Controlling Excited State Localization in Bichromophoric Photosensitizers via the Bridging Group

A series of photosensitizers comprised of both an inorganic and an organic chromophore are investigated in a joint synthetic, spectroscopic, and theoretical study. This bichromophoric design strategy provides a means by which to significantly increase the excited state lifetime by isolating the excited state away from the metal center following intersystem crossing. A variable bridging group is incorporated between the donor and acceptor units of the organic chromophore, and its influence on the excited state properties is explored. The Franck–Condon (FC) photophysics and subsequent excited state relaxation pathways are investigated with a suite of steady-state and time-resolved spectroscopic techniques in combination with scalar-relativistic quantum chemical calculations. It is demonstrated that the presence of an electronically conducting bridge that facilitates donor–acceptor communication is vital to generate long-lived (32 to 45 μs), charge-separated states with organic character. In contrast, when an insulating 1,2,3-triazole bridge is used, the excited state properties are dominated by the inorganic chromophore, with a notably shorter lifetime of 60 ns. This method of extending the lifetime of a molecular photosensitizer is, therefore, of interest for a range of molecular electronic devices and photophysical applications.


■ INTRODUCTION
In the design of photoactive systems, whether they are for photocatalytic, 1−4 environmental sensing, 5−7 photodynamic therapy, 8−11 or photovoltaic applications, 12,13 the light-harvesting molecule or moiety plays a crucial role in the overall functionality and efficiency of the system.−25 Whatever the desired photophysical behavior, a fundamental understanding of the photophysical and photochemical processes involved is vital to the photosensitizer's design.
The design of photoactive transition-metal complexes has greatly expanded from a simple donor−acceptor structure, with numerous structural combinations frequently involving multiple donating and accepting groups present in the literature. 2,26−30 Increasing the excited state lifetime of the photosensitizer is desirable in many applications, allowing greater time for additional processes and/or reaction steps to occur, 1,2,8 e.g., subsequent electron or energy transfer events that may lead to the activation of a catalytic unit.One means by which this can be achieved is through the incorporation of large aromatic chromophores.−36 For example, a recent study by Choroba et al. investigated pyrene-substituted Re(I) complexes with room-temperature phosphorescent lifetimes greater than 1000 μs. 23−54 Thiophenes have also been successfully incorporated into donor−acceptor dyes and have been shown to red-shift as well as increase the intensity of the charge-transfer transition in both organic and inorganic systems. 42,45,55,56−60  terpyridine complexes with triphenylamine (TPA) donors connected through ethynyl and thiophene bridges. 49They found that the lowest energy state was ILCT in nature and was red-shifted by the inclusion of both ethynyl and thiophene groups.−62 Our recent work has examined the effect of a range of bridging groups on the electronic transitions in a series of Ru(II) and Re(I) complexes containing a dipyrido[3,2-a:2′,3′c]phenazine (dppz) acceptor and a TPA donor. 38,42The bridge was shown to have a significant effect on the electronic properties of the ground state; however, the lowest energy excited state was consistently 3 ILCT in character.Herein, the role that a bridging group plays on the photophysical properties of a new series of bichromophoric transition-metal complexes is investigated.The structures shown in Figure 1 contain a metal center and an organic ligand composed of a 1,10-phenanthroline (phen) acceptor, a variable bridging unit, and a TPA donor group.Previous investigations have demonstrated that increasing the energy of the acceptor by changing it from a dppz to a phen moiety can have a significant impact on the excited state properties in Re(I) and Pt(II) phen-TPA systems without a bridging unit. 39,63We therefore endeavor to explore the role of a bridging group in these already photophysically rich systems by applying both steadystate and time-resolved spectroscopic methods in combination with scalar-relativistic quantum chemical calculations.Good communication between donor and acceptor moieties is desirable in many applications, so the role of a conducting thiophene bridge is initially investigated, exploring its effect on the FC photophysics as well as the excited state behavior.The properties of complexes containing an ethynyl bridging group, which has similar electronic properties while avoiding geometric factors such as bridge rotation, are also explored.Finally, in some cases, the ability to isolate excited states or disrupt specific electron transfer pathways may also be of interest; therefore, as a point of comparison, the effect of incorporating the electronically insulating 1,2,3-triazole bridge was also investigated.
