were observed; the one at ca. 600 nm is assigned as the locally excited (LE)-state emission of the rhodamine unit, whereas the weak and broad band in the range of 690–800 nm in DCM is assigned to the CT emission [55]. No such CT emission band can be observed in other solvents, even in non-polar solvents. It is known that the CT state, especially the twisted CT state, is weakly emissive [56,57,58]. To the best of our knowledge, the emissive CT state of rhodamine was rarely reported. The CT emission observed for RB–Pery is unlikely due to the perylene unit; the CT state of perylene should be in a shorter wavelength range (ca. 600 nm, with phenothiazine as the electron donor). The emission wavelength should depend on the electron donor and the electronic coupling between the electron donor and acceptor [28]. The fluorescence excitation spectra for RB–Pery were compared with the UV–vis absorption spectra (Figure S7b,c). In p-XYL, the excitation spectrum shows a smaller magnitude compared to the UV–vis spectrum in the range of 370–460 nm, which indicates that there should be some other non-radiative decay channels. However, RB–Pery shows a noisy excitation spectrum in DCM due to the low fluorescence quantum yield (Figure S7c). In this case, the excitation spectrum gives a weak and distinct band in the 620−800 nm range, but such a band is absent in the absorption spectrum. This result indicates that the excitation in the range of 620−800 nm is more efficient to produce the CT excited state than that with excitation at the LE absorption band.
The luminescence lifetimes of the compounds in different solvents are presented in Table S2. The fluorescence of Pery shows monoexponential decaying kinetics; however, the fluorescence decay traces of RB-Ph and RB–Pery show different features. For instance, RB-Ph shows bi-exponential decay of the fluorescence in both p-XYL and TOL, but monoexponential decay in tetrahydrofuran (THF), DCM, and acetonitrile (ACN). For RB–Pery, bi-exponential decay of the fluorescence was observed in all solvents, which indicates there are other emission decay channels besides the intrinsic emission of the chromophore unit, i.e., CT emission. We compiled the singlet oxygen (1O2) quantum yields of compounds determined in varying polarity solvents according to their ET (30) values (Table S3). Pristine perylene exhibited a weak 1O2 production ability, while no 1O2 was observed for RB-Ph. The main dyad, RB–Pery, showed a low 1O2 production ability (8.0% in p-XYL and 2.7% in TOL); this leads to ineffective SOCT–ISC in the dyad.

3.4. Cyclic Voltammograms of RB-Ph and RB–Pery

Cyclic voltammograms were recorded to study the electrochemical properties of the compounds (Figure 4). Similar to a previous report, only a single reversible oxidation wave at +0.61 V (vs. Fc/Fc+) was observed in perylene and no reduction wave was observed up to the potential window used in the studies (Figure S9) [59]. For another reference, RB-Ph, reversible oxidation and reduction waves were observed at +0.89 V and −1.32 V, respectively (Table 2). For RB–Pery, two reversible oxidation waves at +0.58 V and +0.87 V and a reversible reduction wave at −1.29 V were observed. The first oxidation wave belongs to the perylene moiety, indicating that the perylene in the dyad is easy to oxidize. Therefore, perylene acts as an electron donor and the rhodamine unit acts as electron acceptor in RB–Pery.

