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. 2025 Oct 28;10(44):52473–52482. doi: 10.1021/acsomega.5c05525

Synthesis, Optoelectronic, and Photoelectrochemical Properties of an Azulene-Bridged Porphyrin Dimer

Sahana Nagesh Shet †, Dharmapura H K Murthy ‡,*, Sujana Chandrappa ‡, Simon Joyson Galbao ‡, Mahendra Patil §,*, Vijayendra S Shetti †,*
PMCID: PMC12613113  PMID: 41244410

Abstract

Although azulene-incorporated porphyrin analogues, referred to as ″azuliporphyrins”, are well documented in the literature, the studies available on covalently tethered azulene–porphyrin hybrid molecules are limited. This work reports the synthesis of 1,3-azulene-bridged porphyrin dimers 3 and 4, achieved through the Suzuki–Miyaura cross-coupling reaction. The optoelectronic properties, photophysical behavior, excited-state dynamics, and potential application of dimer 4 in photoelectrochemical water splitting (PEC-WS) are investigated. Dimer 4 exhibits a smaller HOMO–LUMO gap and a lower ionization potential than the corresponding monomer 5. However, despite having an extended π-conjugation, the V-shaped geometry of the dimer restricts its long-range aggregation and charge/exciton delocalization in comparison to its corresponding monomer. The computational electronic structure analysis complements the experimental findings. The results presented in this paper provide rational insights into the structure–property correlation within the less explored azulene–porphyrin hybrid systems.

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Introduction

Light-absorbing organic π-conjugated materials find diverse applications in electronics and photonics. − Among different classes of organic molecules, porphyrins (Figure a) offer chemical stability and intriguing photophysical and optoelectronic properties. − The nature or extent of aggregation critically impacts the efficiency of exciton/charge transport and light absorption behavior in porphyrin-based systems, collectively influencing properties such as solar energy conversion efficiency. The molecular-level aggregation and self-assembly of porphyrin-based systems can rationally be tuned by functionalizing porphyrin’s meso- or β- positions and metalation, in addition to other parameters such as variation in pH, solvents, etc. , However, despite decades of extensive research, obtaining precise control over molecular-level aggregation remains challenging in porphyrin-based molecular systems.

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Chemical structures and numbering schemes of (a) porphyrin and (b) azulene.

Azulene (Figure b) is a nonalternant, nonbenzenoid hydrocarbon having a fusion of electron-rich five-membered and electron-deficient seven-membered rings. Due to this structural feature, azulene possesses an inherent dipole moment (1.08 D), unlike its isomer naphthalene. The narrow HOMO–LUMO gap and anti-Kasha emission make it an exciting molecule for advanced optoelectronic material research. − Although azulene-incorporated porphyrinoids, where one or more macrocyclic core pyrrole units are substituted by azulene, are well studied under the category ‘azuliporphyrins’, the covalently tethered azulene–porphyrin hybrid molecules are only sporadically investigated. −

Porphyrin dimers, one of the most straightforward multiporphyrin systems, are effective model systems for studying energy and electron transport dynamics and artificial photosynthesis. − The two porphyrin units in a porphyrin dimer can either be directly linked or linked through a linker, which enables effective electronic communication between them. − ,−

The availability of limited examples of azulene–porphyrin hybrid molecules, in general, and azulene-bridged porphyrin dimers, in particular, led us to explore azulene-bridged porphyrin dimers. In this work, we present the synthesis of azulene-bridged porphyrin dimers (1,3-azulene-connected) achieved through the Suzuki–Miyaura cross-coupling reaction. The optoelectronic, photoelectrochemical, and computational studies of dimer 4 are presented, which provide insights into the structure–property correlation and its impact on the aggregation behavior and the photoelectrochemical water-splitting (PEC-WS) performance.