■ EXPERIMENTAL SECTION Synthesis.The rhenium(I) complexes were synthesized through the 1:1 combination of [Re(CO) 5 Cl] and the appropriate ligand in toluene with heating at 80 °C overnight.They were isolated through either the removal of the solvent or precipitation.The platinum(II) complexes were synthesized through the 1:1 combination of [Pt(COD)(4-pentylphenylacetylide) 2 ] 63 in 1:5 CH 3 CN/CH 2 Cl 2 with heating at 45 °C for 3 days under a nitrogen atmosphere.They were purified by column chromatography on silica.Full details can be found in the Supporting Information.
Spectroscopy.Electronic absorption and resonance Raman spectroscopy, utilizing excitation wavelengths across the lowest energy absorption band, were used to investigate the FC photophysics.The excited state properties were studied using a combination of nanosecond (ns) transient absorption (TA) spectroscopy with 354.7 nm excitation, photon-counting measurements, and ns time-resolved resonance Raman (TR 3 ) (λ pump = 354.7 nm, λ probe = 354.7 nm).Further experimental details can be found in the Supporting Information.
Computational Details.All quantum chemical calculations were performed using the Gaussian 16 program 64 (B.01) (scalar-relativistic simulations were performed using Orca 65,66 �see details in the Supporting Information).The singlet ground-state geometries of the investigated structures were obtained at the density functional theory (DFT) level.A B3LYP 6 7 , 6 8 -based functional denoted B3LYP35 39,69−73 comprising 35% exact-exchange and 58.5% of nonlocal B88 74 exchange, and the LYP 68 correlation was employed for the complexes, while the CAM-B3LYP 75 functional was used for the organic ligand structures.The split-valence def2-SVP basis set 76 was used unless stated otherwise, alongside Grimme's D3 dispersion correction 77 with Beck−Johnson damping to account for long-range interactions.An implicit CH 2 Cl 2 solvent field was incorporated with the integral equation formalism, employing the SMD model. 78Timedependent DFT (TDDFT) calculations were performed by using the same respective functionals.Further details regarding the rotamers, basis set effects on excited state properties (def2-SVP vs def2-TZVP), and simulated spectra can be found in the Supporting Information.The optimized S 0 and T 1 geometries of the structures, including the various rotamers, are available in ref 79 via the open data repository Zenodo.

■ RESULTS AND DISCUSSION
Franck−Condon Photophysics (M-thio-TPA).The ground-state structures of the ligand and both thiophene complexes were modeled in solution by means of density functional theory (DFT) incorporating an implicit solvent field. 78However, due to the flexible nature of the thiophene bridge, different ligand orientations are possible in solution.A relaxed scan was therefore performed, whereby the dihedral angle between the phen and thiophene groups was altered in a stepwise manner.As shown in Table S1, four possible rotamers were identified in the ligand and both thiophene complexes, with energies that vary by only 0.02 eV for the ligand and 0.03 eV for the complexes.These structures were confirmed to be energetic minima via frequency calculations. 80he simulated electronic absorption spectra, obtained at the TDDFT level of theory, and the oscillator strengths shown in Figure 2 (and Figure S1) were determined via a Boltzmann weighted distribution of the four rotamers, with the transitions shown in bold attributed to the lowest energy rotamer.The higher energy shoulder in the electronic absorption spectrum at approximately 330 nm is attributed to the S 8 electronic state, which possesses a mixture of ILCT and LC character.The calculations predict that the lowest energy absorption band is primarily attributed to an ILCT transition (S 1 ) from the TPA donor to the phen acceptor at just over 400 nm for both the Re(I) and Pt(II) complexes (Figure S2).This is well matched with the experimentally observed λ max at 397 and 406 nm, respectively.Additional weaker transitions of ILCT, MLCT, and mixed character (S 2 −S 4 ) are also predicted to occur in this region.As shown in the charge-density difference (CDD) insets, the ILCT states (S 1 and S 3 ) of the Re(I) and Pt(II) species both involve significant electronic contributions from  the thiophene bridge as well as the TPA donor.Notably, the nature of the phen-localized acceptor orbital differs between the two ILCT states, while the donor orbital remains the same, consistent with previous studies. 39,63Interestingly, unlike in Ptthio-TPA, the S 3 state in Re-thio-TPA is not purely ILCT in nature, as it contains some mixed contribution from the metal.The respective energies, oscillator strengths, and order of the states are slightly affected by the nature of the rotamer (see Tables S2−S11); however, the lowest energy absorption band is primarily attributed to a strongly dipole-allowed ILCT transition.