3.5. Nanosecond Transient Absorption (ns-TA) Spectroscopy

In order to study the excited triplet-state formation, the nanosecond transient absorption (ns-TA) spectra of RB–Pery were recorded in solvents with different polarities. In DCM, a ground-state bleaching (GSB) band centred at ca. 565 nm was observed, which is due to the depletion of the ground state of the rhodamine moiety (Figure 5a). Interestingly, a broad excited-state absorption (ESA) band in the range of 450–550 nm was observed. The ns-TA of the triplet state of the rhodamine chromophore was studied previously [43,44,60], but very weak transient features in the range of 450–550 nm were observed. Instead, it was shown that the triplet state of perylene shows a strong ESA band in this range [27,28,59,61]. Therefore, we propose that the triplet state in the dyad is not solely localized on the rhodamine unit. Contrarily, it is delocalized on both the xanthene chromophore and the perylene moiety, i.e., there is a triplet-state equilibrium. The T1 energy of perylene is ca. 1.53 eV [61], and the T1 state energy of the xanthene moiety is ca. 1.70 eV [43,44]; thus, triplet-state equilibrium between these comparatively close-lying triplet states at room temperature is anticipated.
The excited triplet-state lifetime of RB–Pery was determined as 109.8 μs in deaerated DCM (Figure 5b); however, when the solution was exposed to air, no transient signals were detected which proved the triplet state. Moreover, no transient signal was observed for RB–Pery in deaerated ACN. This highly solvent polarity-dependent triplet formation indicates the SOCT–ISC mechanism for triplet-state population in the dyad.
Previously, it was reported that the T1 state of the rhodamine chromophore shows an absorption band at 420 nm [62], but later, it was proven that the neutral rhodamine radicals show similar absorption bands [63,64]. Upon the nanosecond pulsed laser excitation of native perylene, we detected a long-lived radical cation under both aerated/deaerated conditions (Figure S10). Such long-lived radical species were already reported for native perylene [31,65]. The proposed mechanism was a two-step (bi-photonic) process: firstly, the Pery singlet excited state is populated upon photoexcitation, which undergoes the triplet state, and then the triplet state is excited again to dissociate into ions by releasing/capturing solvated electrons [65]. For RB–Pery, we observed the long-lived rhodamine radical cation in solvents of p-XYL, TOL, and THF in both aerated/deaerated conditions (Figure S11). In this case, we assume the same mechanism for radical cation detection because the quenching of radical anion species with O2 can be observed, whereas radical cations are insensitive to O2 [66]. Moreover, it has been shown previously that rhodamine dye can form long-lived cation radicals (milliseconds) through photoionization in an aqueous solution, likely through both mono- and bi-photonic processes [67,68].
In another previous example, ns-TA spectra of rhodamine 6G radicals were found with the addition of an electron donor, N,N-diisopropylethylamine; the radical absorption band was observed at 421 nm (in DMSO) [64]. However, when we measured the ns-TA spectra of RB–Pery after adding triethylamine (TEA) or N,N-diisopropylethylamine, no signals were found. We further measured the ns-TA spectra of RB–Pery in a deaerated ACN solution and no transient signals were observed. Hence, we conducted the triplet–triplet energy transfer (TTET) experiment in this solvent from diiodo-BDP to RB–Pery to rationalize the triplet excited-state equilibrium (vide infra). For RB-Ph, no transient signals were detected in all studied solvents. However, in DCM, the mixture of RB-Ph and TEA showed long-lived transient species of RB moieties with an ESA at 430 nm and GSB at 554 nm in the deaerated solution, which, upon exposure to air, quenched significantly (Figure S12).
These features are similar to the neutral rhodamine radicals [63,64]. This result also confirmed that the species observed in the case of RB–Pery in non-polar solvents is a cation radical, not a neutral radical. As stated above, rhodamine 6G can easily form a stable radical upon single-electron reduction [63,64]. Stable organic radicals are highly important for many research fields, such as molecular electronics, solar cells, organic light-emitting diodes, photoredox catalysis, and super-resolution microscopy [69,70,71,72,73,74,75].