Materials and Methods

General

General chemicals, solvents, and silica gel were procured from indigenous brands, such as Spectrochem, Avra, and Loba Chemie and used without further purification unless otherwise specified. Freshly distilled pyrrole was used for the porphyrin synthesis. Azulene was procured from Merck and used without further purification. 1,3-Dibromoazulene 1 and borylated Zn-porphyrin 2 were synthesized by following the literature procedures. −

NMR and Mass Measurements

1H and 13C NMR spectra were recorded on a Bruker instrument operating at 400 and 100.6 MHz, respectively, by using tetramethylsilane [Si­(CH3)4] as an internal standard. The NMR measurements were carried out at room temperature in a deuterated chloroform (CDCl3) solvent. The high-resolution mass spectrometry (HRMS) data were obtained using an Agilent 6550 LC/Q-TOF instrument.

Optical Absorption, Emission, and Lifetime Measurements

Absorbance measurements were conducted on a JASCO Model V-770 spectrophotometer. A 0.0625 mg/mL concentration was used to compare the absorbance of dimer 4 and monomer 5. The molar absorption coefficient values of absorption bands of dimers 3 and 4 were computed using 1 × 10–5 and 1 × 10–6 M solutions of toluene for Q and Soret bands, respectively. The steady-state photoluminescence (PL) analysis was performed on a fluorescence spectrophotometer (HITACHI F-4700). Time-resolved photoluminescence measurements were conducted using an Edinburgh Instruments FLS1000 spectrometer. For the solution measurements, 1 mg/mL stock solutions of dimer 4 and monomer 5 were prepared in chlorobenzene. For the thin-film preparation,15 μL of 1 mg/mL solution of both dimer 4 and monomer 5 were drop-cast on clean quartz substrates of area 1 cm2 and dried under an incandescent bulb for 20 min.

Electrochemical Experiments

A potentiostat (Metrohm Autolab PG STAT204) with a three-electrode system was used for electrochemical measurements. Porphyrin (dimer 4 and monomer 5)-coated ITO (indium tin oxide) glass substrates and glassy carbon electrode (GCEs) were used as the working electrode and Pt mesh was used as the counter electrode. A reversible hydrogen electrode and Ag/AgCl electrode (3 M KCl) were used as reference electrodes for photoelectrochemical measurements (linear sweep voltammetry) and HOMO level calculations, respectively.

Electrode Preparation

A 20 μL of 1 mg/mL solution of porphyrins (dimer 4 and monomer 5) in chlorobenzene was drop-cast on the ITO (indium tin oxide)-coated glass substrates. The ITO-coated glass substrates were dried under an incandescent bulb for 20 min and heated on a hot plate at 150 °C for 20 min.

Photoelectrochemical Measurements

Linear sweep voltammetry was conducted in 0.1 M K2SO4 (pH ≈ 5.6) at a scan rate of 50 mV s–1. A solar simulator (Holmarc, HR-SS300WRM1-100A) was equipped with a 300 W xenon short arc lamp (Ushio Inc. Japan), an air mass 1.5 filter, and a 420 nm visible-light filter that was used as the light source.

HOMO Level Calculation from CV

A 10 μL of 1 mg/mL solution of porphyrins (dimer 4 and monomer 5) was drop-cast on the GCE and dried at room temperature. Cyclic voltammetry (CV) was conducted in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile at a scan rate of 25 mV s–1. The HOMO energy levels were calculated according to the equation

EHOMO=−IP=−4.8−(Eox−E1/2ferrocene)eV 1

where IP is the ionization potential and the unit of the oxidation onset potential (E ox) is V vs Ag/AgCl and E1/2ferrocene is calculated using the equation

E1/2ferrocene=12(Ep,a+Ep,c) 2

where E p,a is the anodic peak potential and E p,c is the cathodic peak potential calculated using the CV of ferrocene, as shown in Figure S9.