TDDFT calculations performed along this dihedral angle (Figures S3 and S4) reveal that the energy of the S 1 ILCT state fluctuates in a similar manner to the S 0 energy, reaching minima when the dihedral angle between the phen and the thiophene bridge is close to planar, increasing the electron delocalization throughout the ligand.In contrast, the MLCT energy is stabilized as the planarity is decreased.The S 3 ILCT state is consistently higher in energy and lower in oscillator strength than S 1 within the energetic ground-state minima; however, the relative intensities and energies of these two states are slightly modulated by the dihedral angle.For example, the two lowest energy rotamers of Re-thio-TPA (rotamers A and B) with dihedral angles of ±45°show the greatest and lowest S 1 and S 3 intensities, respectively, while in the slightly less planar rotamers C and D, with dihedral angles of ±125°, the opposite is true.In the case of rotamer D, the lowest ILCT state also gains some metal character and is raised in energy to become the S 2 state.The orbital parentage of these states is discussed in further detail in the following section, as well as how resonance Raman spectroscopy was used to further support their characterization.
Resonance Raman spectra of Re-thio-TPA shown in Figure 3 were obtained in a CH 2 Cl 2 solution at multiple excitation wavelengths encompassing the lowest energy absorption band.Nonresonant 1064 nm FT-Raman spectra of solid samples in KBr disks were also measured for comparison.The edge of the 330 nm shoulder is probed with 351 nm excitation, and predominantly TPA-based vibrations 39,81−84 at 1606 and 1178 cm −1 are enhanced, consistent with a TPA-localized LC state.As the excitation wavelength is tuned to red, a number of other ligand-based vibrations grow in intensity.Delocalized vibrations at 1375 and 1441 cm −1 , which involve the whole ligand, are enhanced as well as phen localized modes at 1422 (coincident with a CH 2 Cl 2 mode), 1520, and 1584 cm −1 . 85,86he strong band at 1467 cm −1 has a significant thiophene character and shows strong enhancement with excitation wavelengths between 407 and 491 nm. 87The enhancement pattern is consistent with the formation of an ILCT state and highlights the active role that the bridge plays in the lowenergy electronic states, consistent with the TDDFT predictions.
While the spectra of the Re(I) and Pt(II) species are very similar, the 1583 cm −1 phen mode is more strongly enhanced at longer excitation wavelengths in Pt-thio-TPA.These slight differences in the resonance enhancement pattern are successfully replicated in the simulated spectra, shown in blue in Figures 3 and S5, for Re-thio-TPA and Pt-thio-TPA, respectively.Examination of the molecular orbitals involved in the low energy, strongly absorbing ILCT transition reveals primary contributions from a TPA-localized highest occupied molecular orbital (HOMO, orbital numbers 173 and 206 for Re-thio-TPA and Pt-thio-TPA, respectively) to two separate π phen * orbitals, corresponding to the LUMO (lowest unoccupied molecular orbital) and LUMO + 1.These unoccupied orbitals have pseudo b 1 and a 2 symmetry, respectively, based on the C 2v description of the isolated phen component of the ligand, 88 with the electron density of the b 1 acceptor orbital (orbital numbers 174(Re-thio-TPA) and 207(Pt-thio-TPA)) localized toward the center of the coordination sphere, while the electron density distribution of the a 2 acceptor orbital (orbital numbers 175(Re-thio-TPA) and 208(Pt-thio-TPA)) is more localized at the periphery of the coordination sphere, i.e., in closer proximity to the bridging moiety.Therefore, as shown in Table S12, the S 1 state of Re-thio-TPA can be described by weighted contributions of electron transfer between the orbitals 173 → 174(b 1 ) and 173 → 175(a 2 ), of 34 and 51%, respectively.Similarly, the S 1 state of Pt-thio-TPA can therefore be described as a transition between 206 → 207(b 1 ) and 206 → 208(a 2 ) with respective weightings of 49 and 33%.Therefore, the π(b 1 ) phen * orbital has a greater contribution to the S 1 ILCT transition in Pt-thio-TPA than in Re-thio-TPA.The greater involvement of the b 1 orbital (with its greater electron density toward the center of the coordination sphere) in the Pt(II) complex is likely responsible for the increased resonance enhancement of the 1583 cm −1 mode, which, as depicted in Figure S5, shows vibrational motion more closely associated with this orbital distribution.In addition, in both complexes, the aforementioned higherlying S 3 ILCT state shows the same orbital contributions but with the opposing dominant acceptor orbital to that involved in the S 1 state.Thus, we observed two ILCT transitions with differently weighted orbital parentage, as seen in previous studies. 39Lastly, it should be noted that the a 2 acceptor orbital exhibits double bond character with respect to the bridge and will, therefore, be heavily involved in the excited state relaxation.