To confirm the assignment of the ESA bands of the RB–Pery dyad, the intermolecular TTET from diiodo-BDP (Scheme 1) to the reference (RB-Ph, Pery) and RB–Pery was studied with ns-TA spectroscopy (Figure 6, Figure 7 and Figure S13). Upon 520 nm pulsed laser excitation, the mixture of diiodo-BDP/RB-Ph (Figure 6a,b) initially showed a different TA spectrum of diiodo-BDP (with a strong GSB band at 530 nm and weak ESA bands in the range of 400–780 nm), which diminished with delay times, and a new spectral feature developed. Note that the photoexcitation of RB-Ph alone does not show any transient signal. The decay trace of the mixture at 565 nm shows a rise phase at the early delay time and then decay at a longer delay time, which indicates the occurrence of TTET. The lifetime of the diiodo-BDP (energy donor) is reduced to 21.7 μs, which is much shorter than that of the pristine diiodo-BDP (99.5 μs, Figure S14), further verifying the TTET process. To better evaluate the transient species produced in the TTET experiment, global fitting of the ns-TA data was carried out, and species-associated difference spectra (SADSs) were obtained. The SADS of the 3RB* state (triplet lifetime of 277.8 μs) is obvious, with an ESA band in the range of 380–470 nm and a GSB band at around 550 nm. Moreover, this result further shows that the triplet energy of rhodamine is lower than 1.67 eV (~1.64 eV, predicted through TD-DFT), making TTET possible from the 3BDP* state (ET1 = 1.67 eV) [76].
By following the same protocol, the TTET experiment from diiodo-BDP to Pery (ET1 = 1.53 eV) was conducted (Figure 6c,d). In this case, new ESA bands in the range of 380–500 nm and a minor GSB at 430 nm developed, and the decay trace at 485 nm shows biphasic characteristics (simultaneous rise and decay), which indicates TTET manifestation. After global fitting, the first SADS shows a triplet state of 3BDP* (τT = 8.7 μs) and the second SADS belongs to the 3Pery* (τT = 66.6 μs).
Next, the TTET of the mixture of diiodo-BDP/RB–Pery was conducted under similar experimental conditions (Figure 7a,b). Note that RB–Pery alone does not give any transient signal upon excitation in ACN. Upon TTET, the GSB band of the energy donor, diiodo-BDP, quickly decreased, and a new GSB band at 565 nm developed, which is the GSB band of the rhodamine moiety. Moreover, the broad ESA band of the energy donor in the range of 380–460 nm transformed from one structureless band to a band with splitting peaks in the range of 380–510 nm. Note that this positive absorption band is different from the triplet absorption of RB-Ph and Pery alone obtained after TTET. In the first SADS, the species with a lifetime of 14 μs is the spectrum of the triplet energy donor. The red SADS with a lifetime of 142 μs can be assigned to the T1→Tn transient absorption of 3RB–Pery*, which is similar to that observed upon direct photoexcitation (shown in Figure 5). Additionally, to allocate and analyse the observed triplet state in the RB–Pery dyad, we normalized the 3RB*, 3Pery*, and 3[RB–Pery]* states populated due to TTET in Figure 8a. The transient features of RB–Pery in the range of 380–510 nm resemble the ESA of pristine perylene, whereas the GSB is nearly similar to the RB-Ph. This result settled the argument that the triplet state in RB–Pery is delocalized on both moieties, which is further shown in the direct comparison of triplet features populated due to TTET and without TTET (Figure 8b). As mentioned above, the triplet energy of the perylene moiety is ~1.53 eV, and the rhodamine moiety is 1.64 eV (TD-DFT estimated), signifying the possibility of triplet energy balance/equilibrium between these two moieties in RB–Pery when measured in deaerated DCM. Previously, our group reported a delocalized triplet state in a bodipy–phenylethyl anthracene dyad; in that case, the triplet excited-state equilibrium was evident in the ns-TA spectra [77]. In the present case, the triplet-state spin density of RB–Pery is delocalized on the whole molecule (Figure 1), which also supports triplet excited-state equilibrium.
To summarize the photophysical processes observed in the RB–Pery in different polarity solvents, we constructed a Jablonski diagram (Scheme 2). In DCM, upon the photoexcitation of the rhodamine moiety, the S1 state of the rhodamine moiety was first populated. Subsequently, charge separation took place and gave a charge transfer state with an energy of 1.65 eV, supported by the CT emission band, followed by charge recombination results in SOCT–ISC, and a long-lived (τT = 109.8 μs) delocalized triplet state was observed. As the perylene and rhodamine triplet-state energy difference is small (~0.11 eV), a triplet-state interconversion or triplet-state energy balance is possible. Resultantly, a delocalized triplet state was observed. On the other side, upon the photoexcitation of RB–Pery in non-polar solvents such as p-XYL or TOL, only a persistent rhodamine radical cation was detected (lifetime up to milliseconds).