Computational Methods

All calculations were performed using the Gaussian16 quantum chemical program. All stationary points were optimized in the solvent phase without any constraints. The B3LYP functional − was employed in combination with the 6-31G** basis set (for C, H, O, N). − Empirical dispersion corrections (D3) for the B3LYP functional were included in the calculations. The optimized stationary points were characterized as local minima by verifying the vibrational frequencies obtained through frequency calculations. The optimized geometries were further used to calculate vertical excitation energies using TDDFT. For this purpose, the long-range corrected CAM-B3LYP functional was employed together with the 6-311G** basis set. Solvent effects were included using the polarizable continuum model (PCM) with chlorobenzene as a solvent. − Interaction energies of the aggregated systems were calculated in the gas phase at the B3LYP-D3/6-311G** level and corrected for basis set superposition error (BSSE) using the counterpoise method of Boys and Bernardi. ,

Synthesis of Dimer 3

A stirred DMF solution (10 mL) containing 1,3-dibromoazulene 1 (10 mg, 35 μmol), borylated Zn-porphyrin 2 (50 mg, 74 μmol), Na2CO3 (31 mg, 296 μmol), and Pd­(PPh3)4 (11.5 mg, 10 μmol) was heated at 80 °C for 6 h under a nitrogen atmosphere. The reaction mixture was then quenched with 20 mL of water and extracted with dichloromethane (2 × 20 mL). The combined organic layers were dried over anhydrous Na2SO4 and then concentrated to dryness under reduced pressure. The resulting residue was purified by neutral alumina column chromatography using petroleum ether/dichloromethane (10:90, v/v) as an eluent, and dimer 3 was obtained as a purple solid (22 mg, 50%).

MP > 300 °C. 1H NMR (400 MHz, CDCl3): δ 2.74 (s, 12H, −CH3), 7.06 (t, J = 10 Hz, 2H, azulene), 7.56–7.63 (m, 9H, Ar + azulene), 8.11 (dd, J = 7.6 Hz, J = 1.6 Hz, 4H, Ar), 8.24 (dd, J = 7.2 Hz, J = 1.2 Hz, 4H, Ar), 8.34 (d, J = 10 Hz, 2H, azulene), 9.10 (d, J = 4.4 Hz, 4H, pyrrole β-H), 9.13 (d, J = 4.4 Hz, 4H, pyrrole β-H), 9.31 (d, J = 4.4 Hz, 4H, pyrrole β-H), 9.40 (d, J = 4.4 Hz, 4H, pyrrole β-H), 9.67 (s, 1H, azulene), 10.24 (s, 2H, meso-H) ppm. 13C­{1H} NMR (100 MHz, CDCl3): δ 29.5, 105.8, 115.7, 120.7, 124.1, 127.3, 129.7, 129.9, 130.0, 131.5, 132.1, 132.3, 132.5, 134.5, 134.6, 137.0, 138.4, 139.3, 139.9, 142.8, 149.8, 150.3, 150.4, 150.7 ppm. LCMS (Q-TOF): C78H52N8Zn2 [M + H]+ Calc. mass 1233.2958, found m/z: 1233.2957. UV–vis [in toluene, λmax/nm, (log ε)]: 419 (5.92), 549 (4.94), 592 (4.37).

Synthesis of Dimer 4

To a stirred solution of dimer 3 (20 mg, 18 μmol) in dichloromethane, trifluoroacetic acid (TFA) (0.1 mL) was added at room temperature, and stirring was continued for a further 10 min. The resulting solution was then neutralized with an aqueous solution of K2CO3, extracted with dichloromethane, dried over anhydrous Na2SO4, and evaporated under reduced pressure to obtain dimer 4 as a purple solid (18 mg, 98%).