Excited State Photophysics (M-thio-TPA).The triplet ground states of the thiophene complexes were also optimized, and as per the singlet structures, four rotamers were identified (Table S1).The spin density distribution of each rotamer in both the Re(I) and Pt(II) species was consistently shown to be 3 ILCT/ 3 LC in nature, delocalized over the entire ligand system, as shown in Figure 4.The fully delocalized nature of the T 1 spin density supports the near-planar ligand geometries obtained, as the planarity facilitates improved conjugation and electron delocalization over the ligand system, thereby minimizing the 3 ILCT/ 3 LC energy.Population of the previously discussed π(a 2 ) phen * acceptor orbital with the double bond character to the bridge leads to the planarization of this highly delocalized triplet state.This is in contrast to the primary dipole-allowed MLCT states, which involve the π(b 1 ) phen * orbital and hence do not promote ligand planarization. 89he excited state properties of the thiophene-containing complexes were investigated with nanosecond transient absorption spectroscopy and time-resolved resonance Raman, supported by TDDFT calculations.The complexes exhibited very weak and short-lived emission that could not be reliably measured with the 10 Hz, nanosecond pulse laser.The ns transient absorption spectra of the two complexes, obtained with 354.7 nm excitation, are given in Figure 4 and are almost identical, with the spectra of Re-thio-TPA and Pt-thio-TPA shown in black and gray, respectively.A strong ground-state bleach is observed at 330 nm, which extends out to 430 nm, after which the transient signal remains positive.A clearly resolved, strong signal is observed at 514 nm, with an additional prominent positive feature present at around 800 nm.−93 The simulated transient absorption spectra are an excellent match to the experimental results, with transitions from the T 1 ground state to the T 4 and T 9 states responsible for the 514 and 800 nm absorption signals, respectively (Figures 4 and S6).The excellent replication of the experimental TA spectra suggests that the 3 ILCT/ 3 LC nature and geometry of the triplet state are well modeled.As illustrated in Figure S7, the transient absorption features do not evolve over time and simply decay to the ground state, with no new bands observed.
As shown in Table 1, kinetic traces obtained at 515 nm show long-lived biexponential decay, with lifetime components of 12.0 and 42.5 μs for Re-thio-TPA, and 14.8 and 45.3 μs for Ptthio-TPA, with relative intensities of the τ 1 and τ 2 components of approximately 1:1 and 1:2 for the two thiophene complexes, respectively.The existence of biexponential decay means that the internal conversion rate between the two states must be slower than their respective nonradiative decay rates to the ground state and that both states have similarly weighted contributions.The long-lived nature of the excited states is attributed to significant 3 LC character and decoupling from the metal center.This is consistent with the previously discussed spin density of the optimized triplet geometry and the fact that the two lowest triplet states have a mixture of 3 LC and 3 ILCT characters.Furthermore, photon-counting measurements of the ligand also showed biexponential decay, with 1.4 and 4.7 ns components (Figure S8); however, the decay is primarily attributed to the short-lived component, so the possibility that the longer-lived component is due to an impurity cannot be excluded.