4. Conclusions

In summary, a rhodamine (RB)–perylene (Pery) compact electron donor/acceptor dyad (RB–Pery) was synthesized and studied for the spin-orbit charge-transfer intersystem crossing (SOCT–ISC) mechanism. The steady-state UV–vis absorption spectra show negligible electronic interaction between the perylene and the rhodamine units; however, the quenched emission of RB–Pery in polar solvents indicated photo-induced electron transfer. This is supported by the frontier molecular orbital calculations; HOMO is localized on the perylene unit and the LUMO is localized on the rhodamine unit. The nanosecond transient absorption (ns-TA) spectra of the dyad in DCM showed mixed characters of both the perylene and rhodamine triplet states. In fact, a delocalized triplet state was observed (i.e., the triplet states localized on the perylene and the rhodamine units are equilibrated); the triplet-state lifetime was determined as 109.8 μs. Thus, the proposed photophysical processes of RB–Pery in deaerated DCM are as follows: 1RB*-Pery→[RB–Pery+•]→[3RB*-Pery↔RB-3Pery*]. A long-lived rhodamine radical cation was observed in deaerated/aerated non-polar or low-polarity solvents (p-xylene or toluene), the lifetime of which is up to a millisecond time scale. The intermolecular triplet–triplet energy transfer, monitored using an ns-TA spectrometer, further verified the delocalized transient triplet features in the RB–Pery dyad. The potential energy curve (PEC) of the torsion between the electron donor and acceptor in the dyad indicates large rotation freedom in RB–Pery, and this may lead to inefficient SOCT–ISC and a low singlet-oxygen quantum yield (ΦΔ) of only 8%. However, the lower torsional freedom can keep the geometry between the donor and acceptor in charge transport favourable manner andvertical configuration can enhance the ISC efficiency. Our results are also useful for both future SOCT–ISC studies and fundamental photochemistry studies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/photochem4010004/s1, Figure S1: 1H NMR spectrum of RB-Ph (CDCl3, 400 MHz); Figure S2; 1H NMR spectrum of RB–Pery (CDCl3, 400 MHz); Figure S3: 13C NMR spectrum of RB-Ph (CDCl3, 126 MHz); Figure S4: 13C NMR spectrum of RB–Pery (CDCl3, 126 MHz); Figure S5: ESI-HRMS of RB–Pery; Figure S6: UV–vis absorption spectra of compound (a) RB-Ph and (b) RB–Pery in different solvents. c = 1.0 × 10−5 M; Figure S7: (a) Fluorescence emission spectra (optically-matched solutions were used, A = 0.10) of RB–Pery in different polarity solvents, λex = 415 nm. Normalized UV–vis absorption and fluorescence excitation