MP > 300 °C. 1H NMR (400 MHz, CDCl3): δ −2.68 (s, 4H, NH), 2.74 (s, 12H, −CH3), 7.10 (t, J = 10 Hz, 2H, azulene), 7.57–7.64 (m, 9H, Ar + azulene), 8.12 (d, J = 7.4 Hz, 4H, Ar), 8.24 (d, J = 7.6 Hz, 4H, Ar), 8.41 (d, J = 10 Hz, 2H, azulene), 9.03 (d, J = 4.8 Hz, 4H, pyrrole β-H), 9.07 (d, J = 4.8 Hz, 4H, pyrrole β-H), 9.22 (d, J = 4.4 Hz, 4H, pyrrole β-H), 9.36 (d, J = 4.4 Hz, 4H, pyrrole β-H), 9.72 (s, 1H, azulene), 10.23 (s, 2H, meso-H) ppm. 13C­{1H} NMR (100 MHz, CDCl3): δ 29.7, 104.8, 114.0, 114.8, 119.9, 124.5, 127.6, 129.5, 131.1, 131.4, 131.8, 134.7, 137.4, 138.7, 138.9, 139.5, 143.1, 150.6 ppm. LCMS (Q-TOF) C78H56N8 [M + H]+ Calc. mass 1105.4701, found m/z: 1105.4700. UV–vis [in toluene, λmax/nm, (log ε)]: 416 (4.78), 514 (3.97), 553 (3.68), 589 (3.79), 648 (3.71).

Results and Discussion

This section is divided into three main parts. The first part addresses the synthesis and structural analysis of dimer 4. The second part explores the optoelectronic and photophysical properties of the dimer along with a computational analysis. Finally, the last part briefly discusses the photoelectrochemical water-splitting (PEC-WS) reaction involving dimer 4.

Synthesis

The azulene-bridged porphyrin dimers 3 and 4 were synthesized by following the reaction steps presented in Scheme . The Suzuki–Miyaura cross-coupling reaction , between 1,3-dibromoazulene 1 and borylated Zn-porphyrin 2 , resulted in the formation of dimer 3 in a 50% yield, which upon treatment with trifluoroacetic acid (TFA) yielded dimer 4 in a near-quantitative yield. Alternatively, dimer 4 can also be obtained directly by the Suzuki–Miyaura cross-coupling reaction between 1 and borylated free-base porphyrin (Zn-free analogue of 2), but the yield obtained for 4, in this case, was lower than that from the earlier method. The dimers 3 and 4 were convincingly characterized by NMR (1H & 13C) spectroscopy, and their molecular formula assignments were done based on the high-resolution mass spectral (HRMS) data (Figures S1–S7). A stacked plot of selected portions of the 1H NMR spectra of dimer 4 and monomer 5 and the partial 1H–1H COSY spectrum of 4 is presented in Figure .

1. Synthesis of Azulene-Bridged Porphyrin Dimers 3 and 4 .

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Selected regions of the 1H NMR spectra of (i) monomer 5 and (ii) dimer 4 and (iii) the partial 1H–1H COSY spectrum of dimer 4.

The resonance signals labeled as ‘a–d’ correspond to the protons of the bridged azulene unit in dimer 4. The signals arising from β-pyrrole protons, aryl ring protons of meso-p-tolyl groups, and meso-H of porphyrin rings of dimer 4 and monomer 5 are labeled as ‘β-P,’ ‘Ar,’ and ‘e,’ respectively. The signals from pyrrole NH of the porphyrin core and methyl protons of meso-p-tolyl groups in 4 and 5 are labeled as ‘NH’ and ‘CH3’, respectively.

In dimer 4, the signals ‘NH’ and ‘e’ were observed at δ −2.68 and 10.23 ppm, respectively. These signals experienced slight downfield (compared to δ −3.11 ppm in monomer 5) and upfield shifts (compared to δ 10.31 ppm in monomer 5). The ‘CH3’ signal appeared as a singlet at δ 2.74 ppm in both dimer 4 and monomer 5. The ‘Ar’ signals for dimer 4 appeared as four doublets at δ 7.58, 7.63, 8.12, and 8.24 ppm, in contrast to the two doublets at δ 7.62 and 8.17 ppm observed in monomer 5. Similarly, the ‘β-P’ signals for dimer 4 were detected as four doublets at δ 9.03, 9.07, 9.22, and 9.36 ppm, while monomer 5 showed two doublets at δ 9.12 and 9.40 ppm. This enhancement in the number of signals for ‘Ar’ and ‘β-P’ protons could presumably arise from the hindered rotation of porphyrins in dimer 4. The azulene unit in dimer 4 exhibited signals ‘b’, ‘c’, and ‘d’ as a triplet at δ 7.1 ppm, a doublet at δ 8.4 ppm, and a singlet at δ 9.7 ppm, respectively. However, the signal for ‘a’ was not observed distinctly and appeared to overlap with the ‘Ar’ signals present in the δ 7.58–7.64 ppm region. To identify signal ″a”, the 1H–1H COSY spectrum of dimer 4 was analyzed. A cross-peak correlation was found between signals ‘b’ and ‘c,’ as well as one of the ‘Ar’ signals in the δ 7.58–7.64 ppm region. Since proton ‘b’ can only couple with protons ‘a’ and ‘c’ through 3 J H–H coupling, it can be concluded that the signal for ‘a’ is indeed overlapped in the δ 7.58–7.64 ppm region, thereby confirming its identity (Figure iii).