In line with our previous investigations, scalar-relativistic (SR) calculations performed utilizing the zero-order regular approximation (ZORA) and the Douglas−Kroll−Hess (DKH) approach 94,95 in the FC geometry (Tables S13−S20) revealed strong spin−orbit coupling (SOC) between the initially populated 1 MLCT states and upper 3 MLCT states.This provides an "MLCT gateway" to allow the low-lying 3 ILCT/ 3 LC states (T 1 , T 2 ) to be subsequently populated via internal conversion.These ligand-localized (triplet) states exhibit low SOC values with low-lying excited singlet states and, thus, are likely inaccessible directly via intersystem crossing.Given the prominent existence of rotamers in the investigation, it is important to consider the possibility that the biexponential nature of the decay could also be attributed to the decay from different rotameric structures present in solution.SR-TDDFT calculations were, therefore, also performed on all ground-state rotamers.However, no notable differences were observed, with excited states of nearequivalent energy, character, and SOC values obtained (Tables S13−S20).Therefore, it is unlikely that the presence of rotamers in solution is responsible for the biexponential decay; however, additional computational and experimental measure- ments were undertaken to examine this possibility and are briefly discussed in the final subsections.
The TA profile, combined with the long-lived excited states of Re-thio-TPA and Pt-thio-TPA, make them suitable complexes for investigation using ns time-resolved resonance Raman (TR 3 ) spectroscopy.354.7 nm pump and 532.0 nm probe pulses utilized to coincide with the ground-state bleach and excited state absorption, respectively, allowed a resonance Raman signal of the excited state to be obtained.The TR 3 spectra of Re-thio-TPA are given in Figure 5 at multiple delays between the pump and probe pulses, where the 532.0 nm probe-only spectrum corresponds to a nonresonant spectrum of the ground state (Figure S9).The upper panel gives the result of subtracting the solvent-normalized probe-only spectrum to show the excited state signature, such that the growth of excited state features at 1173, 1199, 1396, 1494, and 1571 cm −1 can be observed, which grow and decay as a collective group.As shown in Figure 4, a probe wavelength of 532.0 nm is suitable to probe the strong excited state absorption at 515 nm.As this characteristic signal does not feature in the parent complex without the thiophene bridge, it is attributed to delocalized ligand-based transitions involving the thiophene moiety. 39This is also consistent with the energy and character of the TDDFT-predicted T 9 state, which is highly delocalized over the ligand system (CDD in Figure 4).The modes at 1173 and 1574 cm −1 are likely attributed to TPA •+ features, 39,81,83,96 while the remaining modes are likely delocalized ligand and/or thiophene modes.Furthermore, the TR 3 spectra of Pt-thio-TPA (Figure S10) are almost identical to those of Re-thio-TPA, emphasizing the ligand-based nature of the excited state and the lack of involvement of the metal center.
Photophysics of M-CC-TPA.The possibility that the biexponential excited state decay could be due to the presence of different rotamers was further addressed by examining an additional ligand system containing an ethynyl linker between the phen and TPA moieties (Figure 1).This bridge also promotes communication between the donor and acceptor in a similar manner to the thiophene unit 38,57 but maintains a rigid, planar system thereby avoiding ligand-based rotamers.The photophysical behavior of the Re(I) and Pt(II) ethynyl complexes, denoted Re-CC-TPA and Pt-CC-TPA, in the FC region is very similar to that of the thiophene species.Steadystate absorption and resonance Raman spectroscopy reveal that ILCT states, with significant bridge involvement, dominate the FC photophysics (Figures S11−S13 and Tables S21−S22).In the excited state, the photophysical properties are also very similar to the thiophene complexes.Like the thiophene complexes, the emission of the ethynyl complexes was too weak or short-lived for reliable kinetic emission measurements to be taken on our ns system.However, the TA spectra have significant excited state absorption at 476 as well as at ∼800 nm, with minimal differences observed between Re-CC-TPA and Pt-CC-TPA (Figure S14).The TA spectra are also successfully simulated, with the DFT-optimized triplet state possessing both 3 ILCT and 3 LC characters.Finally, long-lived biexponential decay from dark states was also observed, with lifetime components of 12.4 and 43.6 μs for Re-CC-TPA, and 11.2 and 31.7 μs for Pt-CC-TPA, with approximate τ 1 /τ 2 weightings of 1:3 and 1:2, respectively.Akin to the thiophene complexes, larger SOC values are observed between upper singlet and triplet states of MLCT character, while the two lowest-lying triplet states retain 3 ILCT/ 3 LC character (Tables S23 and S24).These findings strongly suggest that for electronically conducting bridge species, the excited state decay is attributed to the presence of two noninteracting dark states delocalized over the ligand system and not to decay from different rotamers.Unfortunately, TR 3 spectra could not be obtained for the Re-CC-TPA and Pt-CC-TPA complexes as the strong excited state absorption occurs at 476 nm (Figure S14), compared to the 515 nm absorption of the thiophene species and is therefore not in resonance with the 532.0 nm probe pulse.