spectra of RB–Pery (b) in p-XYL monitored at 650nm and (c) in DCM monitored at 800 nm. c ≈ 1.0 × 10−5 M. 25 °C; Figure S8: Fluorescence decay traces of the compounds in different solvents (a) RB–Pery monitored at 585 nm and (b) RB-Ph monitored at 585 nm. Fluorescence decay trace of the RB–Pery in DCM (c) monitored at 630 nm and (d) monitored at 690 nm. Excited with a 405 nm picosecond pulsed laser. c ≈ 1.0 × 10−5 M. 25 °C; Figure S9: Cyclic voltammogram of perylene. Conditions: in deaerated DCM containing 0.10 M Bu4NPF6 as supporting electrode, Ag/AgNO3 as reference electrode, redox potentials are versus Fc/Fc+. Scan rates: 50 mV/s. c = 1.0 × 10−3 M. 25 °C; Figure S10: (a) Transient absorption spectra of Pery in deaerated DCM and (b) decay traces at 540 nm in DCM, c = 2.0 × 10−5 M, λex = 432 nm. 25 °C; Figure S11: Transient absorption spectra of RB–Pery, (a) in deaerated p-XYL, c = 1.0 × 10−5 M; (b) in deaerated TOL, c = 1.0 × 10−5 M; (c) in deaerated THF, c = 2.0 × 10−5 M; (d-f) respective decay traces at 575 nm under aerated and deaerated conditions. λex = 570 nm. 25 °C; Figure S12: Transient absorption spectra of RB-Ph (c = 1.0 × 10−4 M) upon adding TEA (c = 4.0 × 10−3 M) (a) in deaerated DCM, (b) in aerated DCM and (c) decay traces at 560 nm in both cases. λex = 550 nm. 25 °C; Figure S13: Intermolecular triplet-triplet energy transfer (TTET): from diiodo-BDP to RB-Ph, Pery and RB–Pery measured by ns-TA spectroscopy; (a) selected kinetic traces of the mixture of the ns-TA spectra of diiodo-BDP and RB-Ph. (b) selected kinetic traces of the mixture of the ns-TA spectra of diiodo-BDP and Pery. (c) selected kinetic traces of the mixture of the ns-TA spectra of diiodo-BDP and RB–Pery. The concentration of diiodo-BDP was fixed c[diiodo-BDP] = 5.0 × 10−6 M and 1:1 molar ratios were used in every case. The ns-TA spectra are given in the main text. In deaerated ACN, λex = 520 nm. 25 °C; Figure S14: (a) Nanosecond transient absorption spectra of diiodo-BDP and (b) decay trace at 530 nm; c = 5.0 × 10−6 M in deaerated ACN. λex = 520 nm. 25 °C; Figure S15: Selected kinetic decay traces obtained after global fitting analysis (a) from ns-TA data of diiodo-BDP/RB-Ph mixture (b) from ns-TA data of diiodo-BDP/Pery mixture and (c) from ns-TA data of diiodo-BDP/RB–Pery. The respective raw data and SADS spectra are given in the main text Figure 6 and Figure 7; Table S1: Absolute Photo-luminescence Quantum Yield of the Compounds in different solvents; Table S2: Luminescence Lifetime of the Compounds in different solvents; Table S3: Singlet Oxygen Quantum Yields of Compounds in Different Solvents.