Optoelectronic and Photophysical Properties and Computational Analysis

The comparison of solution-state absorption spectra in the J-aggregate region of dimer 4 and monomer 5 (Figure a) revealed that dimer 4 exhibits a red-shifted absorption onset compared to 5 due to the extended conjugation (increased electron cloud) arising because of covalently bridged azulene in 4. The optical absorption spectra of dimer 4 and monomer 5 in the Soret band region around 410 nm also displayed such a red shift (Figure S8). In such a scenario, a change in the ionization potential (ease of removing an electron) is anticipated between dimer 4 and monomer 5. To investigate this and the origin of red-shifted absorption, cyclic voltammetry (CV) measurements were conducted. Figure b shows the CV data of dimer 4 and monomer 5 thin films coated on the GCE with ferrocene as a standard (Figure S9). From the data presented in Figure b, the HOMO level of monomer 5 was deduced to be −5.62 eV, which is in agreement with the earlier report. After validation of this approach for monomer 5, a similar method was employed for dimer 4, which showed an earlier onset of current than that of monomer 5. This observation indicated that it is easier to remove an electron, i.e., the lower ionization potential of dimer 4, compared to monomer 5, which can be attributed to the increased electron cloud in dimer 4 (due to azulene bridging). Calculation of the potentials and energetics involved indicates a ∼0.15 eV upward shift of the HOMO level position toward the vacuum for dimer 4 and corroborates the red-shifted absorption. Such a change in the HOMO level upon inclusion of electron-donating substituents was also observed in conducting polymers.

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(a) Optical absorption spectra of dimer 4 and monomer 5 recorded in chlorobenzene at a 0.0625 mg/mL concentration. The inset shows a comparison of Soret bands measured at an approximately 0.0065 mg/mL concentration. (b) Cyclic voltammograms of dimer 4 and monomer 5 thin films coated on the GCE to estimate the HOMO levels (the inset shows an enlarged view to understand the change in the current onset). Comparative steady-state PL profiles of (c) dimer 4 and (d) monomer 5 in thin films coated on the quartz substrate and in chlorobenzene solution (0.625 × 10–3 mg/mL).

In porphyrins, observing energetic shifts (blue or red shifts) in the electronic transitions can be a good indicator to suggest the type of aggregation (H- or J-type). − To probe the aggregation behavior of dimer 4, the photoluminescence (PL) data of solution and thin films coated on quartz were compared and this comparison is shown in Figure c,d. In solution, molecules can freely move, while in the solid state (thin film), molecules have fewer degrees of freedom and are forced to aggregate. Porphyrin monomer 5 showed a 10 nm red shift in emission for the thin film compared to the solution. However, dimer 4 showed a 4 nm blue shift in emission for the thin film compared to its solution counterpart, in contrast to the behavior exhibited by monomer 5. This comparison emphasizes the difference in the excited-state reorganization and inefficient delocalization behavior for dimer 4 compared to porphyrin 5, suggesting a hindered aggregation of the dimer.

Density functional theory (DFT) calculations were performed to gain deeper insights into the structure and optoelectronic properties of porphyrin dimer 4. Different structures of dimer 4, its dimeric aggregates (4D1, 4D2), and the dimeric aggregate of monomer 5 were considered for the optimization using the B3LYP level of theory, including Grimmes dispersion correction (Figure ).