Photophysics of Re-trz-TPA.The importance of a conductive bridge between the TPA donor and phen acceptor to establish a long-lived excited state has been clearly demonstrated.As a point of contrast, the photophysical properties of a Re(I) complex with a triazole bridging group, denoted Re-trz-TPA, were also investigated, and four groundstate rotamers were identified (Table S25).As discussed previously, triazole groups�despite their aromatic nature�are well known to function as electronic insulators and can thereby disrupt donor−acceptor communication and, thus, the population of the desired long-lived charge-separated (e.g., 3 ILCT) states. 56,57,61,97As anticipated, the presence of the triazole group slightly blue-shifts the electronic absorption spectrum relative to the thiophene and ethynyl complexes, with a λ max of 392 nm and decreases the molar absorptivity (Figure S15).TDDFT calculations predict that, unlike the other complexes, the S 1 state is MLCT in nature but with minimal oscillator strength (Tables S26−S29).Resonance Raman spectra of Re-trz-TPA are notably different from the other species, with the totally symmetric CO stretching mode at 2026 cm −1 showing appreciable enhancement at low excitation energies, indicative of the presence of such a lowlying MLCT state.This trend was also successfully replicated in the Boltzmann weighted simulated resonance Raman spectra (Figure S16).
Additional differences are also observed in the excited state properties.Unlike the thiophene and ethynyl complexes, the excited state absorption of Re-trz-TPA is weak and poorly resolved (Figure S17) and possesses a comparatively short lifetime of approximately 60 ns.In addition, unlike the other complexes that exhibited very weak and/or short emission, Retrz-TPA emits at around 600 nm.This emissive lifetime is equivalent to that obtained in the TA measurements, implying that the same state is responsible for both signals.Furthermore, photocounting measurements of the ligand showed that, unlike thio-TPA, the ligand emission decay of trz-TPA was monoexponential (Figure S8).Four triplet ground-state rotamers of Re-trz-TPA were identified, and analysis of the spin density of each Re-trz-TPA rotamer reveals that the triplet state is 3 MLCT in nature, in contrast to the 3 ILCT/ 3 LC states of thiophene and ethynyl complexes.Furthermore, the triplet geometries are significantly less planar, which is in line with the different nature of the triplet state between the complexes with insulating vs conducting bridges.SR calculations show coupling values of around 500 cm −1 between singlet and triplet MLCT states of similar energy within the FC geometry (Table S30).The differing photophysical properties, namely, the 3 MLCT character and shorter excited state lifetime of this insulating-bridge complex highlight the importance of the presence of low-lying 3 ILCT/ 3 LC states (populated via MLCT gateway states) to achieve desired long lifetimes.

■ CONCLUSIONS
A thorough spectroscopic and computational investigation has been performed to understand the photophysical behavior exhibited by a series of bichromophoric complexes.Steadystate techniques combined with DFT and TDDFT simulations allowed the underlying FC photophysics to be investigated, while the nature of the excited state relaxation pathways was elucidated with time-resolved techniques in conjunction with (SR)-TDDFT.The nature of the bridging group has been shown to dominate the excited state photophysics in the investigated Re(I) and Pt(II) complexes.Incorporating an electronically conducting thiophene or ethynyl unit facilitates the generation of strongly absorbing dark states with long biexponential lifetimes.The presence of rotamers in solution was identified, but based on the combination of the aforementioned techniques and a careful comparison of the predicted behavior of each rotamer, the biexponential excited state decay was attributed to the presence of two highly delocalized 3 ILCT/ 3 LC states and not to separate rotamers.Such long-lived states have not been observed in previous studies involving phen-TPA complexes, 39,63 or even within similar dppz-B-TPA complexes incorporating the same bridging units. 38,42hese findings illustrate how the use of a bichromophoric molecule, with both metal and organic components, can be utilized to successfully generate long-lived excited states.Strong SOC facilitates 3 MLCT population, but if low energy 3 ILCT/ 3 LC states are also present (achieved via incorporation of a conducting bridge species), these are subsequently populated via internal conversion.We therefore highlight the importance of energetic and electronic matching of the structural components to achieve prolongated excited states and that although click-chemistry is a strong tool for the synthesis of triazoles, the use of such electronically insulating bridging groups should be avoided if long-lived excited states are the desired outcome.Such long-lived triplet states have the potential to be used in additional electron and/or energy transfer steps, such as within photocatalytic reaction processes, e.g., in the context of clean energy generation, to allow the activation of the catalytic unit, either in a heterogeneous or homogeneous fashion.