Author Contributions

Conceptualization, J.Z.; methodology, M.I., D.L. and K.Y.; writing—original draft preparation, J.Z., M.I. and X.Z.; supervision, J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

We thank the Ministry of Science and Technology (2023YFE0197600), the NSFC (U2001222), the Research and Innovation Team Project of the Dalian University of Technology (DUT2022TB10), the Fundamental Research Funds for the Central Universities (DUT22LAB610), and the State Key Laboratory of Fine Chemicals for financial support.

Data Availability Statement

The original data of this work can be requested from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Photochem 04 00004 sch001
Scheme 1. Synthesis of RB derivatives: (a) p-TsOH, AcOH, 9 0 °C, 12 h; DCM, chloranil, reflux, 4.5 h, yield: 5%. (b) p-TsOH, AcOH, 70 °C, 7 h; DCM, chloranil, rt, 2 h, yield: 15%. The molecular structures of two reference compounds of Pery and 2,6-diiodo-BDP used in the study are also presented.
Scheme 1. Synthesis of RB derivatives: (a) p-TsOH, AcOH, 9 0 °C, 12 h; DCM, chloranil, reflux, 4.5 h, yield: 5%. (b) p-TsOH, AcOH, 70 °C, 7 h; DCM, chloranil, rt, 2 h, yield: 15%. The molecular structures of two reference compounds of Pery and 2,6-diiodo-BDP used in the study are also presented.
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Figure 1. (a) The ground-state optimized geometry with a dihedral angle indicated (between C4–C7-C51–C53 atoms) of RB–Pery and (b) the ground-state potential energy curve (as a function of the rotational dihedral angle about the C–C connection between donor and acceptor units in RB–Pery). The blue dotted line denotes the thermal energy of room temperature (0.026 eV). Calculated using DFT (B3LYP/6-31G(d)) level with Gaussian 09.
Figure 1. (a) The ground-state optimized geometry with a dihedral angle indicated (between C4–C7-C51–C53 atoms) of RB–Pery and (b) the ground-state potential energy curve (as a function of the rotational dihedral angle about the C–C connection between donor and acceptor units in RB–Pery). The blue dotted line denotes the thermal energy of room temperature (0.026 eV). Calculated using DFT (B3LYP/6-31G(d)) level with Gaussian 09.
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Figure 2. The ground-state electron spin-density distributions of the HOMO/LUMO and triplet-state spin-density distributions of RB–Pery and RB-Ph. Isovalues are 0.001. All the calculations were based on the DFT (B3LYP/6-31G(d)) level with Gaussian 09.
Figure 2. The ground-state electron spin-density distributions of the HOMO/LUMO and triplet-state spin-density distributions of RB–Pery and RB-Ph. Isovalues are 0.001. All the calculations were based on the DFT (B3LYP/6-31G(d)) level with Gaussian 09.
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Figure 3. (a) UV–vis absorption spectra of the compounds in TOL; c = 1.0 × 10−5 M. (b) Comparison of the fluorescence emission spectra of Pery and RB–Pery with λex = 415 nm. (c) Comparison of the fluorescence emission spectra of RB-Ph and RB–Pery with λex = 520 nm; note that in both cases, optically matched solutions were used (A = 0.10; in TOL). (d) Normalized emission spectrum of RB–Pery in different solvents. λex = 520 nm. 25 °C.
Figure 3. (a) UV–vis absorption spectra of the compounds in TOL; c = 1.0 × 10−5 M. (b) Comparison of the fluorescence emission spectra of Pery and RB–Pery with λex = 415 nm. (c) Comparison of the fluorescence emission spectra of RB-Ph and RB–Pery with λex = 520 nm; note that in both cases, optically matched solutions were used (A = 0.10; in TOL). (d) Normalized emission spectrum of RB–Pery in different solvents. λex = 520 nm. 25 °C.
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Figure 4. Cyclic voltammograms of RB-Ph and RB–Pery in deaerated DCM containing 0.10 M Bu4NPF6 as supporting electrode and Ag/AgNO3 as reference electrode; redox potential is versus Fc/Fc+. Scan rates: 50 mV/s. c = 1.0 × 10−3 M; 25 °C.