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Optimized geometries of 4, 5D, 4D1, and 4D2. 4D1 and 4D2 represent different stackings of dimer 4 in its dimeric aggregate.

The interaction energies in the aggregates 4D1 and 4D2 were calculated to be −41.3 and −34.5 kcal/mol, respectively, suggesting that dimer 4 was coordinated to each other in 4D1 and 4D2 through noncovalent interactions. The calculated interaction energy for the parallel displaced alignment of 5 in 5D was −25.2 kcal/mol. These interaction energies, computed in the gas phase with BSSE correction at the B3LYP-D3­(DMF)/6-311G** level of theory, reveal that dimer 4 can form stable aggregates despite the strictly prohibited parallel end-to-end stacking of its porphyrin rings imposed by the overall bent molecular geometry.

To further understand the factors responsible for the observed red shift in absorption spectra for dimer 4 compared to monomer 5 (Figure a), the frontier molecular orbitals (MOs) were analyzed. As depicted in Figure a, the MOs of dimer 4 exhibited typical Goutermann-type four-orbital compositions on porphyrin with substantial delocalization of the electron density on the bridged azulene. The plot of HOMO and LUMO energies (Figure b) indicated that compounds 4 and 5 and their dimeric aggregates 4D1,4D2, and 5D have HOMO–LUMO gaps ranging from 4.0 to 4.5 eV. Evidently, dimer 4 and its aggregates (4D1 and 4D2) have a smaller HOMO–LUMO gap compared to monomer 5 and its aggregate 5D. The increased conjugation induced due to the bridging azulene in 4 led to a rise in the energy of the HOMO and a lowering in the energy of the LUMO compared to 5. The effect of conjugation was relatively more significant in HOMO – 1, HOMO – 2, LUMO + 1, and LUMO + 2, leading to lower energy transitions in which these orbitals are involved.

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(a) Representative set of MOs of dimer 4 and (b) plot of HOMO and LUMO energies of 4, 4D1, 4D2, 5, and 5D.

The time-resolved PL decay profiles (in nanoseconds) of dimer 4 and monomer 5 are shown in Figure . A single-exponential function fit (dotted line on the transient) indicated the decay to follow a monomolecular-type recombination. The idea behind comparing lifetimes is to shed insight into the role of the dimer’s structure on the exciton delocalization and decay behavior. A pronounced decrease in the exciton lifetime for dimer 4 by ≈4.8 ns compared to monomer 5 was observed. This significant reduction in the lifetime suggests an inefficient exciton diffusion/transport for dimer 4 such that the exciton decay is primarily mediated via intermolecular interaction within the localized small aggregates. This observation suggests the formation of hindered J-aggregates in the dimer, which can be attributed to its V-shaped geometry, as predicted by the computational analysis. It should be noted that the time scale probed here was in nanoseconds, and most of the ultrafast relaxation processes, such as vibronic relaxation to the lowest excited state, are difficult to visualize. ,

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Time-resolved PL decay curves for dimer 4 and monomer 5. Dotted gray lines on the transients are the result of a single-exponential decay function. Excitation was at 405 nm, and emission was probed at 650 nm. Both compounds were dissolved in chlorobenzene with a 0.1 mg/mL concentration.