Figure 1 .
Figure 1.Structure of the complexes studied.The transition metal (M) corresponds to a ReCl(CO) 3 or a Pt(R) 2 core, and the ligand is comprised of a 1,10-phenanthroline acceptor (phen), a variable bridging unit (B, indicated in red), and a triphenylamine (TPA) donor group.The dashed lines indicate the connection positions.The complexes are subsequently labeled with the notation M−L, where M = Re or Pt, L = phen-thiophene-TPA (thio-TPA), phen-ethynyl-TPA (CC-TPA) and phen-1,2,3-triazole-TPA (trz-TPA).The structure of Re-thio-TPA is shown for reference.

Figure 2 .
Figure 2. Experimental (in black) and simulated (in blue; B3LYP35, def2-SVP, CH 2 Cl 2 solvent field) electronic absorption spectra of Re-thio-TPA (left) and Pt-thio-TPA (right) in CH 2 Cl 2 .The simulated spectra were obtained via a Boltzmann weighted distribution of the four identified rotamers, with the bold blue vertical lines corresponding to the transition energies for the lowest energy rotamer (rotamer A).The charge-density differences (CDDs) are shown for key transitions for rotamer A, with the electron density moving from red to blue.

Figure 3 .
Figure 3. Simulated (left; B3LYP35, def2-SVP, CH 2 Cl 2 solvent field) and experimental resonance Raman spectra of Re-thio-TPA (center) and Ptthio-TPA (right) obtained at a range of excitation wavelengths, indicated on the left-hand side.A nonresonant 1064 nm FT-Raman spectrum obtained in a KBr disk is included for comparison.The simulated spectra were obtained from a Boltzmann weighted distribution of rotamers' spectra; frequencies are scaled by 0.95 (left); and experimental resonance Raman spectra were obtained in CH 2 Cl 2 (solvent bands indicated with an *).

Figure 4 .
Figure 4. Boltzmann weighted simulated (blue; B3LYP35, def2-SVP, CH 2 Cl 2 solvent field) and experimental (black) transient absorption spectra of Re-thio-TPA obtained with 354.7 nm pulse excitation in degassed CH 2 Cl 2 .The downward vertical bars correspond to ground-state bleach (singlet−singlet transitions in the S 0 equilibrium), while upward vertical bars are associated with excited state absorption (triplet−triplet transitions in the T 1 structure).Bold bars correspond to rotamer A. The oscillator strengths have been scaled based on the Boltzmann weighting.The top inset shows the CDD for the T 9 state.The lower inset shows the spin density distribution of Re-thio-TPA (left) and Pt-thio-TPA (right) in their optimized T 1 geometry, which would likely be obtained from excitation of the S 0 rotamer A. The experimental spectrum of Pt-thio-TPA is also shown in faded gray for comparison.

Figure 5 .
Figure 5. TR 3 spectra of Re-thio-TPA obtained with different pump pump = 354.7 nm) and probe (λ probe = 532.0nm) delays in a degassed CH 2 Cl 2 solution.The lower panel shows the pump + probe spectra normalized to the 1157 cm −1 solvent band, while the upper panel shows the result of subtraction of the "probe-only" spectrum.Excited state bands are indicated in bold.
Robson et al. studied a family of Ru(II)

Table 1 .
Transient Absorption Kinetics Data Obtained for the Complexes in Degassed CH 2 Cl 2 Solutions with 354.7 nm Excitation a a λ indicates where the kinetic trace was measured.The approximate relative intensities of the biexponential components are given in brackets.b Obtained through photon-counting measurements.