Figure 4. Cyclic voltammograms of RB-Ph and RB–Pery in deaerated DCM containing 0.10 M Bu4NPF6 as supporting electrode and Ag/AgNO3 as reference electrode; redox potential is versus Fc/Fc+. Scan rates: 50 mV/s. c = 1.0 × 10−3 M; 25 °C.
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Figure 5. (a) Nanosecond transient absorption spectra of RB–Pery and (b) decay trace at 565 nm; c = 2.0 × 10−5 M in deaerated DCM. λex = 570 nm; 25 °C.
Figure 5. (a) Nanosecond transient absorption spectra of RB–Pery and (b) decay trace at 565 nm; c = 2.0 × 10−5 M in deaerated DCM. λex = 570 nm; 25 °C.
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Figure 6. Intermolecular triplet–triplet energy transfer (TTET) from diiodo-BDP to different triplet energy acceptors, measured using ns-TA spectroscopy; (a) ns-TA spectra of a mixture of diiodo-BDP and RB-Ph and (b) SADS obtained after global analysis. (c) ns-TA spectra of the mixture of diiodo-BDP and Pery and (d) SADS obtained after global analysis. In all cases, the concentration of diiodo-BDP was fixed (c[diiodo-BDP] = 5.0 × 10−6 M) and 1:1 molar ratios of donor and acceptor were used. The selected kinetic traces of the TTET are supplied in the Supplementary Information. In deaerated ACN, λex = 520 nm; 25 °C.
Figure 6. Intermolecular triplet–triplet energy transfer (TTET) from diiodo-BDP to different triplet energy acceptors, measured using ns-TA spectroscopy; (a) ns-TA spectra of a mixture of diiodo-BDP and RB-Ph and (b) SADS obtained after global analysis. (c) ns-TA spectra of the mixture of diiodo-BDP and Pery and (d) SADS obtained after global analysis. In all cases, the concentration of diiodo-BDP was fixed (c[diiodo-BDP] = 5.0 × 10−6 M) and 1:1 molar ratios of donor and acceptor were used. The selected kinetic traces of the TTET are supplied in the Supplementary Information. In deaerated ACN, λex = 520 nm; 25 °C.
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Figure 7. Intermolecular triplet–triplet energy transfer (TTET) from diiodo-BDP to RB–Pery, measured using ns-TA spectroscopy; (a) ns-TA spectra of a mixture of diiodo-BDP and RB–Pery and (b) SADS obtained after global analysis. The concentration of diiodo-BDP was fixed (c[diiodo-BDP] = 5.0 × 10−6 M) and 1:1 molar ratio of donor and acceptor was used. The selected kinetic traces of the TTET are supplied in the Supplementary Information. In deaerated ACN, λex = 520 nm; 25 °C.
Figure 7. Intermolecular triplet–triplet energy transfer (TTET) from diiodo-BDP to RB–Pery, measured using ns-TA spectroscopy; (a) ns-TA spectra of a mixture of diiodo-BDP and RB–Pery and (b) SADS obtained after global analysis. The concentration of diiodo-BDP was fixed (c[diiodo-BDP] = 5.0 × 10−6 M) and 1:1 molar ratio of donor and acceptor was used. The selected kinetic traces of the TTET are supplied in the Supplementary Information. In deaerated ACN, λex = 520 nm; 25 °C.
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Figure 8. (a) Comparison of the normalized SADSs of 3RB*, 3Pery*, and 3[RB–Pery]* after TTET with 2,6-diiodo-BDP. (b) Normalized triplet-state spectra of RB–Pery upon direct photoexcitation and due to TTET with diiodo-BDP/RB–Pery mixture (the experimental spectrum was taken from Figure 5a).
Figure 8. (a) Comparison of the normalized SADSs of 3RB*, 3Pery*, and 3[RB–Pery]* after TTET with 2,6-diiodo-BDP. (b) Normalized triplet-state spectra of RB–Pery upon direct photoexcitation and due to TTET with diiodo-BDP/RB–Pery mixture (the experimental spectrum was taken from Figure 5a).