Photoelectrochemical Performance

Porphyrins (and derived compounds) have recently been employed for photocatalytic/photoelectrochemical water-splitting and CO2 reduction reactions owing to their extended visible-light absorption and tunable optoelectronic properties. − To test possible solar energy conversion, the photoelectrochemical activity of thin films of dimer 4 and monomer 5 were compared. Cyclic voltammograms presented in Figure a, measured under dark conditions, reveal a factor 3.4 times increment in the cathodic current at −0.1 V vs RHE for monomer 5 compared to dimer 4, indicating an impeded charge transport and transfer behavior in the latter. Upon comparing the magnitude of photoinduced enhancement in the cathodic current, as shown in Figure b,c, monomer 5 showed more than a factor of 2 increment associated with a lower onset potential than dimer 4. Despite absorbing light efficiently, inadequate photoinduced enhancement in the cathodic current indicated inefficient charge transport in dimer 4, likely due to the lack of the long-range order critical to promote carrier transport to the electrolyte/water interface. This notion, again, is in good agreement with the computationally predicted V-shaped nonplanar geometry of dimer 4 (Figure ), which inhibits the formation of a larger aggregation compared to that of monomer 5. Overall, the data presented in Figure highlights the critical role of molecular packing (aggregation), besides the good light absorption behavior, in promoting efficient charge transport toward the porphyrin/water interface and their potential applications in solar energy harvesting.

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(a) Comparison of cyclic voltammograms of monomer 5 and dimer 4 under dark conditions. Effect of light on the photocathodic current for thin films of (b) dimer 4 and (c) monomer 5 coated on ITO. Linear sweep voltammetry was conducted in 0.1 M K2SO4 (pH ≈ 5.6) at a scan rate of 50 mV s–1. The light source used was a solar simulator (AM 1.5) equipped with a 420 nm cutoff filter to study the visible-light excitation effect.

Conclusions

The Suzuki–Miyaura cross-coupling protocol was used as a key step to synthesize the azulene-bridged Zn-porphyrin dimer 3, which, upon treatment with TFA, yielded dimer 4 containing free-base porphyrins. Both dimers were characterized by NMR spectroscopy and HRMS. Dimer 4 displayed red-shifted absorption and emission bands in the solution state compared to its corresponding monomer 5, indicative of an extended π-electron delocalization arising from azulene incorporation. However, the blue-shifted absorption of dimer 4 in the thin-film state and the significant reduction in its fluorescence lifetime suggested the absence of long-range J-aggregation, consequently reducing its photoelectrochemical water-splitting (PEC-WS) efficacy compared to monomer 5. The computational calculations suggested a V-shaped geometry for dimer 4, resembling the phenylene-bridged porphyrin dimer previously reported by Osuka and co-workers. This V-shaped geometry inhibits long-range aggregation, which is essential for facilitating exciton or charge transport, thereby corroborating the experimental observations. These findings provide valuable insights into the relationship between the structure and optoelectronic properties in a relatively unexplored class of covalently tethered azulene–porphyrin hybrid molecules. Furthermore, they emphasize the need for precise structural modifications of the dimer to enhance control over molecular aggregation, which is crucial for optoelectronic applications. Constructing porphyrin dimers linked by a 2,6-azulene connection could provide a better understanding of how the azulene connection pattern influences the aggregation behavior and optoelectronic properties.

Supplementary Material

ao5c05525_si_001.pdf (1.7MB, pdf)

Acknowledgments

S.N.S. thanks the Anusandhan National Research Foundation (ANRF, erstwhile SERB), Government of India, for junior and senior research fellowships. V.S.S. thanks the ANRF, Government of India, for a Core Research Grant (File No. CRG/2021/001652). The authors thank the Central Research Facility (CRF), NITK Surathkal, for providing various instrumental facilities. S.N.S. and V.S.S. acknowledge the help of Shubham Tiwari and Manoj N. Shet during the NMR and mass data acquisition. S.C. acknowledges the Junior Research Fellowship from the Department of Science & Technology. D.H.K.M. acknowledges funding from the Technology Mission Division (Energy, Water & all Others), Department of Science & Technology, Ministry of Science & Technology, Government of India (Reference Number DST/TMD/IC-MAP/2K20/02), project titled Integrated Clean-Energy Material Acceleration Platform (IC-MAP) on bioenergy and hydrogen.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c05525.

  • Copies of 1H, 13C NMR, and high-resolution mass spectra of compounds 3 and 4; solution-state absorption and cyclic voltammetry plot of ferrocene; and Cartesian coordinates of compounds 4 and 5. (PDF)

The authors declare no competing financial interest.

This article is dedicated to Professor M. Ravikanth on his 60th birthday.

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