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Scheme 2. Simplified Jablonski diagram illustrating the photophysical processes of RB–Pery upon photoexcitation in different-polarity solvents: The energy level of 1RB*-Pery is calculated from the cross point of the normalized UV–vis absorption spectrum and normalized fluorescence emission spectrum in DCM. The energy level of 1(RB–Pery+•) is calculated based on the CT emission band, in DCM. The triplet excited-state energy level of RB-3Pery* is based on literature values [61]. The triplet energy level of 3RB*-Pery is estimated through TD-DFT calculations of the rhodamine moiety at the B3LYP/6-31G level using Gaussian 09. p-XYL stands for p-xylene and DCM is dichloromethane.
Scheme 2. Simplified Jablonski diagram illustrating the photophysical processes of RB–Pery upon photoexcitation in different-polarity solvents: The energy level of 1RB*-Pery is calculated from the cross point of the normalized UV–vis absorption spectrum and normalized fluorescence emission spectrum in DCM. The energy level of 1(RB–Pery+•) is calculated based on the CT emission band, in DCM. The triplet excited-state energy level of RB-3Pery* is based on literature values [61]. The triplet energy level of 3RB*-Pery is estimated through TD-DFT calculations of the rhodamine moiety at the B3LYP/6-31G level using Gaussian 09. p-XYL stands for p-xylene and DCM is dichloromethane.
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Table 1. Photophysical parameters of the compounds a.
Table 1. Photophysical parameters of the compounds a.
Compoundsλabsb (nm)ε cλemd (nm)ΦΔ e (%)ΦF f (%)τF g (ns)τT h (μs)
Pery4384.24434.8 ± 0.577.73.9 ± 0.5596 ± 5 i
RB-Ph5677.0590– j14.21.3 ± 0.5 (97%)
8.5 ± 0.5 (3%)		– j
RB–Pery445, 5752.2, 6.3453, 5988.0 ± 12.4 (RB),
6.0 (Pery)		3.3 (96%)
16.7 (4%)± 0.2		109.8 ± 5
a In TOL, c = 1.0 × 10−5 M, 25 °C. b Absorption maxima. c Molar absorption coefficient at absorption maxima, ε: 104 M−1 cm−1. d Maximal emission wavelength, λex = 415 nm, A = 0.10, 25 °C. e Singlet oxygen quantum yield in p-XYL; Rose Bengal was used as standard (ΦΔ = 76% in MeOH) for RB-Ph and RB–Pery; Ru(bpy)3(PF3)2 was used as standard (ΦΔ = 57% in DCM) for Pery. f Absolute photo-luminescence quantum yield; error bar: ±0.1%; Pery and RB–Pery (λex = 415 nm), RB–Pery (λex = 506 nm), A = 0.10. g Luminescence lifetime, λex = 405 nm. h Triplet excited-state lifetime determined with nanosecond transient absorption spectroscopy in DCM. i Lifetime of the perylene radical cation determined with nanosecond transient absorption spectroscopy in deaerated DCM. j Not observed.
Table 2. Electrochemical redox potentials of the compounds a.
Table 2. Electrochemical redox potentials of the compounds a.
CompoundsEOX (V)ERED (V)
Pery+0.61 b– c
RB-Ph+0.89−1.32
RB–Pery+0.58, +0.87−1.29
a Cyclic voltammetry in N2-saturated DCM containing a 0.10 M Bu4NPF6, with Pt electrode as the counter electrode, glassy carbon electrode as the working electrode, and Ag/AgNO3 couple as the reference electrode versus Fc/Fc2+. b Literature value [59]. c Not observed.
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Imran, M.; Liu, D.; Ye, K.; Zhang, X.; Zhao, J. The Rhodamine–Perylene Compact Electron Donor–Acceptor Dyad: Spin-Orbit Charge-Transfer Intersystem Crossing and the Energy Balance of the Triplet Excited States. Photochem 2024, 4, 40-56. https://doi.org/10.3390/photochem4010004

AMA Style

Imran M, Liu D, Ye K, Zhang X, Zhao J. The Rhodamine–Perylene Compact Electron Donor–Acceptor Dyad: Spin-Orbit Charge-Transfer Intersystem Crossing and the Energy Balance of the Triplet Excited States. Photochem. 2024; 4(1):40-56. https://doi.org/10.3390/photochem4010004

Chicago/Turabian Style

Imran, Muhammad, Dongyi Liu, Kaiyue Ye, Xue Zhang, and Jianzhang Zhao. 2024. "The Rhodamine–Perylene Compact Electron Donor–Acceptor Dyad: Spin-Orbit Charge-Transfer Intersystem Crossing and the Energy Balance of the Triplet Excited States" Photochem 4, no. 1: 40-56. https://doi.org/10.3390/photochem4010004

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